C in 100 Seconds: Stack

Stack — Push, Pop, Peek

A stack is last in, first out (LIFO). The last item pushed is the first one popped. We build one using an array and a top index.

The Stack Struct

typedef struct {
  int items[MAX];
  int top;
} Stack;

items holds the data, top tracks the current position. Start at -1 (empty).

Push — Add to Top

void push(Stack *s, int value) {
  if (is_full(s)) { printf("Stack overflow!\n"); return; }
  s->items[++s->top] = value;
}

++s->top increments first, then stores the value. One line does the move and the store.

Pop — Remove from Top

int pop(Stack *s) {
  if (is_empty(s)) { printf("Stack underflow!\n"); return -1; }
  return s->items[s->top--];
}

s->top-- returns the value at top, then decrements. The opposite of push.

Peek — Look Without Removing

int peek(Stack *s) {
  if (is_empty(s)) return -1;
  return s->items[s->top];
}

Same as pop but without the decrement. The stack stays unchanged.

Student Code

Try it yourself: episode46/stack.c