README.md

Concepts

For this project, we expect you to look at these concepts:

• Struggling with the sandbox? Try this: Using Docker & WSL on your local host
• Pointers and arrays
• Data Structures

Resources

Read or watch:

• C - Arrays
• C - Pointers
• C - Strings
• Memory Layout

Additional Resource

• Arrays & Pointers in C Programming Explained

Learning Objectives

At the end of this project, you are expected to be able to explain to anyone, without the help of Google:

General

• What are pointers and how to use them
• What are arrays and how to use them
• What are the differences between pointers and arrays
• How to use strings and how to manipulate them
• Scope of variables

Requirements

General

• Allowed editors: vi, vim, emacs
• All your files will be compiled on Ubuntu 20.04 LTS using gcc, using the options -Wall -Werror -Wextra -pedantic -std=gnu89
• All your files should end with a new line
• A README.md file, at the root of the folder of the project is mandatory
• Your code should use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
• You are not allowed to use global variables
• No more than 5 functions per file
• You are not allowed to use the standard library. Any use of functions like printf, puts, etc… is forbidden
• You are allowed to use _putchar
• You don’t have to push _putchar.c, we will use our file. If you do it won’t be taken into account
• In the following examples, the main.c files are shown as examples. You can use them to test your functions, but you don’t have to push them to your repo (if you do we won’t take them into account). We will use our own main.c files at compilation. Our main.c files might be different from the one shown in the examples
• The prototypes of all your functions and the prototype of the function _putchar should be included in your header file called main.h
• Don’t forget to push your header file

More Info

You do not need to learn about pointers to functions, pointers to pointers, multidimensional arrays, arrays of structures, malloc and free - yet.

Quiz questions

Question #0

We declare the following variable

int arr[5];

What is the equivalent of typing arr[2]?

• arr + 2

• *arr + 2

• *(arr + 2)

Question #1

We declare the following variable

int arr[5];

What is the size in memory of the variable arr?

• 20 bytes

• 32 bytes

• 4 bytes

• 5 bytes

• 10 bytes

• 8 bytes

Question #2

What is the value of n after the following code is executed?

int n = 98;
int *p = &n;

• 402

• 0

• 98

• 99

Question #3

What is the value of n after the following code is executed?

int n = 98;
int *p = &n;

p = 402;

• 402

• 0

• 98

• 99

Question #4

What is the size of a pointer to a char (on a 64-bit architecture)

• 8 bytes

• 1 byte

• 2 bytes

• 4 bytes

Question #5

What is the identifier to print an address with printf?

• %x

• %a

• %d

• %p

Question #6

What is the size of a pointer to an int (on a 64-bit architecture)

• 8 bytes

• 1 byte

• 2 bytes

• 4 bytes

Question #7

What is the value of n after the following code is executed?

int n = 98;
int *p = &n;

*p = 402;

• 402

• 0

• 98

• 99

Question #8

If we have a variable called var of type int, how can we get its address in memory?

• *var

• *(var)

• &var

Question #9

The process of getting the value that is stored in the memory location pointed to by a pointer is called:

• Casting

• Pointing

• Accessing

• Dereferencing

Question #10

What happens when one tries to access an illegal memory location?

• There’s a chance for the computer to catch fire, and sometimes even explode

• The operation is ignored

• Segmentation fault

• The computer shuts down

Question #11

Is it possible to declare a pointer to a pointer?

• Yes

• No

• It depends on the type the pointer is pointing to

Question #12

What is the value of n after the following code is executed?

int n = 98;
int *p = &n;

*p++;

• 402

• 0

• 98

• 99

Tasks

0. 98 Battery st.

Write a function that takes a pointer to an int as parameter and updates the value it points to to 98.

• Prototype: void reset_to_98(int *n);

julien@ubuntu:~/0x05$ cat 0-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    int n;

    n = 402;
    printf("n=%d\n", n);
    reset_to_98(&n);
    printf("n=%d\n", n);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 0-main.c 0-reset_to_98.c -o 0-98
julien@ubuntu:~/0x05$ ./0-98
n=402
n=98
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 0-reset_to_98.c

1. Don't swap horses in crossing a stream

Write a function that swaps the values of two integers.

