Thread: Evaluation of postfix expression

hey everyone, i have written a C program to evaluate the value of a postfix expression, but im not getting the desired output. The output always comes out to be ZERO at runtime, there is no compilation error,

please help me out quickly to identify the error, the program is as follows:->


#include<stdio.h>
#include<conio.h>
#include<ctype.h>

#define MAX 50
float stack[MAX];
int top=-1;


void push(float a)
{
if(top==MAX-1)
return;
else
stack[++top]=a;
}

float pop()
{
if(top==-1)
return -999;
else
return stack[top--];
}

void main()
{
char pos[50];
float a, b, c,p;
int i=0,j=0;
clrscr();
printf("\n Enter the postfix expression");
gets(pos);
while(pos!='\0')
{
if(isdigit(pos))
{
p = (float)pos - 48;
push(p);
}
else if( (pos=='*') || (pos=='/') || (pos=='-') || (pos=='+'))
{
a = pop();
b = pop();
if(pos=='+')
c = b+a;
if(pos=='-')
c = b-a;
if(pos=='*')
c = b*a;
if(pos=='/')
c = b/a;
push(c);
}
else if(pos==32)
continue;
else
{
printf("\n Enter only digits and 4 operators");
j=1;
break;
}
i++;
}
if(j==1)
exit();
else
printf("\n The value of the postfix expression is = %d",stack[0]);
getch();
}

The code you provide doesn't compile, there are several errors:

1: Main should probably return int unless you're running on some older embedded platform.
2: The way you're "iterating" over the char array isn't doing what you think you're doing.(I'm too lazy to point out everything here, but there are MANY problems)
3: Your method is incorrect, look at the example of 2+2, if you code worked as intended:

Push 2.0f to the stack. So far so good.
Encounter operator '+'. OOPS!
Pop 2.0f from the stack. OOPS!
Pop value from empty stack. OOPS!
Calculate result. OOPS!
Push result to stack. OOPS!
Push 2.0f to the stack. OOPS!
Print value at the top of the stack. Incorrect result.

The method should probably be a 2-step one including:

Shunting-yard algorithm - Wikipedia, the free encyclopedia
Reverse Polish notation - Wikipedia, the free encyclopedia

4: When you print a float using printf, don't use the %d format flat, that's for a signed integer, that'll not work with a float, use %f instead.

Hey with the help of one of my friend, im able to identify the errors, they are:

1. as JacoBn has suggested, since i want to print a float value i need to use %f instead of %d as formatting string

2. while checking for gape if any as in the following line

else if(pos[i]==32)
continue;

it is not incrementing the value of i, so the program is going in an infinite loop, thus to avoid it instead of "while", "for" should be used.


3. if the expression contains sumthing other than operators, digits and spaces it should display the message and exit,

however the exit function doesnt allow the displaying of the message so the following thing shud be done:



instead of

if(j==1)
exit();
else
printf("\n The value of the postfix expression is = %f",stack[0]);

use

if(j==0)
printf("\n The value of the postfix expression is = %f", stack[0]);