I want to evaluate a binary expression tree. This is the code that I have mustered so far. I am destroying nodes of the tree as I evaluate it. But the problem is that during recursion it looks for a data that it doesn't have, I think. It simply keeps on returning 0.
void calc(bnode *&b)
{
bnode *c;
int m;
switch (b->data.ch)
{
case '+':m=b->lchild->data.in+b->rchild->data.in;
break;
case '-':m=b->rchild->data.in-b->lchild->data.in;
break;
case '*':m=b->lchild->data.in*b->rchild->data.in;
break;
case '/':m=b->lchild->data.in/b->rchild->data.in;
break;
case '%':m=b->lchild->data.in%b->rchild->data.in;
break;
}
c=new(bnode);
c->lchild=NULL;
c->rchild=NULL;
c->tag=1;
c->data.in=m;
b=c;
}
int eval(bnode *b)
{
if (b->tag==1)
return b->data.in;
else
{
if (b->lchild->tag==0)
eval(b->lchild);
if (b->rchild->tag==0)
eval(b->rchild);
if (b->lchild->tag==1&&b->rchild->tag==1)
calc(b);
}
}
And the structure I used is
union un
{
int in;
char ch;
};
struct bnode{
bnode *lchild;
un data;
int tag;
bnode *rchild;
};
The function eval() is broken. In the "else" branch, you never return any value, which is undefined behavior.
[Edit] The eval function should be changed to something like this.
int eval(bnode *b)
{
if (b->lchild && b->lchild->tag == 0)
eval(b->lchild);
if (b->rchild && b->rchild->tag == 0)
eval(b->rchild);
if (b->lchild && b->rchild && b->lchild->tag == 1 && b->rchild->tag == 1)
calc(b);
if (b->tag == 1)
return b->data.in;
else
throw "Evaluation error";
}
You also have memory leaks, because the bnode objects are never deleted.
Related
I was trying to solve this problem
https://practice.geeksforgeeks.org/problems/construct-an-expression-tree/1?utm_source=gfgpractice#
Problem: To create expression tree for given postfix expression
My Approach: I tried to maintain a stack of nodes and generated a tree using general postfix evaluation approach where the resultant for the given operator would be the node itself but with modified left and right pointers.
But I'm getting error.
Segmentation Fault (SIGSEGV)
Learn More about
Seg Fault
// { Driver Code Starts
#include<bits/stdc++.h>
using namespace std;
struct et
{
char value;
et* left, *right;
et(char x){
value = x;
left = right = NULL;
}
};
bool isOperator(char c)
{
if (c == '+' || c == '-' ||
c == '*' || c == '/' ||
c == '^')
return true;
return false;
}
void inorder(et *t)
{
if(t)
{
inorder(t->left);
printf("%c ", t->value);
inorder(t->right);
}
}
et* constructTree(char []);
int main()
{
int t;
cin>>t;
while(t--){
char postfix[25];
cin>>postfix;
et* r = constructTree(postfix);
inorder(r);
cout<<endl;
}
return 0;
}// } Driver Code Ends
/*struct et
{
char value;
et* left, *right;
};*/
//function to construct expression tree
et* constructTree(char postfix[])
{
//code here
stack<et> operand;
et *root=NULL;
for(int i=0;postfix[i] != '\0';++i){
if(postfix[i]>='a' && postfix[i]<='z'){
et node = et(postfix[i]);
//cout<<node.value<<" "<<node.left<<" "<<node.right<<"\n";
operand.push(node);
/*et nod = operand.top();
cout<<nod.value<<" "<<nod.left<<" "<<nod.right<<"\n";
operand.pop();
operand.push(nod);
et *no = &(operand.top());
cout<<no->value<<" "<<no->left<<" "<<no->right<<'\n';*/
}
else{
et right1=operand.top();
operand.pop();
et left1=operand.top();
operand.pop();
et node = (et(postfix[i]));
node.left = &left1;
node.right= &right1;
operand.push(node);
root=&(operand.top());
//cout<<node.value<<" "<<node.left<<" "<<node.right<<"\n";
//inorder(root);
}
}
//root=NULL;
return root;
}
Can anyone please tell me what to do so that I won't get an error ?
The problem is here
node.left = &left1;
left1 is a local variable and will be destroyed. So after that you're accessing an invalid address.
You can use something like this
node.left = new er(left1);
but you should prefer to store pointers in stack at first.
In function constructTree, you do not do any dynamic memory allocations.
So whatever memory pointer you return from that function, is pointing to an invalid memory by the time you're outside the function.
