Append certain number of characters to char[]?

Code:

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char s1[] = "Lorem Ipsum";
char *s2 = s1;
while (*s2 != 'e') ++s2;
size_t len = 5;
char *s3;

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In the above code fragment, what would be the cleanest way to add len characters from s2 to s3?

First, s3 doesn't point to writeable memory. Second, better example?

First, you'd need to make s3 an array so it can hold any characters that you assign to it.

You could use strncat with an =0 at the end. (strncpy if there are no other characters in the target string.)

Code:

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strncat(s3, s2, len);
s3[len] = 0;

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I think you want to pointers way, however:

Code:

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while(len--) *s3++ = *s2++;
*s3 = 0;

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That code modifies s3 and s2, of course. (You'd need to put the array elsewhere since you can't increment a pointer.)

strncpy &| strncat with actual writable memory.


Quzah.

Quote:

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Originally Posted by Dave_Sinkula

First, s3 doesn't point to writeable memory. Second, better example?

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How about

Code:

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char s1[] = "This is an example sentence.";
char *start = s1;
while (*start != ' ') ++start;
char *end = start;
while (*end != ' ') ++end;
size_t len = end - start;
char *s3 = malloc(len + 1);

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In this case, what would be the cleanest way to have s3 == "is" (preferably, including the '\0')?

See my example and Quzah's cryptic reply; strncpy().

Or you could use sprintf().

Code:

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sprintf(s3, "%.*s", len, s2);

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Code:

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strncpy(s3, s2, len);

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Code:

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sprintf(s3, "%.*s", len, s2);

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Code:

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while(len--) *s3++ = *s2++;
*s3 = 0;

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Which is most preferable?

Quote:

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Originally Posted by Queue

Code:

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strncpy(s3, s2, len);

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Code:

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sprintf(s3, "%.*s", len, s2);

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Code:

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while(len--) *s3++ = *s2++;
*s3 = 0;

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Which is most preferable?

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Whichever you choose of the last two -- there's a bug in the first.

Well, strncpy() [edit] (+ the =0 as Dave has pointed out) [/edit] would probably be faster than sprintf() because that's its job; printf() would have to parse "%.*s" to figure out what to do. And the second may be slightly faster (or it may not, depending on optimisations); however, it changes s3, which is probably a bad idea, since that's your pointer to the malloc()'d memory.

As an alternative, try this or a variation:

Code:

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s3[len] = 0;
while(len--) s3[len] = s2[len];

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Or even memcpy plus null termination.

There are hundreds of ways to do it, take your pick. They pretty much boil down to a library function (such as strncpy, sprintf, memcpy, memmove, etc) or a loop of some kind.

Quote:

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Originally Posted by dwks

There are hundreds of ways to do it, take your pick. They pretty much boil down to a library function (such as strncpy, sprintf, memcpy, memmove, etc) or a loop of some kind.

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So it's basically a style/personal choice? Thanks everyone for all the input.

Yes, pretty much. I usually use a function, possibly one that I wrote myself, because it's easier to read.

Quote:

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Originally Posted by Queue

So it's basically a style/personal choice? Thanks everyone for all the input.

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I like minimalistic standard functions, because they may be implemented in hand-optimized assembly to do things better than I know, or they might do the same as a simple function I might write -- generally as-good or better.

I guess memcpy() would be the fastest (in theory) because it doesn't need to look for a NULL as strncpy does, it just has to copy the right number of bytes. strncpy() might be better if the string to be copied might contain NULLs.