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I need to form a string, inside each iteration of the loop, which contains the loop index i:

for(i=0;i<100;i++) { // Shown in java-like code which I need working in c! String prefix = "pre_"; String suffix = "_suff"; // This is the string I need formed: // e.g. "pre_3_suff" String result = prefix + i + suffix; }

I tried using various combinations of strcat and itoa with no luck.

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    show us what you've tried instead of what you want, you'll learn much more with comments on your code than people telling you what to do Commented Mar 2, 2011 at 19:09
  • State your problem in full... it seems you are having a problem but which problem you didn't mentioned? the string is not forming or what? Commented Mar 2, 2011 at 19:11
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    @SMKamran: This is not his code. It's Java-style pseudocode. His problem is that he doesn't know how to do this in C. Commented Mar 2, 2011 at 19:12

3 Answers 3

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Strings are hard work in C.

#include <stdio.h> int main() { int i; char buf[12]; for (i = 0; i < 100; i++) { snprintf(buf, 12, "pre_%d_suff", i); // puts string into buffer printf("%s\n", buf); // outputs so you can see it } }

The 12 is enough bytes to store the text "pre_", the text "_suff", a string of up to two characters ("99") and the NULL terminator that goes on the end of C string buffers.

This will tell you how to use snprintf, but I suggest a good C book!

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6 Comments

I want to give +1, but the first line of your answer rather contradicts the rest of it. You've shown that in fact the solution is easy; you just have to throw out the notion that inefficient string concatenation idioms from script languages translate over to C. snprintf is the answer to almost any C string-assembly question.
@R. What I've shown is that the solution is not quite as easy as the OP was hoping. "Just" throwing out that notion is harder for some than for others.
Also you should be using snprintf, not sprintf. I missed that on the first read. Your code is very dangerous as written, since if the 100 is changed without updating the buffer size, you'll clobber the stack.
@R. When you change the 100, you change the buffer size. Using snprintf does not change that; it just means that you have one more place to write and update the buffer size.
The difference is that your code crashes (or worse yet yields a privilege compromise) when somebody forgets to change the buffer size, while the version with snprintf just truncates the string. In any case I would make the buffer size 12+3*sizeof(int) and then you don't have to worry... but it would still be better to use snprintf.
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Use sprintf (or snprintf if like me you can't count) with format string "pre_%d_suff".

For what it's worth, with itoa/strcat you could do:

char dst[12] = "pre_"; itoa(i, dst+4, 10); strcat(dst, "_suff");
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4 Comments

"what's itoa"?
According to a C++ reference, regarding itoa, it is stated that "This function is not defined in ANSI-C and is not part of C++, but is supported by some compilers."
@AnttiHaapala--СлаваУкраїні, itoa converts an int to a char string.
@Koldsoar the quotes mean: the function is not part of the standard C library.
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Look at snprintf or, if GNU extensions are OK, asprintf (which will allocate memory for you).

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1 Comment

Allocating the memory "for you" is hardly doing you a favor. There's a small constant bound on the size needed, so it makes much more sense to provide the buffer yourself. With asprintf you'd have to add a test for allocation failure and code to free the buffer later.

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