Thread: char *ptr

Declared char array (read-only):
const char alpha [] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

Given a char pointer called ptr and the alpha array declared above, how can I make ptr point to the letter 'D'?

Hi L F, my nick is Nefisto and i'm not an american ppl, so if u can't understand my english, let me know and i'll try to explain in another way ;P

When u create an variable, or a vector in ur case, even if it is an const var, in rlly it is not more than a position in memory, so let's say the computer decide to give to you a memory space number 100 when u've created ur "alpha" const vector, so the letter 'A' will exist in position 100 in ur memory and because char cost 1 byte the letter B will be in position 101, like u can see in code below.

Code:

#include <stdio.h>

int main()
{
    const char alpha[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        char* ptr = NULL;
    int i;
    
    for(i = 0; i < 26; i++)
        printf("Memory POS: %p\tValue in this POS: %c\n", alpha + i, *(alpha + i));
    
    return 0;
}

So if u have a char pointer called ptr, u can make it point to letter 'D' in some ways, lets try some of them:

Here we get the mem position of the const char, and made an implicity conversion for (char*) to remove const properties, after we use * to get the value that is storaged in this mem position

Code:

#include <stdio.h>


int main()
{
    const char alpha[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char* ptr = NULL;
    
    int i;
    
    for(i = 0; i < 26; i++)
    {
        printf("Memory POS: %p\tValue in this POS: %c\n", alpha + i, *(alpha+i));
        
        if(*(alpha+i) == 'D')
            ptr = (char*)(alpha+i);
    }
    
    
    printf("Memory POS for D: %p\tValue in this POS: %c\n", ptr, *ptr);
    return 0;
}

here we get the first pos of const alpha vector, and go three position ahead

Code:

int main()
{
    const char alpha[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char* ptr = NULL;
    
    ptr = (char*)alpha;
    
    ptr += 3;
    
    printf("Memory POS for D: %p\tValue in this POS: %c\n", ptr, *ptr);
    return 0;
}

this another one will be usefull if u want to change the value of an const vector

Code:

int main()
{
    const char alpha[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char* ptr = NULL;
    
    int i;
    
    printf("Before:\n");
    for(i = 0; i < 26; i++)
    {
        printf("Memory POS: %p\tValue in this POS: %c\n", alpha + i, *(alpha+i));
        
        if(*(alpha+i) == 'D')
            *(char*)(alpha+i) = 'U';
    }
    
    printf("After:\n");
    for(i = 0; i < 26; i++)
        printf("Memory POS: %p\tValue in this POS: %c\n", alpha + i, *(alpha+i));
    
    return 0;
}

i hope i've helped good luck in ur studies ;P