strncpy adavnced :P

This is a discussion on strncpy adavnced :P within the C Programming forums, part of the General Programming Boards category; Hello everybody, I just started to learn C, so I hope you don't mind that I will post here the ...

  1. #1
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    Question strncpy adavnced :P

    Hello everybody,

    I just started to learn C, so I hope you don't mind that I will post here the newbie-problems I do run into. I am learning C as a hobby, just as I learned Basic, Pascal and VB(A) in the past. So now I want to learn the "famous C" :P.

    Well, here we go....

    You can use "strncpy' to copy a number of characters from one string to another. That is handy, but what I really would like is that I can copy a number of characters from a given position in the source string to the destination string. I tried some things....

    Having in mind that a string in C is an array of char, I tried this:

    strncpy(dest_str, source_str[3], 10)

    I hoped that strncpy would start at position 4 of the source string and copied 10 characters. But when I compiled the program, the compiler told me that it hated me and stopped :P.

    Then I thought that maybe a pointer could help me. So I tried the next something

    Code:
    #include <stdio.h>
    #include <string.h>
    
    int main()
    {
    	char regel1[30];
    	char regel2[30];
    
    	char *ptr;
    
    	strcpy(regel1, "Programing is fun and relaxing to do");
    	
    
    	ptr = &regel1[10];
    	strncpy(regel2, *ptr, 3);
    	
    }
    But after that I got an casting error.

    Okay, so how to solve this problem... can I use "strncpy" at all to get done what I want? I mean, something like that would be nice for other functions as "memset" too.

    Thank you already for your answering and advices.

    Joke.
    (BTW the name Joke is no joke . It is very common Dutch girls name and is pronounced as "jo-ke" and not as the English word joke).

  2. #2
    and the hat of wrongness Salem's Avatar
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    strncpy( regel2, &regel1[10], 3 );
    You were pretty close.

    BTW, strncpy doesn't append a \0 (like strcpy does), so you have to do that yourself.
    regel2[3] = '\0';
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    Registered User slingerland3g's Avatar
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    Not sure of your intent on using

    Code:
    ptr = &regel1[10];
    Remove that and simply call

    Code:
    strncpy(regel2, regel1, 3);
    Should do the trick

  4. #4
    Registered User slingerland3g's Avatar
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    Sorry, noticed my flaw. Just change your ptr* to simply be ptr. Overlooked that you wanted to start at position 10 of your string array.

    Code:
    strncpy(regel2, ptr, 3);

    Also

    If the array pointed to by s2 is a string that is shorter than n bytes, null bytes are appended to the copy in the array pointed to by s1, until n bytes in all are written.

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