I have the following class when I get a pointer to a member function according to some condition and then call the function.
class Test
{
public:
bool isChar(char ch) { return (ch >= 'a' && ch <= 'z'); }
bool isNumeric(char ch) { return (ch >= '0' && ch <= '0'); }
enum class TestType
{
Undefined,
Char,
Numeric,
AnotherOne,
};
bool TestFor(TestType type, char ch)
{
typedef bool (Test::*fptr)(char);
fptr f = nullptr;
switch(type)
{
case TestType::Char:
f = &Test::isChar;
break;
case TestType::Numeric:
f = &Test::isNumeric;
break;
default: break;
}
if(f != nullptr)
{
return (this->*f)(ch);
}
return false;
}
};
But actually I don't like the syntax. Is there a way to replace
(this->*f)(ch)
with
f(ch)
?
In my real code the function a big enough and it's not so clear what (this->*f) is. I'm looking for some c++11 solution. I know about std::function and I will use it if if no solution will be found.
Update
The solution that I decided to use, if suddenly someone needs it: (thanks for #StoryTeller - Unslander Monica)
bool TestFor(TestType type, char ch)
{
bool(Test::* fptr)(char) = nullptr;
switch(type)
{
case TestType::Char:
fptr = &Test::isChar;
break;
case TestType::Numeric:
fptr = &Test::isNumeric;
break;
default: break;
}
if(fptr != nullptr)
{
auto caller = std::mem_fn(fptr);
return caller(this, ch);
}
return false;
}
If the syntax bothers you so much, you can always use std::mem_fn to generate a cheap one-time wrapper around a member function.
auto caller = std::mem_fn(f);
caller(this, ch);
I'm currently working on a project converting from postfix to infix using stacks in the form of linked lists. I'm currently trying to read in the whole line as a string then placing it into a character array then when a symbol is found placing one element into a right operand another into a left operand then printing it back out inlcuding the operator. however after placing the first item into the left operand and then popping the stack im not able to place the other item into the right operand. What could be the problem? I its with my pop fucntion.
Here is my code:
#include "stack.h"
stack::~stack()
{
cout<<"Inside !stack \n";
while(s_top != 0)
{
pop();
}
}
void stack::pop()
{
cout<<"Inside pop \n";
stack_node *p;
if (s_top != 0)
{
p = s_top;
s_top = s_top->next;
delete p;
}
}
void stack::push(char a)
{
cout<<"Inside push \n";
stack_node *p = new stack_node;
p->data = a;
p->next = s_top;
s_top = p;
}
void stack::print()
{
cout<<"Inside print \n";
for(stack_node *p = s_top; p!=0; p=p->next)
{
cout<<p->data<<endl;
}
}
stack_element stack::top()
{
cout<<"Inside top \n";
if (s_top == 0)
{
exit(1);
}
else
{
return s_top->data;
}
}
/*stack::stack(const stack & Org)
{
cout<<"Inside the Copy Constructor\n";
stack_node *p=Org.s_top;
(*this).s_top = 0;
while(p!=0)
{
(*this).push(p->data);
p=p->next;
}
}
and here is my cpp where it doesnt completely work
#include "stack.h"
string convert(string expression){
stack c;
string post = " ";
string rightop="";
string leftop="";
string op =" ";
for (int i =0; i<expression.length();i++){
c.push(expression[i]);
if(expression[i]=='*'||'+'||'-'||'/'){
cout<<c.top()<<endl;
leftop=c.top();
c.pop();
rightop=c.top();
cout<<rightop<<endl;
c.pop();
op=c.top();
c.pop();
}
}
}
int main(){
string expression;
cout<<" Enter a Post Fix expression: ";
getline(cin,expression);
convert(expression);
return 0;
}
Here's an issue:
(expression[i]=='*'||'+'||'-'||'/'
This does not do what you think it does.
The fix:
(expression[i] == '*' ||
expression[i] == '+' ||
expression[i] == '-' ||
expression[i] == '/')
Edit 1: Searching strings
Another method is:
char c = expression[i];
const std::string operators="*+-/";
if (operators.find(c) != std::string::npos)
{
// expression[i] is an operator character
}
The commonly posted solution is to use switch:
switch (expression[i])
{
case '+': Process_Operator_Plus(); break;
case '-': Process_Operator_Minus(); break;
case '*': Process_Operator_Multiply(); break;
case '/': Process_Operator_Divide(); break;
}
Remember, you will need to handle operator precedence when evaluating expressions.
I have written the following infix to postfix program but it's not working.
My program takes input but doesn't show any result. Can anyone help find the problem in my program.