• Prototype: void swap_int(int *a, int *b);

julien@ubuntu:~/0x05$ cat 1-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    int a;
    int b;

    a = 98;
    b = 42;
    printf("a=%d, b=%d\n", a, b);
    swap_int(&a, &b);
    printf("a=%d, b=%d\n", a, b);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 1-main.c 1-swap.c -o 1-swap
julien@ubuntu:~/0x05$ ./1-swap
a=98, b=42
a=42, b=98
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 1-swap.c

2. This report, by its very length, defends itself against the risk of being read

Write a function that returns the length of a string.

• Prototype: int _strlen(char *s);

FYI: The standard library provides a similar function: strlen. Run man strlen to learn more.

julien@ubuntu:~/0x05$ cat 2-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *str;
    int len;

    str = "My first strlen!";
    len = _strlen(str);
    printf("%d\n", len);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 2-main.c 2-strlen.c -o 2-strlen
julien@ubuntu:~/0x05$ ./2-strlen
16
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 2-strlen.c

3. I do not fear computers. I fear the lack of them

Write a function that prints a string, followed by a new line, to stdout.

• Prototype: void _puts(char *str);

FYI: The standard library provides a similar function: puts. Run man puts to learn more.

julien@ubuntu:~/0x05$ cat 3-main.c
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *str;

    str = "I do not fear computers. I fear the lack of them - Isaac Asimov";
    _puts(str);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 _putchar.c 3-main.c 3-puts.c -o 3-puts
julien@ubuntu:~/0x05$ ./3-puts
I do not fear computers. I fear the lack of them - Isaac Asimov
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 3-puts.c

4. I can only go one way. I've not got a reverse gear

Write a function that prints a string, in reverse, followed by a new line.

• Prototype: void print_rev(char *s);

julien@ubuntu:~/0x05$ cat 4-main.c
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *str;

    str = "I do not fear computers. I fear the lack of them - Isaac Asimov";
    print_rev(str);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 _putchar.c 4-main.c 4-print_rev.c -o 4-print_rev
julien@ubuntu:~/0x05$ ./4-print_rev
vomisA caasI - meht fo kcal eht raef I .sretupmoc raef ton od I
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 4-print_rev.c

5. A good engineer thinks in reverse and asks himself about the stylistic consequences of the components and systems he proposes

Write a function that reverses a string.

• Prototype: void rev_string(char *s);

julien@ubuntu:~/0x05$ cat 5-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char s[10] = "My School";

    printf("%s\n", s);
    rev_string(s);
    printf("%s\n", s);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 5-main.c 5-rev_string.c -o 5-rev_string
julien@ubuntu:~/0x05$ ./5-rev_string
My School
loohcS yM
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 5-rev_string.c

6. Half the lies they tell about me aren't true

Write a function that prints every other character of a string, starting with the first character, followed by a new line.

• Prototype: void puts2(char *str);

julien@ubuntu:~/0x05$ cat 6-main.c
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *str;

    str = "0123456789";
    puts2(str);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 _putchar.c 6-main.c 6-puts2.c -o 6-puts2
julien@ubuntu:~/0x05$ ./6-puts2
02468
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 6-puts2.c

7. Winning is only half of it. Having fun is the other half

Write a function that prints half of a string, followed by a new line.

• Prototype: void puts_half(char *str);
• The function should print the second half of the string
• If the number of characters is odd, the function should print the last n characters of the string, where n = (length_of_the_string - 1) / 2

julien@ubuntu:~/0x05$ cat 7-main.c
#include "main.h"

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char *str;

    str = "0123456789";
    puts_half(str);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 _putchar.c 7-main.c 7-puts_half.c -o 7-puts_half
julien@ubuntu:~/0x05$ ./7-puts_half
56789
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 7-puts_half.c

8. Arrays are not pointers

Write a function that prints n elements of an array of integers, followed by a new line.

