Postfix expression evaluation using stack - c++

Question

I am writing code for postfix expression evaluation with +,-,*,/ and ^ operators. It should take input from the command line as a string which is a postix notation of an expression (whitespace is used as a delimiter). The program should print the result of the expression on the console.
The code I have written works fine for all test cases with integer values. E.g. it works for input: 2 2 2 * +
I have tried to change the stack input value to float rather than an int but it still doesn't work.
using namespace std;
struct node
{
float data ;
node *next ;
};
class stack
{
private :
node *head;
public :
stack()
{
head = NULL;
}
void push(float a)
{
node * temp = new node();
temp ->data = a ;
temp -> next = head ;
head = temp ;
}
int pop()
{
node *temp = head ;
head = temp->next ;
int a = temp->data ;
delete temp;
return a;
}
int see_top()
{
if(is_empty())
return 0 ;
node * temp = head ;
return (temp->data);
}
int is_empty()
{
if(head == NULL)
return 1;
else
return 0 ;
}
};
int is_digit(char a)
{
if(a >= '0' && a<= '9')
return 1;
else
return 0;
}
int is_operand(char a)
{
switch(a)
{
case '+' :
case '-' :
case '*' :
case '/' :
case '^' : return 1;
default : return 0;
}
}
float operation(float a , float b , char sym)
{
float ans ;
switch(sym)
{
case '+' : ans = a + b ;
break;
case '-' : ans = a - b ;
break ;
case '*' : ans = a*b ;
break ;
case '/' : ans = a/b ;
break ;
case '^' : ans = pow(a,b) ;
break ;
default : break ;
}
return ans ;
}
int main()
{
char exp[100];
stack s ;
float num=0 , num1=0 , num2=0 ;
int l , i ;
cout << "Enter the posfix expression : ";
cin.getline(exp,100);
l=strlen(exp);
for(i=0;i<l;i++)
{
if(is_digit(exp[i]))
{
num = 0;
while(exp[i]!=' ')
{
int a = exp[i++]- '0';
num = 10*num + a ;
}
s.push(num);
}
else if(is_operand(exp[i]))
{
num2 = s.see_top();
s.pop();
num1 = s.see_top();
s.pop();
num = operation(num1 , num2 , exp[i]);
s.push(num);
}
else
continue;
}
num = s.see_top();
s.pop();
cout << "Answer : " << num ;
return 0 ;
}
When I try to input an expression with some float values, it doesn't return the correct value. E.g. ideally for an expression 0.5 0.5 *, it should return 0.25 but it rather returns 225. For expression 0.1 0.1 + it returns -36.

