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i want to traverse the expression from right to left.

What I have tried:

#include<stdio.h>
#include<ctype.h>
#include<string.h>
int stack[20],top=-1;
void push(int x)
	{   
		top++;
		stack[top]=x;
	}
int pop()
	{   
	    return stack[top--];
	}
void prefixeval()
{   
    
    char exp[20];
    char *e;
    int n1,n2,n3,num,len;
    len=strlen(exp);
    printf("Enter the expression :: ");
    scanf("%s",exp);
    e = exp;
    for(*e=len-1;*e>=0;*e--)
    {
        if(isdigit(*e))
        {
            num = *e - 48;
            push(num);
        }
        else
        {
            n1 = pop();
            n2 = pop();
            switch(*e)
            {
            case '+':
            {
                n3 = n1 + n2;
                break;
            }
            case '-':
            {
                n3 = n1 - n2;
                break;
            }
            case '*':
            {
                n3 = n1 * n2;
                break;
            }
            case '/':
            {
                n3 = n1/ n2;
                break;
            }
            }
            push(n3);
        }
        e++;
    }
    printf("\nThe result of expression %s  =  %d\n\n",exp,pop());
}
int main()
{
	prefixeval();
	
}
C++

Posted
Updated 19-Aug-22 6:24am
Comments
Rick York 19-Aug-22 14:22pm    
I already gave you one suggestion and it was just an extension of something you had already done yourself. Why are you so resistant to it? That fact is YOU need to do this work. You can not expect anyone to do it for you.

BTW: that was two questions ago.

It is already clear at first glance that something is overwritten here:
C
char exp[20];
char *e;
int n1, n2, n3, num, len;
len = strlen(exp);
printf("Enter the expression :: ");
scanf("%s", exp);
e = exp;

for (*e = len - 1; *e >= 0; *e--) { ...

You declare a pointer e that points first to nothing, then to the entered data.
Then the data is overwritten with the length.

In the further course then naturally still further goes wrong. Pop is called several times without push having been called before ...

The variables n1 and n2 have nonsensical values afterwards and although the calculation of n3 has not taken place the undefined value of n3 is then also pushed.

Almost every line of the program seems to go sief somehow ...

The two global variables should also be avoided, better encapsulate everything in a class or keep local.
int stack[20], top = -1;


//Edit:
Further references to a similar program of yours have already been given here:

A code for prefix expression evaluation in C using stacks not running[^]

The current program is not an improvement over the source code you presented there. The error even occurs at the same place.
C
int len;
char prefix[20];
int res,i;
gets(prefix);
len=strlen(prefix);
for(i=len-1;prefix[i]>=0;i--) { ...

So I repeat here the urgent suggestion to start the loop at 0.

//edit2:
Quote:
i want to traverse the expression from right to left

If that is the goal, as already mentioned, quite a few changes might be necessary.
But then this place cannot work, because the second operand has not been processed at that time.
n1 = pop();
n2 = pop();

Here's an example based on your code how it might work:
C
char exp[20];
char op, *e;
int num, len;
// len = strlen(exp);       ???
printf("Enter the expression :: ");
scanf("%s", exp);
len = strlen(exp);
// e = exp;
for (e = &exp[len-1]; e >= exp; e--)
  {
  if (isdigit(*e)) {
    num = *e - 48;
	push(num);
  }
  else {
    op = *e;
  }
//e++;
}

// TODO:
int n1, n2, n3;

It might be more obvious to push the operand instead of the numbers.

And before I get hit, here's an urgent hint: NEVER use the scanf() function with strings! Better use fgets() or another function that can handle constrained arrays.
 
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v8
Comments
Rick York 19-Aug-22 3:56am    
I made the same suggestion the last time he asked about this. He seems to not like that idea although he had it himself first. Check out the previous C question that he posted.
merano99 19-Aug-22 5:23am    
Thanks for the tip. The code was also somehow familiar to me.
There are often several ways to get a valid solution. Here there seem to be difficulties with the implementation. As usual, OriginalGriff has already posted the reference to "Testing and Debugging".
Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C
int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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This is the third time you've posted a question about the same code:

A code for prefix expression evaluation in C using stacks not running[^]

I am doing a mini project on stack application.basically I have combined various small functions and code..please help me structure this program.[^]

And now this question. You keep playing with this code, expecting us to debug it when it doesn't work. Learn how to use a debugger instead of immediately turning to other people. This is getting abusive, so your account will be closed if this behavior persists.
 
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Quote:
Why is this program not returning any output

What about seeing by yourself what is going on by using the debugger ?

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.
 
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