Programming

$ man malloc

void *malloc(size_t size);

$ man strtok

char *strtok(char *str, const char *delim);

$ man strcpy

char *strcpy(char *dest, const char *src);

$ man sprintf

int sprintf(char *str, const char *format, ...);

$

char *replace_str(char *str, char *orig, char *rep)
{
    static char buffer[1024];
    char *p;                             //Line 4: 
    if (!(p = strstr(str, orig)))        //Line 5: p is the position of first occurrence of 'orig' in 'str', i.e. 6 (real position maybe something as 5f0076) at "Hello world!"
            return str;                  //Line 6: 'str' is a string, returned if 'orig' not found
    }   
    strncpy(buffer, str, p - str);      //Line 8: Copy 'p-str' (6=5f0076 - 5f0070) characters from 'str'(where 'orig' starts at) to 'buffer', i.e. from the "start"(letter 'H') to the first occurrence of "orig" (letter 'w')
    buffer[p - str] = '\0';              //Line 9: put '\0' to the end of buffer, now '\0' is at the position letter "w" of world used to stay. buffer[] becomes "Hello \0".

    sprintf(buffer + (p - str), "%s%s", rep, p + strlen(orig)); //Line 11: Now 'rep' is copied to buffer, buffer[] becomes "Hello Miami" but what about p+strlen(orig)?????
    return buffer;
}

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

// Try the calculation of char pointers cf. array subscription
// I did not figure out the char pointer can be implicitly converted to integer number
//

int main()
{
    char str[] = "The quick brown fox jumps over the lazy dog!";
    char *line;
    char *p, *q, *l, *m;

    line = (char *) malloc(sizeof(str));    //Line 15 allocate memory 

    strcpy(line, str);            //Line 17 copy char array to char pointer 

    p = strstr(line, "fox");    // Find the position of "fox" in line
    q = strstr(line, "dog");    // Find the position of "dog" in line

    l = strstr(line, "quick");    // Find the position of "quick" in line
    m = strstr(line, "lazy");   // Find the position of "lazy" in line

    printf("Postion subtraction q(%p) - p(%p) = %d\n", p, q, q - p);        //Line 25
    printf("Postion subtraction m(%p) - l(%p) = %d\n", m, l, m - l);        //Line 26

    printf("Char value subtractn: o(%c) - q(%c) = %d\n", str[12], str[4], str[12] - str[4]);
    printf("Char Value subtractn: q(%c) - e(%c) = %d\n", str[4], str[2], str[4] - str[2]);
    printf("Position subtraction: &q - &e = %d\n", &str[4] - &str[2]);        //Line 30

    return 0;
}

$ gcc -Wall ptr_substraction001b.c
ptr_substraction001b.c: In function ‘main’:
ptr_substraction001b.c:25:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘long int’ [-Wformat]
ptr_substraction001b.c:26:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘long int’ [-Wformat]
ptr_substraction001b.c:30:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘long int’ [-Wformat]

$ ./a.out
Postion subtraction q(0x987020) - p(0x987038) = 24
Postion subtraction m(0x987033) - l(0x987014) = 31
Char value subtractn: o(o) - q(q) = -2
Char Value subtractn: q(q) - e(e) = 12
Position subtraction: &q - &e = 2