Checking Balanced Parentheses (an Application of Stacks)

This lesson covers checking whether parentheses are balanced in C using a stack, so you understand a practical application of the stack data structure. It's for anyone searching "C balanced parentheses stack application."

Using a stack (last in, first out), you can check whether parentheses are balanced by pushing an opening bracket like ( or [ onto the stack, and checking whether it matches the top of the stack every time a closing bracket comes up.

The example builds a stack by combining an array, char stack[100], with an index, top — pushing when an opening bracket comes up, and checking against the top of the stack when a closing bracket comes up. If the stack is empty once everything's processed, the parentheses are balanced.

A common early mistake is handling several types of brackets at once (parentheses, square brackets, and curly braces mixed together). Simply counting opens and closes isn't enough — you also need to confirm they're nested in the correct order.

A similar idea is used when a compiler parses syntax too — a widely applicable way of thinking.

📖 Reference code

#include <stdio.h>

int isBalanced(const char *str) {
    char stack[100];
    int top = -1;
    for (int i = 0; str[i]; i++) {
        char ch = str[i];
        if (ch == '(' || ch == '[' || ch == '{') {
            stack[++top] = ch;
        } else if (ch == ')' || ch == ']' || ch == '}') {
            if (top < 0) return 0;
            char open = stack[top--];
            if ((ch == ')' && open != '(') ||
                (ch == ']' && open != '[') ||
                (ch == '}' && open != '{')) {
                return 0;
            }
        }
    }
    return top == -1;
}

int main() {
    printf("%d\n", isBalanced("(a[b]{c})"));
    printf("%d\n", isBalanced("(a[b)]"));
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).