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So, I was doing Data Structure and ALgorithm using C++ and STL, I was trying to implement Infix to postfix using stack. I am not sure what is this issue with the code? There is no compile error and when the code runs it returns -1073741510. I have rechecked the whole code, couldn't found any issues
#include<iostream>
#include<stack>
#include<string>
using namespace std;
int isOperator(char ch)
{
if(ch=='+' || ch == '-' || ch == '*' || ch == '/')
return 1;
else
return 0;
}
int precedence(char ch)
{
//for limited input only
if(ch == '*' || ch == '/')
return 3;
else if(ch=='+' || ch == '-' )
return 2;
else
return 0;
}
string infixtopostfix(string infix)
{
stack <char> st;
int i=0;
string postfix;
while(infix[i]!='\0')
{
if(!isOperator(infix[i]))
{
postfix.push_back(infix[i]);
i++;
}
else
{
if(precedence(infix[i])>precedence(st.top()))
{
st.push(infix[i]);
i++;
}
else{
postfix.push_back(st.top());
st.pop();
}
}
}
while(!st.empty())
{
postfix.push_back(st.top());
st.pop();
}
return postfix;
}
int main()
{
string infix= "a+b";
cout<<"Postfix-->"<<infixtopostfix(infix)<<endl;
return 0;
}
Your postfix string has a size of zero. This means that any attempt to access the characters of that string is an error. If you want to add characters to a string use push_back.
postfix[j] = infix[i];
should be
postfix.push_back(infix[i]);
and (twice)
postfix[j] = st.top();
should be
postfix.push_back(st.top());
and
postfix[j]='\0';
should be removed.
There is no need to nul terminate std::string. Once you have made all these changes you will also see that the j variable can be removed. A std::string knows it's own size, you don't need a separate variable to keep track. It seems that you are programming a std::string as if it's like a C string.
It seems to be a very common misunderstanding that std::string (or a std::vector) automatically grows when you subscript it. This is not true.
EDIT
You have another error here,
if(precedence(infix[i])>precedence(st.top()))
The stack maybe empty when executing this statement, leading to a crash. I'm guessing the code should read
if(st.empty() || precedence(infix[i])>precedence(st.top()))
With that change your code works for me.
Thank You everyone for the help. Here is the final answer which is running without any error.
#include<iostream>
#include<stack>
#include<string>
using namespace std;
int isOperator(char ch)
{
if(ch=='+' || ch == '-' || ch == '*' || ch == '/')
return 1;
else
return 0;
}
int precedence(char ch)
{
//for limited input only
if(ch == '*' || ch == '/')
return 3;
else if(ch=='+' || ch == '-' )
return 2;
else
return 0;
}
string infixtopostfix(string infix)
{
stack <char> st;
int i=0;
string postfix;
while(infix[i]!='\0')
{
if(!isOperator(infix[i]))
{
postfix.push_back(infix[i]);
i++;
}
else{
if(st.empty() || precedence(infix[i])>precedence(st.top()))
{
st.push(infix[i]);
i++;
}
else{
postfix.push_back(st.top());
st.pop();
}
}
}
while(!st.empty())
{
postfix.push_back(st.top());
st.pop();
}
return postfix;
}
int main()
{
string infix= "a+b";
cout<<"Postfix-->"<<infixtopostfix(infix)<<endl;
return 0;
}
My lecturer gave me an assignment to create a program to convert and infix expression to postfix using Stacks. I've made the stack classes and some functions to read the infix expression.
But this one function, called convertToPostfix(char * const inFix, char * const postFix) which is responsible to convert the inFix expression in the array inFix to the post fix expression in the array postFix using stacks, is not doing what it suppose to do. Can you guys help me out and tell me what I'm doing wrong?
The following is code where the functions to convert from inFix to postFix is and convertToPostfix(char * const inFix, char * const postFix) is what I need help fixing:
void ArithmeticExpression::inputAndConvertToPostfix()
{
char inputChar; //declaring inputChar
int i = 0; //inizalize i to 0
cout << "Enter the Arithmetic Expression(No Spaces): ";
while( ( inputChar = static_cast<char>( cin.get() ) ) != '\n' )
{
if (i >= MAXSIZE) break; //exits program if i is greater than or equal to 100
if(isdigit(inputChar) || isOperator(inputChar))
{
inFix[i] = inputChar; //copies each char to inFix array
cout << inFix[i] << endl;
}
else
cout << "You entered an invalid Arithmetic Expression\n\n" ;
}
// increment i;
i++;
convertToPostfix(inFix, postFix);
}
bool ArithmeticExpression::isOperator(char currentChar)
{
if(currentChar == '+')
return true;
else if(currentChar == '-')
return true;
else if(currentChar == '*')
return true;
else if(currentChar == '/')
return true;
else if(currentChar == '^')
return true;
else if(currentChar == '%')
return true;
else
return false;
}
bool ArithmeticExpression::precedence(char operator1, char operator2)
{
if ( operator1 == '^' )
return true;
else if ( operator2 == '^' )
return false;
else if ( operator1 == '*' || operator1 == '/' )
return true;
else if ( operator1 == '+' || operator1 == '-' )
if ( operator2 == '*' || operator2 == '/' )
return false;
else
return true;
return false;
}
void ArithmeticExpression::convertToPostfix(char * const inFix, char * const postFix)
{
Stack2<char> stack;
const char lp = '(';
stack.push(lp); //Push a left parenthesis ‘(‘ onto the stack.
strcat(inFix,")");//Appends a right parenthesis ‘)’ to the end of infix.