Answer 1

The code looks very good! .
You are almost there. There are three problems in the code:
pop() casts everything to int
see_top() casts everything to int
The decimal point parsing does not work.
Here is an updated pop() and see_top(). Notice that they now return float.
float pop()
{
node *temp = head;
head = temp->next;
float a = temp->data;
delete temp;
return a;
}
float see_top()
{
if (is_empty())
return 0;
node *temp = head;
return (temp->data);
}
Parsing of decimal points is probably best fixed by adding a switch statement.
You really have a new state once you see a decimal point. See the code below. I have also added code to handle '-' character. I use the pow() math function to add in the decimal part of the number. I would move the number parsing into a function, but did not want to completely change your code. Here is the full solution:
#include <cmath>
#include <iostream>
using namespace std;
struct node {
float data;
node *next;
};
class stack {
private :
node *head;
public :
stack()
{
head = NULL;
}
void push(float a)
{
node *temp = new node();
temp->data = a;
temp->next = head;
head = temp;
}
float pop()
{
node *temp = head;
head = temp->next;
float a = temp->data;
delete temp;
return a;
}
float see_top()
{
if (is_empty())
return 0;
node *temp = head;
return (temp->data);
}
int is_empty()
{
if (head == NULL)
return 1;
else
return 0;
}
};
bool is_digit(char a)
{
return (a >= '0' && a <= '9') || a == '.' || a == '-';
}
bool is_operand(char a)
{
switch (a) {
case '+' :
case '-' :
case '*' :
case '/' :
case '^' : return true;
default : return false;
}
}
float operation(float a, float b, char sym)
{
float ans;
switch (sym) {
case '+' : ans = a + b;
break;
case '-' : ans = a - b;
break;
case '*' : ans = a * b;
break;
case '/' : ans = a / b;
break;
case '^' : ans = pow(a, b);
break;
default : break;
}
return ans;
}
int main()
{
char exp[100];
stack s;
float num = 0, num1 = 0, num2 = 0;
int l, i;
cout << "Enter the posfix expression : ";
cin.getline(exp, 100);
l = strlen(exp);
for (i = 0; i < l; i++) {
int exponent = 0;
int sign = 1;
if (is_digit(exp[i])) {
num = 0;
while (exp[i] != ' ') {
switch (exp[i]) {
case '.':exponent = -1;
break;
case '-':
sign = -1; // handle the negative sign
break;
default:int a = exp[i] - '0';
if(exponent >= 0) {
num = pow(10, exponent) * num + a;
++exponent;
} else {
num = pow(10, exponent) * a + num;
--exponent;
}
}
++i;
}
num *= sign; // If there was a '-', multiply by -1
s.push(num);
} else if (is_operand(exp[i])) {
num2 = s.see_top();
s.pop();
num1 = s.see_top();
s.pop();
num = operation(num1, num2, exp[i]);
s.push(num);
} else
continue;
}
num = s.see_top();
s.pop();
cout << "Answer : " << num;
return 0;
}
Alternative Solution Using std::istringstream
For a little extra fun, I rewrote some of the input processing to use std::istringstream. stringstream is very powerful, and should always be considered when doing parsing. I find that the string to double conversion functions in c++ is confusing and error prone. So, if you are allowed to use stringstream, this may be helpful. It removes the need to search for decimal points because stringstream will do the conversion for you. it will handle all the edge cases. Here is the stringstream solution:
#include <iostream>
#include <cmath>
#include <sstream>
using namespace std;
struct node {
float data;
node *next;
};
class stack {
private :
node *head;
public :
stack()
{
head = NULL;
}
void push(float a)
{
node *temp = new node();
temp->data = a;
temp->next = head;
head = temp;
}
float pop()
{
node *temp = head;
head = temp->next;
float a = temp->data;
delete temp;
return a;
}
float see_top()
{
if (is_empty())
return 0;
node *temp = head;
return (temp->data);
}
bool is_empty()
{
if (head == NULL)
return true;
else
return false;
}
};
int is_digit(char a)
{
if (a >= '0' && a <= '9')
return 1;
else
return 0;
}
int is_operand(char a)
{
switch (a) {
case '+' :
case '-' :
case '*' :
case '/' :
case '^' : return 1;
default : return 0;
}
}
float operation(float a, float b, char sym)
{
float ans;
switch (sym) {
case '+' : ans = a + b;
break;
case '-' : ans = a - b;
break;
case '*' : ans = a * b;
break;
case '/' : ans = a / b;
break;
case '^' : ans = pow(a, b);
break;
default : break;
}
return ans;
}
int main()
{
// char exp[100];
stack s;
float num = 0, num1 = 0, num2 = 0;
cout << "Enter the posfix expression : ";
std::string input_line;
std::getline(cin, input_line);
std::istringstream token_stream(input_line); // Parse words from input_line
std::string token;
while (token_stream >> token) {
// Check the first character of the token. It is n operand, then use it
// You could move the size() check to the is_operand() function
if (token.size() == 1 && is_operand(token[0])) {
char operand = token[0];
num2 = s.see_top();
s.pop();
num1 = s.see_top();
s.pop();
num = operation(num1, num2, operand);
s.push(num);
} else {
std::istringstream number_stream(token); // convert number tokens to floats
if(number_stream >> num) {
s.push(num);
}
}
}
num = s.see_top();
s.pop();
cout << "Answer : " << num;
return 0;
}