using “push” and “pop” in a stack

Question

I have an assignment that is asking me to fill up a stack with random variables and pop them out in a FILO order. Whilst I managed to get it to fill the stack, it seems to be popping out the last element and nothing else. I'm not sure why. Any help would be appreciated.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define STACK_SIZE 10
#define STACK_EMPTY -1
void push(char [], // input/ouput - the stack
          char,  // input - data being pushed onto the stack
          int *, // input/output - pointer to the index of the top of stack
          int); // constant - maximum size of stack
char     // output - data being popped out from the stack
pop(char [], // input/output - the stack
    int *); // input/output - pointer to the index of the top of stack
void push(char stack[],char item,int *top,int max_size){
    stack[*top++] =item;
}
char pop(char stack[],int *top){
    return stack[*top--];
}
int main(){
    char s[STACK_SIZE];
    int s_top = STACK_EMPTY; // Pointer points to the index of the top of the stack

    char randChar = ' ';
    int i = 0;
    int j=0;
    int randNum = 0;

    srand(time(NULL));

    for (i = 0; i < STACK_SIZE; i++){
        randNum = 33 + (int)(rand() % ((126-33)+ 1 ));
        randChar = (char) randNum;
        push(s,randChar, &s_top, STACK_SIZE);

        printf ("Random char: %c\n", randChar);

    }
    printf("-----------\n");

    for(j=STACK_SIZE; j>0; j--){
        printf("Random chars:%c\n", pop(s, &s_top));
    }
    return 0;
}

Answer 1

Your push should be

(*top)++;
stack[*top] = value;

That is first increment to the next empty position and then insert. The top variable always points to the top element. Therefore to push, first increment then assign. To pop, first extract the value at top and then decrement.

Note: the above line can be clubbed to stack[++(*top)] = value

In the current code, at the first push, your code with stack[*top++] = item, with the post increment attempts to assign the value to the current value of *top which is -1 and then increment, which is wrong.

With respect to this modification of push routine the pop routine is okay.