// int i = 0;
int j = 0;
if(!stack.isEmpty())
{
for(int i = 0;i < 100;){
if(isdigit(inFix[i]))
{
postFix[j] = inFix[i];
cout << "This is Post Fix for the first If: " << postFix[j] << endl;
i++;
j++;
}
if(inFix[i] == '(')
{
stack.push(inFix[i]);
cout << "The InFix was a (" << endl;
i++;
//j++;
}
if(isOperator(inFix[i]))
{
char operator1 = inFix[i];
cout << "CUrrent inFix is a operator" << endl;
if(isOperator(stack.getTopPtr()->getData()))
{
cout << "The stack top ptr is a operator1" << endl;
char operator2 = stack.getTopPtr()->getData();
if(precedence(operator1,operator2))
{
//if(isOperator(stack.getTopPtr()->getData())){
cout << "The stack top ptr is a operato2" << endl;
postFix[j] = stack.pop();
cout << "this is post fix " << postFix[j] << endl;
i++;
j++;
// }
}
}
else
stack.push(inFix[i]);
// cout << "Top Ptr is a: "<< stack.getTopPtr()->getData() << endl;
}
for(int r = 0;r != '\0';r++)
cout << postFix[r] << " ";
if(inFix[i] == ')')
{
while(stack.stackTop()!= '(')
{
postFix[j] = stack.pop();
i++;
j++;
}
stack.pop();
}
}
}
}
Note the function convertToPostfix was made using this algorithm:
Push a left parenthesis ‘(‘ onto the stack.
Append a right parenthesis ‘)’ to the end of infix.
While the stack is not empty, read infix from left to right and do the following:
If the current character in infix is a digit, copy it to the next element of postfix.
If the current character in infix is a left parenthesis, push it onto the stack.
If the current character in infix is an operator,
Pop operator(s) (if there are any) at the top of the stack while they have equal or higher precedence than the current operator, and insert the popped operators in postfix.
Push the current character in infix onto the stack.
If the current character in infix is a right parenthesis
Pop operators from the top of the stack and insert them in postfix until a left parenthesis is at the top of the stack.
Pop (and discard) the left parenthesis from the stack.
This is basically a comment to the answer from Yuushi.
The outer while(!stack.empty()) loop is wrong. just remove it. (keep the loop body ofc). At the end of the function, check that the stack is empty, else the expression had syntax errors.
As Yuushi already said the precedence function looks bogus. First you should give the parameters better names: one is the operator to the left, and the other to the right. (Right now you call it precedence(rightOp, leftOp)). Then you should document what the result means - right now you return true if a rOp b lOp c == (a rOp b) lOp c (yes, the operator order doesn't match what you call - "+" and "-" are not the same in both orders for example).
If you find a new operator you need to loop over the old operators on the stack, for example after reading a - b * c your output is a b c and the stack is [- *]. now you read a +, and you need to pop both operators, resulting in a b c * -. I.e., the input a - b * c + d should result in a b c * - d +
Update: appended complete solution (based on Yuushi's answer):
bool isOperator(char currentChar)
{
switch (currentChar) {
case '+':
case '-':
case '*':
case '/':
case '^':
case '%':
return true;
default:
return false;
}
}
// returns whether a `lOp` b `rOp` c == (a `lOp` b) `rOp` c
bool precedence(char leftOperator, char rightOperator)
{
if ( leftOperator == '^' ) {
return true;
} else if ( rightOperator == '^' ) {
return false;
} else if ( leftOperator == '*' || leftOperator == '/' || leftOperator == '%' ) {
return true;
} else if ( rightOperator == '*' || rightOperator == '/' || rightOperator == '%' ) {
return false;
}
return true;
}
#include <stdexcept>
#include <cctype>
#include <sstream>
#include <stack>
std::string convertToPostfix(const std::string& infix)
{
std::stringstream postfix; // Our return string
std::stack<char> stack;
stack.push('('); // Push a left parenthesis ‘(‘ onto the stack.
for(std::size_t i = 0, l = infix.size(); i < l; ++i) {
const char current = infix[i];
if (isspace(current)) {
// ignore
}
// If it's a digit or '.' or a letter ("variables"), add it to the output
else if(isalnum(current) || '.' == current) {
postfix << current;
}
else if('(' == current) {
stack.push(current);
}
else if(isOperator(current)) {
char rightOperator = current;
while(!stack.empty() && isOperator(stack.top()) && precedence(stack.top(), rightOperator)) {
postfix << ' ' << stack.top();
stack.pop();
}
postfix << ' ';
stack.push(rightOperator);
}
// We've hit a right parens
else if(')' == current) {
// While top of stack is not a left parens
while(!stack.empty() && '(' != stack.top()) {
postfix << ' ' << stack.top();
stack.pop();
}
if (stack.empty()) {
throw std::runtime_error("missing left paren");
}
// Discard the left paren
stack.pop();
postfix << ' ';
} else {
throw std::runtime_error("invalid input character");
}
}
// Started with a left paren, now close it:
// While top of stack is not a left paren
while(!stack.empty() && '(' != stack.top()) {
postfix << ' ' << stack.top();
stack.pop();
}
if (stack.empty()) {
throw std::runtime_error("missing left paren");
}
// Discard the left paren
stack.pop();
// all open parens should be closed now -> empty stack
if (!stack.empty()) {
throw std::runtime_error("missing right paren");
}
return postfix.str();
}
#include <iostream>
#include <string>
int main()
{
for (;;) {
if (!std::cout.good()) break;
std::cout << "Enter the Arithmetic Expression: ";
std::string infix;
std::getline(std::cin, infix);
if (infix.empty()) break;
std::cout << "Postfix: '" << convertToPostfix(infix) << "'\n";
}
return 0;
}
So there are a number of problems with your code. I'll post what (should be) a corrected solution, which has copious comments to explain what's happening and where you've made mistakes. A few things up front:
I'll use std::string instead of char * because it makes things much cleaner, and honestly, you should be using it in C++ unless you have a very good reason not to (such as interoperability with a C library). This version also returns a string instead of taking a char * as a parameter.
I'm using the stack from the standard library, <stack>, which is slightly different to your home-rolled one. top() shows you the next element without removing it from the stack, and pop() returns void, but removes the top element from the stack.
It's a free function, not part of a class, but that should be easy to modify - it's simply easier for me to test this way.
I'm not convinced your operator precedence tables are correct, however, I'll let you double check that.
