Week 09 Tutorial Sample Answers

Part 0: Welcome, Assignment 1 & Assignment 2

Great job getting through Assignment 1!

Assignment 2 will be released late this week!

Part 1: The parts of a pointer

Look at this C code:

int n = 42;
int *p, *q;
p = &n;
*p = 5;
*q = 17;
q = p;
*q = 8;

Explain what does actually happen in the memory after each line in order?

Part 2: Functions and Pointers

Have a quick look at the following code:

#include <stdio.h>

void halve_values(int num_1, int num_2, int num_3);

int main(void) {
    int num_1 = 4;
    int num_2 = 10;
    int num_3 = 16;

    printf("Values before halved:\n");
    printf("Num 1: %d\n", num_1);
    printf("Num 2: %d\n", num_2);
    printf("Num 3: %d\n", num_3);

    halve_values(num_1, num_2, num_3);

    printf("Values after halved:\n");
    printf("Num 1: %d\n", num_1);
    printf("Num 2: %d\n", num_2);
    printf("Num 3: %d\n", num_3);

    return 0;
}

void halve_values(int num_1, int num_2, int num_3) {
    num_1 = num_1 / 2;
    num_2 = num_2 / 2;
    num_3 = num_3 / 2;
}

Note that function halve_values doesn't seem to be working correctly.

Fixing the program

Use your knowledge of how to work with pointers to fix the program.

The following diagram may be useful as a guide for explaining how pointers work in functions:

Part 3: Struct pointers

Have a quick look at the following code which contains a struct book.

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

struct book {
    char title[100];
    char author[100];
    int year;
};

void modify_book(struct book book);

int main(void) {
    struct book book;

    strcpy(book.title, "To Kill a Mockingbird");
    strcpy(book.author, "Harper Lee");
    book.year = 1960;

    printf("Before modification:\n");
    printf("Title: %s, Author: %s, Year: %d\n", book.title, book.author, book.year);

    modify_book(book);

    printf("After modification:\n");
    printf("Title: %s, Author: %s, Year: %d\n", book.title, book.author, book.year);

    return 0;
}

void modify_book(struct book book) {
    book.year = 1925;
    strcpy(book.title, "The Great Gatsby");
    strcpy(book.author, "F. Scott Fitzgerald");
}

Note that function modify_book in this example doesn't seem to be working correctly.

Part 4: Command line arguments

Tutor demo!

In this section we'll have a look at command line arguments and how they can be used to pass input to a C program when it is executed.

We'll be using this starter code:

#include <stdio.h>

int main(int argc, char *argv[]) {

    return 0;
}

The following diagram may be useful as a guide for explaining how command line arguments work:

Your turn!

In groups we will write pseudocode or a flowchart for one of the following programs:

Sum of Command Line Arguments: Write a C program that takes multiple integers as command-line arguments and prints their sum.

Count Characters in Command Line Arguments: Write a C program that counts the total number of characters in all the command-line arguments passed to it.

Reverse Command Line Arguments: Write a C program that prints all the command-line arguments passed to it in reverse order.

Check for Command Line Arguments: Write a C program that checks if any command-line arguments were provided except for the program name. If none were provided, print a message indicating so; otherwise, print the number of arguments.