c tutorial :)

delete-numbers.c

#include <stdio.h>

int is_number(char c)
{
    return c >= '0' && c <= '9';
}

int difference()
{
    return 'A' - 'a';
}

int main(int argc, char** argv)
{
    char* argument;
    int i;
    int non_numeric_character_index;
    if (argc != 2)
    {
        printf("Usage: lowercase <string>\n");
        return 0;
    }
    argument = argv[1];

    non_numeric_character_index = 0;
    for (i = 0; argument[i] != 0 ; i++)
    {
        if (!is_number(argument[i]))
        {
            argument[non_numeric_character_index++] = argument[i];
        }
    }
    argument[non_numeric_character_index] = 0;

    printf("%s\n", argument);
}

lowercase.c

#include <stdio.h>

char is_uppercase_letter(char c)
{
    return c >= 'A' && c <= 'Z';
}

char difference()
{
    return 'a' - 'A';
}

int main(int argc, char** argv)
{
    char* argument;
    int i;
    if (argc != 2)
    {
        printf("Usage: %s <string>\n", argv[0]);
        return 0;
    }
    argument = argv[1];

    for (i = 0; argument[i] != 0 ; i++ )
    {
        if (is_uppercase_letter(argument[i]))
        {
            argument[i] = argument[i] + difference();
        }
        // printf("%d\n", argument[i]);
    }

    printf("%s\n", argument);
}

string-compare.c

#include <stdio.h>

char is_ended(char* str, int i)
{
    return str[i] == 0;
}

int main(int argc, char** argv)
{
    if (argc != 3)
    {
        printf("Usage: string_compare <string1> <string2>\n");
        return 0;
    }
    char* str1 = argv[1];
    char* str2 = argv[2];

    for (int i = 0 ; ; i++)
    {
        if (is_ended(str1, i))
        {
            if (is_ended(str2, i))
            {
                printf("Equal length\n");
                return 0;
            }
            else
            {
                printf("Str1 is shorter.\n");
                return 0;
            }
        }
        if (is_ended(str2, i))
        {
            printf("str2 is shorter\n");
            return 0;
        }
    }
}