calc.c

#include <stdio.h>
#include <string.h>

int do_operation(int a, int b, char operator) {
    switch (operator) {
        case '+': return a + b;
        case '-': return a - b;
        case '*': return a * b;
        case '/': return a / b;
        default: return 0;
    }
}

int main() {
    char buffer[256];
    size_t buffer_idx = 0;

    scanf("%s", buffer);

    int result = 0;
    int a, b;
    char operator;
    char tmp[32];

    int read;
    do {
        read = sscanf(buffer + buffer_idx, "%d%c%d", &a, &operator, &b);

        if (read == 3) {
            result = do_operation(a, b, operator);
            printf("%d\n", result);

            sprintf(tmp, "%d", a);
            buffer_idx += strlen(tmp);   
            sprintf(tmp, "%d", b);
            buffer_idx += strlen(tmp);  
            buffer_idx += 1;
            continue;
        }

        read = sscanf(buffer + buffer_idx, "%c%d", &operator, &a);

        if (read == 2) {
            result = do_operation(result, a, operator);
            printf("%d\n", result);

            sprintf(tmp, "%d", a);
            buffer_idx += strlen(tmp); 
            buffer_idx += 1;
            continue;
        }
    } while (read > 1);

    printf("Result is %d\n", result);
    return 0;
}