#include <stack>
#include <cctype>
#include <iostream>
std::string convertToPostfix(std::string& infix)
{
std::string postfix; //Our return string
std::stack<char> stack;
stack.push('('); //Push a left parenthesis ‘(‘ onto the stack.
infix.push_back(')');
//We know we need to process every element in the string,
//so let's do that instead of having to worry about
//hardcoded numbers and i, j indecies
for(std::size_t i = 0; i < infix.size(); ++i) {
//If it's a digit, add it to the output
//Also, if it's a space, add it to the output
//this makes it look a bit nicer
if(isdigit(infix[i]) || isspace(infix[i])) {
postfix.push_back(infix[i]);
}
//Already iterating over i, so
//don't need to worry about i++
//Also, these options are all mutually exclusive,
//so they should be else if instead of if.
//(Mutually exclusive in that if something is a digit,
//it can't be a parens or an operator or anything else).
else if(infix[i] == '(') {
stack.push(infix[i]);
}
//This is farily similar to your code, but cleaned up.
//With strings we can simply push_back instead of having
//to worry about incrementing some counter.
else if(isOperator(infix[i]))
{
char operator1 = infix[i];
if(isOperator(stack.top())) {
while(!stack.empty() && precedence(operator1,stack.top())) {
postfix.push_back(stack.top());
stack.pop();
}
}
//This shouldn't be in an else - we always want to push the
//operator onto the stack
stack.push(operator1);
}
//We've hit a right parens - Why you had a for loop
//here originally I don't know
else if(infix[i] == ')') {
//While top of stack is not a right parens
while(stack.top() != '(') {
//Insert into postfix and pop the stack
postfix.push_back(stack.top());
stack.pop();
}
// Discard the left parens - you'd forgotten to do this
stack.pop();
}
}
//Remove any remaining operators from the stack
while(!stack.empty()) {
postfix.push_back(stack.top());
stack.pop();
}
}
Here's mine using C with multiple digits evaluation.
#include <stdio.h>
#include <math.h>
#define MAX 50
void push(char[],char);
void in_push(double[], double);
int pop();
int prec(char);
double eval(char[],int,double[]);
int top = 0;
void main() {
double eval_stack[MAX];
int op_count=0;
char stack[MAX], exps[MAX], symbols[MAX];
int i=0,j=0,len,check;
while((symbols[i]=getchar())!='\n') {
if(symbols[i]!=' ' || symbols[i]!='\t') {
if(symbols[i]=='+' || symbols[i]=='-' || symbols[i]=='/' || symbols[i]=='*' || symbols[i]=='^')
op_count++;
i++;
}
}
symbols[i]='#';
symbols[++i]='\0';
len = strlen(symbols);
stack[top] = '#';
for(i=0; i<=len; i++) {
if(symbols[i]>='a' && symbols[i]<='z') {
exps[j]=symbols[i];
j++;
}
switch(symbols[i]) {
case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9':
//if(symbols[i]>='a' && symbols[i]<='z') {
exps[j]=symbols[i];
j++;
break;
case '+': case '-': case '*': case '/': case '^':
exps[j++] = ' ';
while(prec(symbols[i]) <= prec(stack[top])) {
exps[j] = stack[top];
pop();
//printf("\n\t\t%d\t\t%d\n", top,j);
j++;
}
if(prec(symbols[i]) > prec(stack[top])) {
push(stack,symbols[i]);
}
break;
case '(':
push(stack,symbols[i]);
break;
case ')':
while(stack[top]!='(') {
exps[j] = stack[top];
pop();
j++;
}
pop();
break;
case '#':
exps[j++] = ' ';
while(stack[top]!='#') {
exps[j] = stack[top];
pop();
j++;
}
pop();
break;
}
}
exps[j]='\0';
printf("Postfix: %s", exps);
for(i=0; i<j; i++)
if(exps[i]=='a')
check = 1;
if(check!=1)
printf("\nSolution: %.1f", eval(exps,j,eval_stack));
}
double eval(char exps[],int exps_len,double eval_stack[]) {
int i; int len=exps_len,temp;
double in_temp[MAX],t;
int count,power,j,in_count;
count=power=j=t=in_count=0;
double result;
for(i=0; i<len; i++) {
switch(exps[i]) {
case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9':
in_temp[i] = exps[i]-'0';
j=i+1;
while(exps[j]>='0' && exps[j]<='9') {
in_temp[j] = exps[j]-'0';
j++; // 2
}
count = i; // 3
while(in_temp[count]<='0' && in_temp[count]<='9') {
power = (j-count)-1;
t = t + in_temp[count]*(pow(10,power));
power--;
count++;
}
in_push(eval_stack,t);
i=j-1;
t=0;
break;
case '+':
temp = pop();
pop();
result = eval_stack[temp] + eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '-':
temp = pop();
pop();
result = eval_stack[temp] - eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '*':
temp = pop();
pop();
result = eval_stack[temp] * eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '/':
temp = pop();
pop();
result = eval_stack[temp] / eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '^':
temp = pop();
pop();
result = pow(eval_stack[temp],eval_stack[temp+1]);
in_push(eval_stack,result);
break;
}
}
return eval_stack[top];
}
int prec(char a) {
if(a=='^')
return 3;
else if(a=='*' || a=='/' || a=='%')
return 2;
else if(a=='+' || a=='-')
return 1;
else if(a=='(')
return 0;
else
return -1;
}
void push(char stack[], char ele) {
if(top>=MAX) {
printf("\nStack Overflow");
exit(1);
}
stack[++top] = ele;
}
void in_push(double stack[], double ele) {
if(top>=MAX) {
printf("\nStack Overflow");
exit(1);
}
stack[++top] = ele;
}
int pop() {
if(top<0) {
printf("\nStack Underflow");
exit(1);
}
top = top - 1;
return top;
}
This is my implementation of converting infix to postfix expression
//Infix to Postfix conversion
#include <bits/stdc++.h>
using namespace std;
bool isoperator(char c) // function to check if the character is an operator
{
if(c=='+'||c=='-'||c=='*'||c=='/'||c=='^')
return true;
else
return false;
}
int precedence(char c) // function to given the precedence of the operators
{
if(c == '^')
return 3;
else if(c == '*' || c == '/')
return 2;
else if(c == '+' || c == '-')
return 1;
else
return -1;
}
