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

int compare(char* num1, char* num2) {
    int length1 = strlen(num1);
    int length2 = strlen(num2);

    if (length1 < length2) {
        return 1;
    }
    else if (length1 > length2) {
        return 0;
    }

    else {
        for (int i = 0; i < length1; i++) {
            if (num1[i] < num2[i]) {
                return 1;
            }
            else if (num1[i] > num2[i]) {
                return 0;
            }
        }
    }
    return 0;
}

void add(char* num1, char* num2, char* result) {
    int length1 = strlen(num1);
    int length2 = strlen(num2);
    int i = length1 - 1;
    int j = length2 - 1;
    int k = 0;
    int up = 0;
    int sum;

    while (i >= 0 || j >= 0 || up) {
        int digit1, digit2;

        if (i >= 0) {
            digit1 = num1[i] - '0';
            i--;
        }
        else {
            digit1 = 0;
        }

        if (j >= 0) {
            digit2 = num2[j] - '0';
            j--;
        }
        else {
            digit2 = 0;
        }

        sum = digit1 + digit2 + up;
        up = sum / 10;
        result[k++] = (sum % 10) + '0';
    }

    result[k] = '';
    for (int i = 0, j = k - 1; i < j; i++, j--) {
        char temp = result[i];
        result[i] = result[j];
        result[j] = temp;
    }  
}

void subtract(char* num1, char* num2, char* result) {
    int length1 = strlen(num1);
    int length2 = strlen(num2);
    int borrow = 0, diff;
    int i = length1 - 1;
    int j = length2 - 1;
    int k = 0;

    while (i >= 0 || j >= 0) {
        int digit1, digit2;

        if (i >= 0) {
            digit1 = num1[i] - '0';
            i--;
        }
        else {
            digit1 = 0;
        }

        if (j >= 0) {
            digit2 = num2[j] - '0';
            j--;
        }
        else {
            digit2 = 0;
        }

        diff = digit1 - digit2 - borrow;
        if (diff < 0) {
            diff += 10;
            borrow = 1;
        }
        else {
            borrow = 0;
        }
        result[k++] = diff + '0';
    }

    while (k > 1 && result[k - 1] == '0') {
        k--;
    }

    result[k] = '';
    for (int i = 0, j = k - 1; i < j; i++, j--) {
        char temp = result[i];
        result[i] = result[j];
        result[j] = temp;
    }
}

void processLine(char* line) {
    char num1[100], num2[100], optr, result[101];
    sscanf(line, "%[^+-]%c%s", num1, &optr, num2);

    if (optr == '+') {
        add(num1, num2, result);
    }
    else if (optr == '-') {
        int isNegative = 0;
        if (compare(num1, num2)) {
            isNegative = 1;
            subtract(num2, num1, result);
        }
        else {
            subtract(num1, num2, result);
        }
        if (isNegative) {
            char tempResult[102];
            tempResult[0] = '-';
            strcpy(tempResult + 1, result);
            strcpy(result, tempResult);
        }
    }
    printf("%sn", result);
}

int main() {
    FILE* file = fopen("input.txt", "r");
    if (file == 0) {
        printf("Error opening file.");
        exit(1);
    }

    int n;
    fscanf(file, "%d", &n);
    fgetc(file);

    char line[204];
    for (int i = 0; i < n; i++) {
        if (fgets(line, sizeof(line), file)) {
            if (line[strlen(line) - 1] == 'n') {
                line[strlen(line) - 1] = ''; 
            }
            processLine(line);
        }
    }

    fclose(file);
    return 0;
}   

I wonder if the cords continue naturally.
I’m also curious if I wrote the name of the variable efficientl, andabout the role of differ and borrow in the subtract function.
Also, since it hasn’t been long since I started coding, I’d like you to let me know if there’s a way to modify it in the easiest way possible.