And also it would be a great help if you tell if my Algorithm for converting infix to postfix is correct or not.
using namespace std;
class Stack
{
private:
int top;
char s[mx];
public:
Stack()
{
top=-1;
}
void push(char c)
{
if(!stackFull())
s[++top]=c;
}
void pop()
{
if(!stackEmpty())
top--;
else cout<<"Stack is empty"<<endl;
}
char topShow()
{
if(!stackEmpty())
return s[top];
}
bool stackEmpty()
{
if(top==-1)
return 1;
else return 0;
}
bool stackFull()
{
if(top == (mx-1))
return 1;
else return 0;
}
};
class Expression
{
private:
char entry2;
int precedence;
char infix[mx];
char postfix[mx];
public:
int prec(char symbol)
{
switch(symbol)
{
case '(':return 0; break;
case '-':return 1; break;
case '+':return 2; break;
case '*':return 3; break;
case '/':return 4; break;
}
}
void Read()
{
cout<<"Enter the infix expression: ";cin>>infix;
for(int i=0;infix[i]!='\0';i++)
{
convertToPostfix(infix[i]);
}
}
void ShowResult()
{
cout<<"Postfix expression"<<endl;
for(int j=0;postfix[j]!='\0';j++)
{
cout<<postfix[j];
}
}
void convertToPostfix(char c)
{
int p=0;
Stack myStack;
precedence=prec(c);
entry2=myStack.topShow();
if(isdigit(c))
{
postfix[++p]=c;
}
if(precedence>prec(entry2))
{
myStack.push(c);
}
if(precedence<prec(entry2))
{
switch(c)
{
case '(': myStack.push(c); break;
case ')': while(myStack.topShow()!= '(')
{
postfix[++p]=myStack.topShow();
myStack.pop();
};myStack.pop();break;
case '+':
case '-':
case '*':
case '/': while(prec(myStack.topShow())>=precedence)
{
postfix[++p]=myStack.topShow();
myStack.pop();
};break;
}
}
}
};
int main()
{
Expression myExp;
myExp.Read();
myExp.ShowResult();
return 0;
}
Here are some issues I found:
Boolean Functions Return true or false
Match return types with return values. The numbers 1 and 0 are not Boolean values.
Precedence table
Add and subtract have same precedence.
Multiply and divide have same precedence.
Multiply and divide have higher precedence than add and subtract.
Stack disappears
Since the stack is declared as a local variable in the function, it will be created fresh when entering the function and destroyed before exiting the function.
Solution: move it to the class as a class member or declare it as static.
Multiple statements per line are not more efficient
Blank lines and newlines do not affect performance, and add negligible time to the build.
However, they make your program more readable which helps when inspecting or debugging. Use them.
And similarly with space before and after operators.
Build the habit now rather than correcting when you get a job.
Call function once and store the value
You call prec(entry2) twice, which is a waste of time. Call it once and save the value in a variable. Similarly with stack.TopShow().
Use std::vector not an array
The std::vector will grow as necessary and reduce the chance of buffer overflow.
With an array, you must check that your indices are always within range. Also, array capacities don't change; you have to declare a new instance and copy the data over.
The variable mx is not declared
The compiler should catch this one. You use mx as the capacity for an array and comparing for full. However, it is never declared, defined nor initialized. Prefer std::vector and you won't have to deal with these issues.
Input is not validated
You input a letter, but don't validate it.
Try these characters: space, #, #, A, B, etc.
Missing default for switch
Crank up your compiler warnings to maximum.
Your switch statements need defaults.
What precedence do numeric characters ('0'..'9') have?
(You check the precedence of numeric characters.)
Check all paths through your functions and program.
Using a debugger (see below) or pen and paper, check your program flow through you functions. Include boundary values and values not within the bounds.
Case statements: break or return
You don't need a break after a return statement. Think about it. Can the program continue executing at the line after a return statement?
Use a debugger or print statements
You can print variables at different points in your program. This is an ancient technique when debuggers are not available.
Learn to use a debugger. Most IDEs come with them. You can single step each statement and print out variable values. Very, very, useful.
class infixToPostfix{
public static void postfix(String str){
Stack<Character> stk = new Stack<Character>();
for(Character c : str.toCharArray()){
// If operands appears just print it
if(c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z'){
System.out.print(c);
}else{
// Open paranthesis push is
if(c == '('){
stk.push(c);
//Close paranthesis pop until close paranthesis
}else if( c == ')'){
while(stk.peek() != '(')
System.out.print(stk.pop());
stk.pop();
// check the precedence of operator with the top of stack
}else if(c == '+' || c == '-'){
if(!stk.isEmpty()){
char top = stk.peek();
if(top == '*' || top == '/' || top == '+' || top == '-'){
System.out.print(stk.pop());
}
}
stk.push(c);
}else{
if(!stk.isEmpty()){
char top = stk.peek();
if(top == '/' || top == '*'){
System.out.print(stk.pop());
}
}
stk.push(c);
}
}
}
//Print all the remaining operands
while(!stk.isEmpty()) System.out.print(stk.pop());
System.out.println();
}
public static void main(String args[]){
String str = "A+B-(c+d*Z+t)/e";
postfix(str);
}
}
using stack and map u can solve the problem
1) create a map having operator as key and some integer to set priority. operator with same precedence will have same value something like:
map<char,int>oprMap;
oprMap['^'] = 3;
oprMap['*'] = 2;
oprMap['/'] = 2;
oprMap['+'] = 1;
oprMap['-'] = 1;
2) iterate through given expression call these checks
1) if current element
i) is operand add it to result
ii) not operand do following check
a. while not (stack is empty and element is open bracket and found operator with higher precedence.
add top of the stack to the result and pop()
b. push current element to stack
iii) if open brackets push to stack
iv) if closed brackets pop until get closed bracket and add it to result
3) while stack is not empty pop() and add top element to the result.