void infixToPostfix(string s) // funtion to convert infix to postfix
{
stack<char>st;
string postfix;
for(int i=0;i<s.length();i++)
{
if((s[i]>='a'&&s[i]<='z')||(s[i]>='A'&&s[i]<='Z')) // if the given character is alphabet add it to the postfix string
postfix+=s[i];
else if(s[i]=='(') // if the given character is "(" add it to the postfix string
st.push('(');
else if(s[i]==')') // if we find a closing bracket we pop everything from stack till opening bracket and add it to postfix string
{
while(st.top()=='(' && !st.empty())
{
postfix+=st.top();
st.pop();
}
if(st.top()=='(') // popping the opening bracket
st.pop();
}
else if(isoperator(s[i])) // if we find a operator
{
if(st.empty()) // if stack is empty add it to the stack
st.push(s[i]);
else
{
if(precedence(s[i])>precedence(st.top())) // if operator precedence is grater push it in stack
st.push(s[i]);
else if((precedence(s[i])==precedence(st.top()))&&(s[i]=='^')) // unique case for ^ operator
st.push(s[i]);
else
{
while((!st.empty())&&(precedence(s[i])<=precedence(st.top()))) // if precedence of st.top() is greater than s[i] adding it the postfix string
{
postfix+=st.top();
st.pop();
}
st.push(s[i]); // pushing s[i] in the stack
}
}
}
}
while(!st.empty()) // popping the remaining items from the stack and adding it to the postfix string
{
postfix+=st.top();
st.pop();
}
cout<<postfix<<endl; // printing the postfix string
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
string s;
cin>>s;
infixToPostfix(s);
return 0;
}
Example:
Input: a+b*(c^d-e)^(f+g*h)-i
Output: abcd^efgh*+i-(^-(*+
ps: If you find any mistakes, comment below :)
C++ implementation is given below:
void infix2postfix(string s)
{
stack<char>st;
for(int i=0; i<s.length(); i++)
{
if(isdigit(s[i]) || isalpha(s[i])) cout<<s[i];
else if( s[i]==')' )
{
while(st.top()!='(')
{
cout<<st.top();
st.pop();
}
st.pop();
}
else st.push(s[i]);
}
}
Operator Precedence is the problem in this case. The correct operator precedence in descending order is:
mul, div, mod << *, /, % >>
add, sub << +, - >>
XOR << ^ >>
In the question above consider the precedence function
bool ArithmeticExpression::precedence(char operator1, char operator2)
{
if ( operator1 == '^' )
return true;
else if ( operator2 == '^' )
return false;
else if ( operator1 == '*' || operator1 == '/' )
return true;
else if ( operator1 == '+' || operator1 == '-' )
if ( operator2 == '*' || operator2 == '/' )
return false;
else
return true;
return false;
}
for each value in operator1 corresponding value of operator2 should be checked for precedence, according to OPERATOR PRECEDENCE TABLE mentioned above. Do not return any value without proper comparison.
I just had a small question about my program. So, I have a function that evaluates a postfix expression and returns the integer or float calculated. Here are the following functions involved:
#include <iostream>
#include <sstream>
#include <cstring>
#include <stack>
#include <limits>
float postfixUtility::evaluatePostfix(string pexp)
{
stack<int> S;
int pexpLength = pexp.length();
cout << pexpLength << endl;
for (int i = 0; i < pexpLength; i++)
{
if(pexp[i] == ' ' || pexp[i] == ',')
{
continue;
}
else if(isOperator(pexp[i]))
{
int operand2 = S.top(); S.pop();
int operand1 = S.top(); S.pop();
int result = isOperate(pexp[i], operand1, operand2);
S.push(result);
}
else if(isDigit(pexp[i]))
{
int operand = 0;
while(i<pexp.length() && isDigit(pexp[i]))
{
operand = (operand*10) + (pexp[i] - '0');
i++;
}
i--;
S.push(operand);
}
}
return S.top();
}
bool postfixUtility::isDigit(char C)
{
if(C >= '0' && C <= '9')
{
return true;
}
return false;
}
bool postfixUtility::isOperator(char C)
{
if(C == '+' || C == '-' || C == '*' || C == '/')
{
return true;
}
return false;
}
int postfixUtility::isOperate(char operation, int operand1, int operand2)
{
if(operation == '+')
{
return operand1+operand2;
}
if(operation == '-')
{
return operand1-operand2;
}
if(operation == '*')
{
return operand1*operand2;
}
if(operation == '/')
{
return operand1/operand2;
}
}
These functions work together to solve the postfix expression inputted. The expression inputted is not empty and actually holds a postfix expression. However, every time I run the code, it results in a segfault. I am quite baffled actually because it seems to me that my code should work.
Thank you!
EDIT #1: So, the original input to my function is: "(4+3* 12)/ ( 12+ 3/ 2+ 46 /4)"
Then, I put that through another function to convert it to postfix. This is that function:
int postfixUtility::priority(char a)
{
int temp;
if (a == '^')
temp = 1;
else if (a == '*' || a == '/')
temp = 2;
else if (a == '+' || a == '-')
temp = 3;
return temp;
}
string postfixUtility::getPostfix(string nexp)
{
stack<char> operator_stack;
stringstream output;
for (unsigned i = 0; i < nexp.length(); i++) {
if (nexp[i] == '+' || nexp[i] == '-' || nexp[i] == '*' || nexp[i] == '/' || nexp[i] == '^') {
while (!operator_stack.empty() && priority(operator_stack.top()) <= priority(nexp[i])) {
output << operator_stack.top();
operator_stack.pop();
}
operator_stack.push(nexp[i]);
} else if (nexp[i] == '(') {
operator_stack.push(nexp[i]);
} else if (nexp[i] == ')') {
while (operator_stack.top() != '(') {
output << operator_stack.top();
operator_stack.pop();
}
operator_stack.pop();
} else {
output << nexp[i];
}
}
while (!operator_stack.empty()) {
output << operator_stack.top();
operator_stack.pop();
}
//cin.ignore(numeric_limits<streamsize>::max(), '\n');
return output.str();
}
Which converts it to: "43 12*+ 12 3 2/+ 46 4/+/", is that wrong? Would that be the reason why I am getting a segfault?
