How can I copy from a c string into an array and then limit the amount returned while also adding a null terminator onto the end?

Question

I'm trying to implement a function that copies characters from the c string source and then stores it into the array destination. I know this is strcpy however I am not allowed to call in any functions and am not allowed to use any local variables. I also have to be able add on a null terminator on the end of it so that the array becomes a c string itself. numChars is the size that the array destination is being limited to. for example if source was "apples" and numChar was 2, the first 3 elements would be 'a', 'p' and '\0'. Below is my attempt, how would I go about this?

void copyWord(char * destination, const char * source, int numChars){
    
    while(*destination != numChars){
        *destination = *source;
        destination++;
        source++;
    }
    destination[numChars] = '\0';
}

Just for a start, `*destination` gets you the character pointed to, not a character count, so you should rethink the condition in the `while` statement. — jkb, Sep 05 '22 at 05:57
@Phillip Yes, typo my brain instantly went to b after typing a lol — ajcheng12, Sep 05 '22 at 06:00
And here, I think after calling this function, *destination variable would be changed. How to resolve this issue? — Phillip, Sep 05 '22 at 06:13

selbie · Answer 1 · 2022-09-05T06:11:47.407

0

What you describe is basically strncpy, but your own implementation:

void copyWord(char *destination, const char *source, int destSize) {

    if ((destination == nullptr) || (destSize <= 0)) {
        return;
    }

    while ((destSize > 1) && (*source)) {
        *destination = *source;
        destination++;
        source++;
        destSize--;
    }

    // destSize is at least 1, so this is safe
    *destination = '\0';

}

edited Sep 05 '22 at 06:11

answered Sep 05 '22 at 06:02

selbie

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Thank you for the help, this way is a bit more complicated but I understand the solution. – ajcheng12 Sep 05 '22 at 06:04
It may be "more complicated", but it's more correct than the other suggested answer. – selbie Sep 05 '22 at 06:06

score 0 · Answer 2 · answered Sep 05 '22 at 06:07

Something like this should do the trick, you probably should add some error checks as well checking the arguments to the function.

void copyWord(char * destination, const char * source, int numChars)
{
    while(numChars-- && *source) 
    {
        *destination++ = *source++;
    }
    *destination = '\0';
}

Phillip · Answer 3 · 2022-09-05T06:06:38.627

-1

Here is my attempt solution:

void copyWord(char * destination, const char *source, int numChars){
    if(!numChars){
         *destination = '\0';
         return;
    }
    while(*source != NULL && numChars --){
        *destination = *source;
        destination++;
        source++;
    }
    *destination = '\0';
}

edited Sep 05 '22 at 06:06

answered Sep 05 '22 at 06:00

Phillip

170
4

This is correct, thank you for the help. I see that my condition for the while loop was wrong because I wanted to decrement down given a set size. – ajcheng12 Sep 05 '22 at 06:03
Yep. it's nice to help you. – Phillip Sep 05 '22 at 06:03
There's a bug in the above code. It would be perfectly valid to pass `0` as numChars, but it will still write character to it. – selbie Sep 05 '22 at 06:03
Also, if `source` has fewer characters than numChars, it will read right past the end of the source buffer generating undefined behavior. – selbie Sep 05 '22 at 06:05
@selbie I see what you mean I will look into it. – ajcheng12 Sep 05 '22 at 06:06
@selbie I've made changes to my code. Would it work now? – Phillip Sep 05 '22 at 06:06
@Phillip - In your solution, how many characters get written to `destination` when `numChars` is initially 1? (I'll give you a hint: It's not 1, but it should be) – selbie Sep 05 '22 at 06:08
Here **--** is a suffix operator. It is called once. – Phillip Sep 05 '22 at 06:11

How can I copy from a c string into an array and then limit the amount returned while also adding a null terminator onto the end?

3 Answers3