{
stack<char>S;
for (int i = 0; i < n; i++) {
if(isOperand(exps[i])) {
res = res + exps[i];
} else if(isOperator(exps[i])){
while(!(S.empty() && isOpenParanthesis(S.top()) && isHeigherPrecedence(S.top(),exps[i])){
res = res+S.top();
S.pop();
}
S.push(exps[i]);
} else if(isOpenParanthesis(exps[i])) {
S.push(exps[i]);
} else if(isClosingParanthesis(exps[i])) {
while(!S.empty() && !isOpenParanthesis(S.top())) {
res = res+S.top();
S.pop();
}
S.pop();
}
}
while(!S.empty()) {
res = res + S.top();
S.pop();
}
}
}
#include<bits/stdc++.h>
using namespace std;
// This isHigher function checks the priority of character a over b.
bool isHigher(char a,char b)
{
if(a=='+' || a=='-')
return false;
else if((a=='*' && b=='*') || (a=='*' && b=='/') || (a=='/' && b=='*') ||
(a=='/' && b == '/')|| (a=='^' && b=='^')||(a=='*' && b=='^') || (a=='/' &&
b=='^'))
return false;
return true;
}
int main(){
string s;
cin>>s;
s = s + ")";
//Vector postfix contains the postfix expression.
vector<char>postfix;
stack<char>mid;
mid.push('(');
for(int i=0;i<s.length();i++)
{
if(s[i] == '(')
mid.push(s[i]);
else if(s[i] == '+' || s[i] == '^' || s[i] == '-' || s[i] == '*'||
s[i] == '/')
{
if(mid.top() == '(')
mid.push(s[i]);
else {
if(isHigher(s[i],mid.top()))
mid.push(s[i]);
else
{
while(mid.top()!='(')
{
if(!isHigher(s[i],mid.top()))
{
postfix.push_back(mid.top());
mid.pop();
}
else
break;
}
mid.push(s[i]);
}
}
}
else if(s[i] == ')')
{
while(mid.top() != '(')
{
postfix.push_back(mid.top());
mid.pop();
}
mid.pop();
}
else
postfix.push_back(s[i]);
}
for(int i=0;i<postfix.size();i++)
cout<<postfix[i];
return 0;
}
I am doing my assignment in which I have to modify my previous task (which is prefix expression tree which takes expression and give result)
+ OR
* AND
- NOT
Now I have to make it Logic expression tree which will perform AND OR and NOT operations
char input;
cin.get(input);
if((input == '+')||(input == '-')||(input == '*'))
{
p = new ExprTreeNode(input,NULL,NULL);
buildSub(p->left);
buildSub(p->right);
}
else if(isdigit(input))
{ //create a new node
p = new ExprTreeNode(input,NULL,NULL);
}
else
{
cout <<" invalid expression exiting..." <<endl;
exit (1);
}
above code reads expression and makes tree using recursion...
I am confused how I can add unary operator that is NOT...
after that I have to evaluate expression
int answer;
switch (p->dataItem){
case '*':
// AND
case'+':
// OR
case '-':
// Reverse
default:
answer = (p->dataItem-'0');
break;
}
return answer;
p is ExprTreeNode
// Data members
char dataItem; // Expression tree data item
ExprTreeNode *left, // Pointer to the left child
*right; // Pointer to the right child
I'd say you want to do the following:
if ((input == '+') || (input == '*'))
{
p = new ExprTreeNode(input,NULL,NULL);
buildSub(p->left);
buildSub(p->right);
}
else if (input == '-')
{
p = new ExprTreeNode(input, NULL, NULL);
buildSub(p->left);
}
else if(isdigit(input))
{ //create a new node
p = new ExprTreeNode(input,NULL,NULL);
}
else
{
cout <<" invalid expression exiting..." <<endl;
exit (1);
}
Edit:
Then, the evaluation routine could work like this:
bool ExprTreeNode::evaluate() {
switch (dataItem) {
case '+':
return left->evaluate() || right->evaluate();
case '*':
return left->evaluate() && right->evaluate();
case '-':
return !left->evaluate();
case '0':
return false;
default:
return true;
}
}