Question 6 of 8: File I/O

Question 6: File I/O

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Approach. Scan the file one character at a time; push every opening bracket onto a stack, and on every closing bracket, pop the stack and confirm the popped symbol is the matching opener — any mismatch, or a closing bracket with nothing on the stack, means unbalanced. At end of file, the stack must be empty (no unmatched openers left over).

#include <stdio.h>
#define MAXLEN 10000

int is_balanced(FILE *fp) {
    char stack[MAXLEN];
    int top = -1, c;

    while ((c = fgetc(fp)) != EOF) {
        if (c == '(' || c == '[' || c == '{') {
            stack[++top] = (char) c;
        } else if (c == ')' || c == ']' || c == '}') {
            if (top < 0) return 0;             /* closer with nothing open */
            char open = stack[top--];
            if ((c == ')' && open != '(') ||
                (c == ']' && open != '[') ||
                (c == '}' && open != '{'))
                return 0;                       /* wrong bracket type */
        }
        /* any other character is ignored */
    }
    return top == -1;                            /* nothing left unclosed */
}

int main(void) {
    char filename[256];
    printf("Enter file name: ");
    scanf("%255s", filename);

    FILE *fp = fopen(filename, "r");
    if (fp == NULL) {
        printf("Could not open %s\n", filename);
        return 1;
    }
    printf(is_balanced(fp) ? "All parentheses are balanced.\n"
                            : "Parentheses are NOT balanced.\n");
    fclose(fp);
    return 0;
}

Tracing the two given examples: "( [ ] )" pushes ( then [, pops [ to match ], pops ( to match ) — stack ends empty, balanced. "( [) ]" pushes ( then [, then hits ): the top of the stack is [, not (, so the type check fails immediately — correctly reported as not balanced.