EDIT #2: So, I have commented out 2 lines in my code and I am no longer getting a segfault.
float postfixUtility::evaluatePostfix(string pexp)
{
stack<int> S;
int pexpLength = pexp.length();
for (int i = 0; i < pexpLength; i++)
{
if(pexp[i] == ' ' || pexp[i] == ',')
{
continue;
}
else if(isOperator(pexp[i]))
{
float operand2 = S.top();
//S.pop();
float operand1 = S.top();
//S.pop();
float result = isOperate(pexp[i], operand1, operand2);
S.push(result);
}
else if(isDigit(pexp[i]))
{
int operand = 0;
while(i<pexp.length() && isDigit(pexp[i]))
{
operand = (operand*10) + (pexp[i] - '0');
i++;
}
i--;
S.push(operand);
}
}
return S.top();
}
However, the answer is supposed to be 1.6, but I am getting 1. Any reason why that is happening?
EDIT #3: I changed the isOperate function to the following:
float postfixUtility::isOperate(char operation, float operand1, float operand2)
{
if(operation == '+')
{
return operand1+operand2;
}
else if(operation == '-')
{
return operand1-operand2;
}
else if(operation == '*')
{
return operand1*operand2;
}
else if(operation == '/')
{
return operand1/operand2;
}
}
However, I still get 1 as a result.
Those pops where important:
float operand2 = S.top();
//S.pop();
float operand1 = S.top();
//S.pop();
Without popping the last element, operand1 will always be equal to operand2 and dividing a number by itself usually results in 1.
The reason you where getting a segfault in the first place is that your postfix converter is giving you (very) wrong results (which should be clear from the fact that its output contains numbers not present in its input). Let's have look at the first part of the output you posted: 43 12*+. Your parser will identify 43 and 12 as numbers and push them on the stack. It will identify * as an operator, take the two numbers from the stack, multiply them and push the result on the stack. Then it will encounter + and try to take two operands from the stack. However, there's only one element on the stack, namely the result of the multiplication. Calling top() on an empty stack is causing your segfault.
Edit: It is generally a good idea to do a sanity check before performing a operation that could result in undefined behavior for incorrect input.This allows you to either do something to solve the problem or to produce an error message containing diagnostic information (e.g. in your case the operator, the position in the string, the string itself, etc.). This will help you identify such problems more easily.
tl;dr: Fix your postfix converter.
im studying c++, and im stuck on understanding certain line of code.
bool IsOperator(char C)
{
if(C == '+' || C == '-' || C == '*' || C == '/' || C== '$')
return true;
return false;
}
The program im trying to understand scans if the char in string is operator. If yes, get him a number. Inflix to postflix is the problem here.
int IsRightAssociative(char op)
{
if(op == '$') return true;
return false;
}
Here is what i cant understand, where the program finds the '$' char if i write ((3+2)*3-5)*2??
I will attach the whole code for better understanding
#include<iostream>
#include<stack>
#include<string>
using namespace std;
// Function to convert Infix expression to postfix
string InfixToPostfix(string expression);
// Function to verify whether an operator has higher precedence over other
int HasHigherPrecedence(char operator1, char operator2);
// Function to verify whether a character is operator symbol or not.
bool IsOperator(char C);
// Function to verify whether a character is alphanumeric chanaract (letter or numeric digit) or not.
bool IsOperand(char C);
int main()
{
string expression;
cout<<"Enter Infix Expression \n";
getline(cin,expression);
string postfix = InfixToPostfix(expression);
cout<<"Output = "<<postfix<<"\n";
}
// Function to evaluate Postfix expression and return output
string InfixToPostfix(string expression)
{
// Declaring a Stack from Standard template library in C++.
stack<char> S;
string postfix = ""; // Initialize postfix as empty string.
for(int i = 0;i< expression.length();i++) {
// Scanning each character from left.
// If character is a delimitter, move on.
if(expression[i] == ' ' || expression[i] == ',') continue;
// If character is operator, pop two elements from stack, perform operation and push the result back.
else if(IsOperator(expression[i]))
{
while(!S.empty() && S.top() != '(' && HasHigherPrecedence(S.top(),expression[i]))
{
postfix+= S.top();
S.pop();
}
S.push(expression[i]);
}
// Else if character is an operand
else if(IsOperand(expression[i]))
{
postfix +=expression[i];
}
else if (expression[i] == '(')
{
S.push(expression[i]);
}
else if(expression[i] == ')')
{
while(!S.empty() && S.top() != '(') {
postfix += S.top();
S.pop();
}
S.pop();
}
}
while(!S.empty()) {
postfix += S.top();
S.pop();
}
return postfix;
}
// Function to verify whether a character is english letter or numeric digit.
// We are assuming in this solution that operand will be a single character
bool IsOperand(char C)
{
if(C >= '0' && C <= '9') return true;
if(C >= 'a' && C <= 'z') return true;
if(C >= 'A' && C <= 'Z') return true;
return false;
}
// Function to verify whether a character is operator symbol or not.
bool IsOperator(char C)
{
if(C == '+' || C == '-' || C == '*' || C == '/' || C== '$')
return true;
return false;
}
// Function to verify whether an operator is right associative or not.
int IsRightAssociative(char op)
{
if(op == '$') return true;
return false;
}
// Function to get weight of an operator. An operator with higher weight will have higher precedence.
int GetOperatorWeight(char op)
{
int weight = -1;
switch(op)
{
case '+':
case '-':
weight = 1;
case '*':
case '/':
weight = 2;
case '$':
weight = 3;
}
return weight;
}
// Function to perform an operation and return output.
int HasHigherPrecedence(char op1, char op2)
{
int op1Weight = GetOperatorWeight(op1);
int op2Weight = GetOperatorWeight(op2);
// If operators have equal precedence, return true if they are left associative.
// return false, if right associative.
