Append certain number of characters to char[]?

**Queue** · 09-16-2006

Code:

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

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

**Dave_Sinkula** · 09-16-2006

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

**dwks** · 09-16-2006

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:

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

I think you want to pointers way, however:

Code:

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

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

**quzah** · 09-16-2006

strncpy &| strncat with actual writable memory.

Quzah.

**Queue** · 09-16-2006

Originally Posted by Dave_Sinkula

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

How about

Code:

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);

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

**dwks** · 09-16-2006

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

Or you could use sprintf().

Code:

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

**Queue** · 09-16-2006

Code:

strncpy(s3, s2, len);

Code:

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

Code:

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

Which is most preferable?

**Dave_Sinkula** · 09-16-2006

Originally Posted by Queue

Code:

strncpy(s3, s2, len);

Code:

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

Code:

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

Which is most preferable?

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

**dwks** · 09-16-2006

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:

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

**Dave_Sinkula** · 09-16-2006

Or even memcpy plus null termination.

**dwks** · 09-16-2006

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.

**Queue** · 09-16-2006

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.

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

**dwks** · 09-16-2006

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

**Dave_Sinkula** · 09-16-2006

Originally Posted by Queue

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

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.

**dwks** · 09-16-2006

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.

Thread: Append certain number of characters to char[]?

Thread Tools

Search Thread

Display

Append certain number of characters to char[]?

Similar Threads

HELP!!!!emergency Problem~expert please help

Read the number of characters

Random number + guessing game trouble

Stone Age Rumble

Perfect number...