• Prototype: void print_array(int *a, int n);
• where n is the number of elements of the array to be printed
• Numbers must be separated by comma, followed by a space
• The numbers should be displayed in the same order as they are stored in the array
• You are allowed to use printf

julien@ubuntu:~/0x05$ cat 8-main.c
#include "main.h"

/**
 * main - check the code for
 *
 * Return: Always 0.
 */
int main(void)
{
    int array[5];

    array[0] = 98;
    array[1] = 402;
    array[2] = -198;
    array[3] = 298;
    array[4] = -1024;
    print_array(array, 5);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 8-main.c 8-print_array.c -o 8-print_array
julien@ubuntu:~/0x05$ ./8-print_array
98, 402, -198, 298, -1024
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 8-print_array.c

9. strcpy

• Prototype: char *_strcpy(char *dest, char *src);

Write a function that copies the string pointed to by src, including the terminating null byte (\0), to the buffer pointed to by dest.

• Return value: the pointer to dest

FYI: The standard library provides a similar function: strcpy. Run man strcpy to learn more.

julien@ubuntu:~/0x05$ cat 9-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    char s1[98];
    char *ptr;

    ptr = _strcpy(s1, "First, solve the problem. Then, write the code\n");
    printf("%s", s1);
    printf("%s", ptr);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 9-main.c 9-strcpy.c -o 9-strcpy
julien@ubuntu:~/0x05$ ./9-strcpy
First, solve the problem. Then, write the code
First, solve the problem. Then, write the code
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 9-strcpy.c

10. Great leaders are willing to sacrifice the numbers to save the people. Poor leaders sacrifice the people to save the numbers

Write a function that convert a string to an integer.

• Prototype: int _atoi(char *s);
• The number in the string can be preceded by an infinite number of characters
• You need to take into account all the - and + signs before the number
• If there are no numbers in the string, the function must return 0
• You are not allowed to use long
• You are not allowed to declare new variables of “type” array
• You are not allowed to hard-code special values
• We will use the -fsanitize=signed-integer-overflow gcc flag to compile your code.

FYI: The standard library provides a similar function: atoi. Run man atoi to learn more.

julien@ubuntu:~/0x05$ cat 100-main.c
#include "main.h"
#include <stdio.h>

/**
 * main - check the code
 *
 * Return: Always 0.
 */
int main(void)
{
    int nb;

    nb = _atoi("98");
    printf("%d\n", nb);
    nb = _atoi("-402");
    printf("%d\n", nb);
    nb = _atoi("          ------++++++-----+++++--98");
    printf("%d\n", nb);
    nb = _atoi("214748364");
    printf("%d\n", nb);
    nb = _atoi("0");
    printf("%d\n", nb);
    nb = _atoi("Suite 402");
    printf("%d\n", nb);
    nb = _atoi("         +      +    -    -98 Battery Street; San Francisco, CA 94111 - USA             ");
    printf("%d\n", nb);
    nb = _atoi("---++++ -++ Sui - te -   402 #cisfun :)");
    printf("%d\n", nb);
    return (0);
}
julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra -std=gnu89 -fsanitize=signed-integer-overflow 100-main.c 100-atoi.c -o 100-atoi
julien@ubuntu:~/0x05$ ./100-atoi
98
-402
-98
214748364
0
402
98
402
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 100-atoi.c

11. Don't hate the hacker, hate the code

Create a program that generates random valid passwords for the program 101-crackme.

• You are allowed to use the standard library
• You don’t have to pass the betty-style tests (you still need to pass the betty-doc tests)
• man srand, rand, time
• gdb and objdump can help

julien@ubuntu:~/0x05$ gcc -Wall -pedantic -Werror -Wextra 101-keygen.c -o 101-keygen
julien@ubuntu:~/0x05$ ./101-crackme "`./101-keygen`"
Tada! Congrats
julien@ubuntu:~/0x05$

Repo:

• GitHub repository: alx-low_level_programming
• Directory: 0x05-pointers_arrays_strings
• File: 101-keygen.c