// if operator is left-associative, left one should be given priority.
if(op1Weight == op2Weight)
{
if(IsRightAssociative(op1)) return false;
else return true;
}
return op1Weight > op2Weight ? true: false;
}
So if anyone can help i will be very glad :(.
My lecturer gave me an assignment to create a program to convert and infix expression to postfix using Stacks. I've made the stack classes and some functions to read the infix expression.
But this one function, called convertToPostfix(char * const inFix, char * const postFix) which is responsible to convert the inFix expression in the array inFix to the post fix expression in the array postFix using stacks, is not doing what it suppose to do. Can you guys help me out and tell me what I'm doing wrong?
The following is code where the functions to convert from inFix to postFix is and convertToPostfix(char * const inFix, char * const postFix) is what I need help fixing:
void ArithmeticExpression::inputAndConvertToPostfix()
{
char inputChar; //declaring inputChar
int i = 0; //inizalize i to 0
cout << "Enter the Arithmetic Expression(No Spaces): ";
while( ( inputChar = static_cast<char>( cin.get() ) ) != '\n' )
{
if (i >= MAXSIZE) break; //exits program if i is greater than or equal to 100
if(isdigit(inputChar) || isOperator(inputChar))
{
inFix[i] = inputChar; //copies each char to inFix array
cout << inFix[i] << endl;
}
else
cout << "You entered an invalid Arithmetic Expression\n\n" ;
}
// increment i;
i++;
convertToPostfix(inFix, postFix);
}
bool ArithmeticExpression::isOperator(char currentChar)
{
if(currentChar == '+')
return true;
else if(currentChar == '-')
return true;
else if(currentChar == '*')
return true;
else if(currentChar == '/')
return true;
else if(currentChar == '^')
return true;
else if(currentChar == '%')
return true;
else
return false;
}
bool ArithmeticExpression::precedence(char operator1, char operator2)
{
if ( operator1 == '^' )
return true;
else if ( operator2 == '^' )
return false;
else if ( operator1 == '*' || operator1 == '/' )
return true;
else if ( operator1 == '+' || operator1 == '-' )
if ( operator2 == '*' || operator2 == '/' )
return false;
else
return true;
return false;
}
void ArithmeticExpression::convertToPostfix(char * const inFix, char * const postFix)
{
Stack2<char> stack;
const char lp = '(';
stack.push(lp); //Push a left parenthesis ‘(‘ onto the stack.
strcat(inFix,")");//Appends a right parenthesis ‘)’ to the end of infix.
// int i = 0;
int j = 0;
if(!stack.isEmpty())
{
for(int i = 0;i < 100;){
if(isdigit(inFix[i]))
{
postFix[j] = inFix[i];
cout << "This is Post Fix for the first If: " << postFix[j] << endl;
i++;
j++;
}
if(inFix[i] == '(')
{
stack.push(inFix[i]);
cout << "The InFix was a (" << endl;
i++;
//j++;
}
if(isOperator(inFix[i]))
{
char operator1 = inFix[i];
cout << "CUrrent inFix is a operator" << endl;
if(isOperator(stack.getTopPtr()->getData()))
{
cout << "The stack top ptr is a operator1" << endl;
char operator2 = stack.getTopPtr()->getData();
if(precedence(operator1,operator2))
{
//if(isOperator(stack.getTopPtr()->getData())){
cout << "The stack top ptr is a operato2" << endl;
postFix[j] = stack.pop();
cout << "this is post fix " << postFix[j] << endl;
i++;
j++;
// }
}
}
else
stack.push(inFix[i]);
// cout << "Top Ptr is a: "<< stack.getTopPtr()->getData() << endl;
}
for(int r = 0;r != '\0';r++)
cout << postFix[r] << " ";
if(inFix[i] == ')')
{
while(stack.stackTop()!= '(')
{
postFix[j] = stack.pop();
i++;
j++;
}
stack.pop();
}
}
}
}
Note the function convertToPostfix was made using this algorithm:
Push a left parenthesis ‘(‘ onto the stack.
Append a right parenthesis ‘)’ to the end of infix.
While the stack is not empty, read infix from left to right and do the following:
If the current character in infix is a digit, copy it to the next element of postfix.
If the current character in infix is a left parenthesis, push it onto the stack.
If the current character in infix is an operator,
Pop operator(s) (if there are any) at the top of the stack while they have equal or higher precedence than the current operator, and insert the popped operators in postfix.
Push the current character in infix onto the stack.
If the current character in infix is a right parenthesis
Pop operators from the top of the stack and insert them in postfix until a left parenthesis is at the top of the stack.
Pop (and discard) the left parenthesis from the stack.
This is basically a comment to the answer from Yuushi.
The outer while(!stack.empty()) loop is wrong. just remove it. (keep the loop body ofc). At the end of the function, check that the stack is empty, else the expression had syntax errors.
As Yuushi already said the precedence function looks bogus. First you should give the parameters better names: one is the operator to the left, and the other to the right. (Right now you call it precedence(rightOp, leftOp)). Then you should document what the result means - right now you return true if a rOp b lOp c == (a rOp b) lOp c (yes, the operator order doesn't match what you call - "+" and "-" are not the same in both orders for example).
