Using a stack to do an infix to postfix - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include "stack.h"
#define MAX_EQU_LEN 100
static int prec(char operator)
{
switch (operator)
{
case '*':
return 5;
case '/':
return 4;
case '%':
return 3;
case '+':
return 2;
case '-':
return 1;
default:
break;
}
return 0;
}
static int isNumeric(char* num)
{
if(atoi(num) == 0)
{
return 0;
}
return 1;
}
char* infix_to_postfix(char* infix)
{
char* postfix = malloc(MAX_EQU_LEN);
stack* s = create_stack();
s->size = strlen(infix);
node* tempPtr = s->stack;
unsigned int i;
char symbol,next;
char temp[2] = {0};
for(i = 0; i < s->size ; i++)
{
symbol = *((infix + i));
temp[0] = symbol;
tempPtr = s->stack;
if(isNumeric(temp) != 1)
{
strcat(postfix, temp);
}
else if(symbol == '(')
{
push(s, symbol);
}
else if(symbol == ')')
{
while(s->size != 0 && top(s) != '(')
{
next = tempPtr->data;
temp[0] = next;
pop(s);
strcat(postfix, temp);
tempPtr = s->stack;
if(tempPtr->data == '(')
{
pop(s);
}
}
}
else
{
while(s->size != 0 && prec(top(s)) > prec(symbol))
{
next = tempPtr->data;
temp[0] = next;
pop(s);
strcat(postfix,temp);
push(s,next);
}
}
while(s->size != 0)
{
next = tempPtr->data;
temp[0] = next;
pop(s);
strcat(postfix, temp);
}
}
return postfix;
}
int evaluate_postfix(char* postfix) {
//For each token in the string
int i,result;
int right, left;
char ch;
stack* s = create_stack();
node* tempPtr = s->stack;
for(i=0; i < strlen(postfix); i++){
//if the token is numeric
ch = postfix[i];
if(isNumeric(&ch)){
//convert it to an integer and push it onto the stack
atoi(&ch);
push(s, ch);
}
else
{
pop(&s[0]);
pop(&s[1]);
//apply the operation:
//result = left op right
switch(ch)
{
case '+': push(&s[i],right + left);
break;
case '-': push(&s[i],right - left);
break;
case '*': push(&s[i],right * left);
break;
case '/': push(&s[i],right / left);
break;
}
}
}
tempPtr = s->stack;
//return the result from the stack
return(tempPtr->data);
}
This code is part of a bigger program that is designed to convert simple math from infix to postfix and then evaluate it. This will be aided by a stack to hold the numbers and symbols. However when I run the program it seg faults and all the debugger says is that it is in the infix_to_postfix function and I cannot figure out what part of the code makes it seg fault.