Task 1: Generate Student IDs Based on Random Major Selection

The following program generates five student IDs. Each ID starts with either 20256343 (for one major) or 20256136 (for another), followed by a four-digit number. The selection between majors is randomized.

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

#define COUNT 5
#define MAJOR_A_MAX 80
#define MAJOR_B_MAX 35

int main() {
    int i = 0;
    srand((unsigned int)time(NULL));

    while (i < COUNT) {
        int is_major_a = rand() % 2;
        int suffix;

        if (is_major_a) {
            suffix = rand() % MAJOR_A_MAX + 1;
            printf("20256343%04d\n", suffix);
        } else {
            suffix = rand() % MAJOR_B_MAX + 1;
            printf("20256136%04d\n", suffix);
        }
        i++;
    }

    return 0;
}

Without seeding via srand(time(NULL)), the sequence of generated numbers would be identical across runs. Removing this line results in deterministic output.

Task 2: Beverage Vending Machine Simulator

This program simulates a vending machine that allows users to select drinks, specify quantities, and receive change. The total resets after each transaction.

#include <stdio.h>

int main() {
    int option, units;
    float price = 0.0f, payment, refund;

    while (1) {
        printf("\nBeverage Menu:\n");
        printf("1. Cola - ¥3/unit\n");
        printf("2. Sprite - ¥3/unit\n");
        printf("3. Orange Juice - ¥5/unit\n");
        printf("4. Mineral Water - ¥2/unit\n");
        printf("0. Exit\n");
        printf("Select beverage: ");
        scanf("%d", &option);

        if (option == 0) break;
        if (option < 1 || option > 4) {
            printf("Invalid selection.\n");
            continue;
        }

        printf("Enter quantity: ");
        scanf("%d", &units);
        if (units < 0) {
            printf("Quantity cannot be negative.\n");
            continue;
        }

        switch (option) {
            case 1:
            case 2:
                price = 3 * units;
                break;
            case 3:
                price = 5 * units;
                break;
            case 4:
                price = 2 * units;
                break;
        }

        printf("Insert amount: ");
        scanf("%f", &payment);
        refund = payment - price;

        printf("Total: %.2f\n", price);
        printf("Change: %.2f\n", refund);
        price = 0.0f; // Reset for next transaction
    }

    printf("Thank you!\n");
    return 0;
}

If price = 0.0f; is omitted, totals from previous purchases accumulate, leading to incorrect billing.

Task 3: Traffic Light Command Interpreter

Reads single-character commands (r, g, y) from input and responds accordingyl. Exits on EOF.

#include <stdio.h>

int main() {
    char cmd;
    while ((cmd = getchar()) != EOF) {
        if (cmd == '\n') continue; // Skip newlines

        switch (cmd) {
            case 'r':
                printf("stop!\n");
                break;
            case 'g':
                printf("go go go\n");
                break;
            case 'y':
                printf("wait a minute\n");
                break;
            default:
                printf("something must be wrong...\n");
                break;
        }
    }
    return 0;
}

An extra getchar() was originally used to consume the newline; here, it's handled more cleanly by skipping \n.

Task 4: Daily Expense Tracker

Accumulates expenses until -1 is entered, then reports total, maximum, and minimum expenditures.

#include <stdio.h>

int main() {
    double expense, total = 0.0, max_val = -1.0, min_val = 20001.0;

    printf("Enter daily expenses (-1 to stop):\n");
    while (scanf("%lf", &expense) == 1 && expense != -1) {
        total += expense;
        if (expense > max_val) max_val = expense;
        if (expense < min_val) min_val = expense;
    }

    printf("Total spent: %.1f\n", total);
    printf("Highest expense: %.1f\n", max_val);
    printf("Lowest expense: %.1f\n", min_val);
    return 0;
}

Initial min_val is set high to ensure any real expense will be lower.

Task 5: Triangle Classifier

Takes three side lengths and classifies the triangle as equilateral, isosceles, right-angled, or scalene—after vaildating triangle inequality.

#include <stdio.h>

int main() {
    double a, b, c;

    while (scanf("%lf %lf %lf", &a, &b, &c) == 3) {
        if (a + b <= c || a + c <= b || b + c <= a) {
            printf("Not a valid triangle\n");
        } else if (a == b && b == c) {
            printf("Equilateral triangle\n");
        } else if (a == b || a == c || b == c) {
            printf("Isosceles triangle\n");
        } else if (a*a + b*b == c*c ||
                   a*a + c*c == b*b ||
                   b*b + c*c == a*a) {
            printf("Right-angled triangle\n");
        } else {
            printf("Scalene triangle\n");
        }
    }
    return 0;
}

Note: Floating-point comparison assumes exact equality; in practice, an epsilon tolerance may be needed.

Task 6: Lucky Day Guessing Game

Generates a random day (1–30) in November 2025. The user has three attempts to guess it.

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

int main() {
    srand((unsigned int)time(NULL));
    int lucky_day = rand() % 30 + 1;
    int guess, attempts = 0;

    printf("Guess your lucky day in Nov 2025 (1-30). You have 3 tries:\n");

    while (attempts < 3) {
        printf("Attempt %d: ", attempts + 1);
        scanf("%d", &guess);

        if (guess == lucky_day) {
            printf("Correct! :)\n");
            return 0;
        } else if (guess > lucky_day) {
            printf("Too late! Try an earlier date.\n");
        } else {
            printf("Too early! Your lucky day hasn't come yet.\n");
        }
        attempts++;
    }

    printf("Out of tries! Your lucky day was %d.\n", lucky_day);
    return 0;
}

The original code incorrectly incremented x after each guess; this version tracks attempts separately for clarity.