If you find a new operator you need to loop over the old operators on the stack, for example after reading a - b * c your output is a b c and the stack is [- *]. now you read a +, and you need to pop both operators, resulting in a b c * -. I.e., the input a - b * c + d should result in a b c * - d +
Update: appended complete solution (based on Yuushi's answer):
bool isOperator(char currentChar)
{
switch (currentChar) {
case '+':
case '-':
case '*':
case '/':
case '^':
case '%':
return true;
default:
return false;
}
}
// returns whether a `lOp` b `rOp` c == (a `lOp` b) `rOp` c
bool precedence(char leftOperator, char rightOperator)
{
if ( leftOperator == '^' ) {
return true;
} else if ( rightOperator == '^' ) {
return false;
} else if ( leftOperator == '*' || leftOperator == '/' || leftOperator == '%' ) {
return true;
} else if ( rightOperator == '*' || rightOperator == '/' || rightOperator == '%' ) {
return false;
}
return true;
}
#include <stdexcept>
#include <cctype>
#include <sstream>
#include <stack>
std::string convertToPostfix(const std::string& infix)
{
std::stringstream postfix; // Our return string
std::stack<char> stack;
stack.push('('); // Push a left parenthesis ‘(‘ onto the stack.
for(std::size_t i = 0, l = infix.size(); i < l; ++i) {
const char current = infix[i];
if (isspace(current)) {
// ignore
}
// If it's a digit or '.' or a letter ("variables"), add it to the output
else if(isalnum(current) || '.' == current) {
postfix << current;
}
else if('(' == current) {
stack.push(current);
}
else if(isOperator(current)) {
char rightOperator = current;
while(!stack.empty() && isOperator(stack.top()) && precedence(stack.top(), rightOperator)) {
postfix << ' ' << stack.top();
stack.pop();
}
postfix << ' ';
stack.push(rightOperator);
}
// We've hit a right parens
else if(')' == current) {
// While top of stack is not a left parens
while(!stack.empty() && '(' != stack.top()) {
postfix << ' ' << stack.top();
stack.pop();
}
if (stack.empty()) {
throw std::runtime_error("missing left paren");
}
// Discard the left paren
stack.pop();
postfix << ' ';
} else {
throw std::runtime_error("invalid input character");
}
}
// Started with a left paren, now close it:
// While top of stack is not a left paren
while(!stack.empty() && '(' != stack.top()) {
postfix << ' ' << stack.top();
stack.pop();
}
if (stack.empty()) {
throw std::runtime_error("missing left paren");
}
// Discard the left paren
stack.pop();
// all open parens should be closed now -> empty stack
if (!stack.empty()) {
throw std::runtime_error("missing right paren");
}
return postfix.str();
}
#include <iostream>
#include <string>
int main()
{
for (;;) {
if (!std::cout.good()) break;
std::cout << "Enter the Arithmetic Expression: ";
std::string infix;
std::getline(std::cin, infix);
if (infix.empty()) break;
std::cout << "Postfix: '" << convertToPostfix(infix) << "'\n";
}
return 0;
}
So there are a number of problems with your code. I'll post what (should be) a corrected solution, which has copious comments to explain what's happening and where you've made mistakes. A few things up front:
I'll use std::string instead of char * because it makes things much cleaner, and honestly, you should be using it in C++ unless you have a very good reason not to (such as interoperability with a C library). This version also returns a string instead of taking a char * as a parameter.
I'm using the stack from the standard library, <stack>, which is slightly different to your home-rolled one. top() shows you the next element without removing it from the stack, and pop() returns void, but removes the top element from the stack.
It's a free function, not part of a class, but that should be easy to modify - it's simply easier for me to test this way.
I'm not convinced your operator precedence tables are correct, however, I'll let you double check that.
#include <stack>
#include <cctype>
#include <iostream>
std::string convertToPostfix(std::string& infix)
{
std::string postfix; //Our return string
std::stack<char> stack;
stack.push('('); //Push a left parenthesis ‘(‘ onto the stack.
infix.push_back(')');
//We know we need to process every element in the string,
//so let's do that instead of having to worry about
//hardcoded numbers and i, j indecies
for(std::size_t i = 0; i < infix.size(); ++i) {
//If it's a digit, add it to the output
//Also, if it's a space, add it to the output
//this makes it look a bit nicer
if(isdigit(infix[i]) || isspace(infix[i])) {
postfix.push_back(infix[i]);
}
//Already iterating over i, so
//don't need to worry about i++
//Also, these options are all mutually exclusive,
//so they should be else if instead of if.
//(Mutually exclusive in that if something is a digit,
//it can't be a parens or an operator or anything else).
else if(infix[i] == '(') {
stack.push(infix[i]);
}
//This is farily similar to your code, but cleaned up.
//With strings we can simply push_back instead of having
//to worry about incrementing some counter.
else if(isOperator(infix[i]))
{
char operator1 = infix[i];
if(isOperator(stack.top())) {
while(!stack.empty() && precedence(operator1,stack.top())) {
postfix.push_back(stack.top());
stack.pop();
}
}
//This shouldn't be in an else - we always want to push the
//operator onto the stack
stack.push(operator1);
}
//We've hit a right parens - Why you had a for loop
//here originally I don't know
else if(infix[i] == ')') {
//While top of stack is not a right parens
while(stack.top() != '(') {
//Insert into postfix and pop the stack
postfix.push_back(stack.top());
stack.pop();
}
// Discard the left parens - you'd forgotten to do this
stack.pop();
}
}
//Remove any remaining operators from the stack
while(!stack.empty()) {
postfix.push_back(stack.top());
stack.pop();
}
}
Here's mine using C with multiple digits evaluation.
#include <stdio.h>
#include <math.h>
#define MAX 50
void push(char[],char);
void in_push(double[], double);
int pop();
int prec(char);
double eval(char[],int,double[]);
int top = 0;
void main() {
double eval_stack[MAX];
int op_count=0;
char stack[MAX], exps[MAX], symbols[MAX];
int i=0,j=0,len,check;
while((symbols[i]=getchar())!='\n') {
if(symbols[i]!=' ' || symbols[i]!='\t') {
if(symbols[i]=='+' || symbols[i]=='-' || symbols[i]=='/' || symbols[i]=='*' || symbols[i]=='^')
op_count++;
i++;
}
}
symbols[i]='#';
symbols[++i]='\0';
len = strlen(symbols);
stack[top] = '#';
for(i=0; i<=len; i++) {
if(symbols[i]>='a' && symbols[i]<='z') {
exps[j]=symbols[i];
j++;
}
switch(symbols[i]) {
case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9':
//if(symbols[i]>='a' && symbols[i]<='z') {
exps[j]=symbols[i];
j++;
break;
case '+': case '-': case '*': case '/': case '^':
exps[j++] = ' ';
while(prec(symbols[i]) <= prec(stack[top])) {
exps[j] = stack[top];
pop();
//printf("\n\t\t%d\t\t%d\n", top,j);
j++;
}
if(prec(symbols[i]) > prec(stack[top])) {
push(stack,symbols[i]);
}
break;
case '(':
push(stack,symbols[i]);
break;
case ')':
while(stack[top]!='(') {
exps[j] = stack[top];
pop();
j++;
}
pop();
break;
case '#':
exps[j++] = ' ';
while(stack[top]!='#') {
exps[j] = stack[top];
pop();
j++;
}
pop();
break;
}
}
exps[j]='\0';
printf("Postfix: %s", exps);
for(i=0; i<j; i++)
if(exps[i]=='a')
check = 1;
if(check!=1)
printf("\nSolution: %.1f", eval(exps,j,eval_stack));
}
double eval(char exps[],int exps_len,double eval_stack[]) {
int i; int len=exps_len,temp;
double in_temp[MAX],t;
int count,power,j,in_count;
count=power=j=t=in_count=0;
double result;
for(i=0; i<len; i++) {
switch(exps[i]) {
case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9':
in_temp[i] = exps[i]-'0';
j=i+1;
while(exps[j]>='0' && exps[j]<='9') {
in_temp[j] = exps[j]-'0';
j++; // 2
}
count = i; // 3
while(in_temp[count]<='0' && in_temp[count]<='9') {
power = (j-count)-1;
t = t + in_temp[count]*(pow(10,power));
power--;
count++;
}
in_push(eval_stack,t);
i=j-1;
t=0;
break;
case '+':
temp = pop();
pop();
result = eval_stack[temp] + eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '-':
temp = pop();
pop();
result = eval_stack[temp] - eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '*':
temp = pop();
pop();
result = eval_stack[temp] * eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '/':
temp = pop();
pop();
result = eval_stack[temp] / eval_stack[temp+1];
in_push(eval_stack,result);
break;
case '^':
temp = pop();
pop();
result = pow(eval_stack[temp],eval_stack[temp+1]);
in_push(eval_stack,result);
break;
}
}
return eval_stack[top];
}
int prec(char a) {
if(a=='^')
return 3;
else if(a=='*' || a=='/' || a=='%')
return 2;
else if(a=='+' || a=='-')
return 1;
else if(a=='(')
return 0;
else
return -1;
}
void push(char stack[], char ele) {
if(top>=MAX) {
printf("\nStack Overflow");
exit(1);
}
stack[++top] = ele;
}
void in_push(double stack[], double ele) {
if(top>=MAX) {
printf("\nStack Overflow");
exit(1);
}
stack[++top] = ele;
}
int pop() {
if(top<0) {
printf("\nStack Underflow");
exit(1);
}
top = top - 1;
return top;
}
This is my implementation of converting infix to postfix expression
//Infix to Postfix conversion
#include <bits/stdc++.h>
using namespace std;
bool isoperator(char c) // function to check if the character is an operator
{
if(c=='+'||c=='-'||c=='*'||c=='/'||c=='^')
return true;
else
return false;
}
int precedence(char c) // function to given the precedence of the operators
{
if(c == '^')
return 3;
else if(c == '*' || c == '/')
return 2;
else if(c == '+' || c == '-')
return 1;
else
return -1;
}
void infixToPostfix(string s) // funtion to convert infix to postfix
{
stack<char>st;
string postfix;
for(int i=0;i<s.length();i++)
{
if((s[i]>='a'&&s[i]<='z')||(s[i]>='A'&&s[i]<='Z')) // if the given character is alphabet add it to the postfix string
postfix+=s[i];
else if(s[i]=='(') // if the given character is "(" add it to the postfix string
st.push('(');
else if(s[i]==')') // if we find a closing bracket we pop everything from stack till opening bracket and add it to postfix string
{
while(st.top()=='(' && !st.empty())
{
postfix+=st.top();
st.pop();
}
if(st.top()=='(') // popping the opening bracket
st.pop();
}
else if(isoperator(s[i])) // if we find a operator
{
if(st.empty()) // if stack is empty add it to the stack
st.push(s[i]);
else
{
if(precedence(s[i])>precedence(st.top())) // if operator precedence is grater push it in stack
st.push(s[i]);
else if((precedence(s[i])==precedence(st.top()))&&(s[i]=='^')) // unique case for ^ operator
st.push(s[i]);
else
{
while((!st.empty())&&(precedence(s[i])<=precedence(st.top()))) // if precedence of st.top() is greater than s[i] adding it the postfix string
{
postfix+=st.top();
st.pop();
}
st.push(s[i]); // pushing s[i] in the stack
}
}
}
}
while(!st.empty()) // popping the remaining items from the stack and adding it to the postfix string
{
postfix+=st.top();
st.pop();
}
cout<<postfix<<endl; // printing the postfix string
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
string s;
cin>>s;
infixToPostfix(s);
return 0;
}
Example:
Input: a+b*(c^d-e)^(f+g*h)-i
Output: abcd^efgh*+i-(^-(*+
ps: If you find any mistakes, comment below :)
C++ implementation is given below:
void infix2postfix(string s)
{
stack<char>st;
for(int i=0; i<s.length(); i++)
{
if(isdigit(s[i]) || isalpha(s[i])) cout<<s[i];
else if( s[i]==')' )
{
while(st.top()!='(')
{
cout<<st.top();
st.pop();
}
st.pop();
}
else st.push(s[i]);
}
}
Operator Precedence is the problem in this case. The correct operator precedence in descending order is:
mul, div, mod << *, /, % >>
add, sub << +, - >>
XOR << ^ >>
In the question above consider the precedence function
bool ArithmeticExpression::precedence(char operator1, char operator2)
{
if ( operator1 == '^' )
return true;
else if ( operator2 == '^' )
return false;
else if ( operator1 == '*' || operator1 == '/' )
return true;
else if ( operator1 == '+' || operator1 == '-' )
if ( operator2 == '*' || operator2 == '/' )
return false;
else
return true;
return false;
}
for each value in operator1 corresponding value of operator2 should be checked for precedence, according to OPERATOR PRECEDENCE TABLE mentioned above. Do not return any value without proper comparison.