Convert integer to be used in strcat

Question

I'm trying to open different files by having a for loop increment a counter, then appending that counter to the filename to be opened, but I'm stuck on how to use strcat to do this. If I understand right, strcat takes 2 strings, but my counter is an int. How can I make it so that it becomes a string?

for(a = 1; a < 58; a++) {
    FILE* inFile;
    int i;

    char filename[81];

    strcpy(filename, "./speeches/speech");
    strcat(filename, a);
    strcat(filename, ".txt");

Definitely doesn't work since a is an int. When I try casting it to char, because a starts at 1 and goes to 57, I get all the wrong values since a char at 1 is not actually the number 1.. I'm stuck.

Sorry for my previous answer, just realized it's C and not C++.. — Marco A., Apr 02 '14 at 08:14
@michaeltang itoa() is not a standard function, so you need to be careful using it. — Arthur Dent, Feb 23 '18 at 18:08

unwind · Accepted Answer · 2014-04-02T08:19:53.337

You can't cast an integer into a string, that's just not possible in C.

You need to use an explicit formatting function to construct the string from the integer. My favorite is snprintf().

Once you realize that, you can just as well format the entire filename in a single call, and do away with the need to use strcat() (which is rather bad, performance-wise) at all:

snprintf(filename, sizeof filename, "./speeches/speech%d", a);

will create a string in filename constructed from appending the decimal representation of the integer a to the string. Just as with printf(), the %d in the formatting string tells snprintf() where the number is to be inserted. You can use e.g. %03d to get zero-padded three-digits formatting, and so on. It's very powerful.

Wow, you're fast, haha. Thanks for the tip!! snprintf does seem more efficient than trying to build a string with strcpy and strcat. This was exactly what I was looking for. :) — Gary M, Apr 02 '14 at 08:18

score 3 · Answer 2 · answered Apr 02 '14 at 08:26

3

you can use a single statement for this,

snprintf(filename,sizeof(filename),"./speeches/speech%d.txt",a);

answered Apr 02 '14 at 08:26

LearningC

3,182
1
12
19

score 2 · Answer 3 · answered Apr 02 '14 at 08:23

2

You are right about the strcat function. It only works with strings.

You could use the 'sprintf' function. A modification to your code below:

char append[2]; //New variable
for(a = 1; a < 58; a++) {
  FILE* inFile;
  int i;

  char filename[81];

  strcpy(filename, "./speeches/speech");
  sprintf(append,"%d",a); // put the int into a string
  strcat(filename, append); // modified to append string
  strcat(filename, ".txt");

answered Apr 02 '14 at 08:23

vasanth89

36
4

I would also recommend moving the `char filename[81]` outside the loop and just keep clearing the filename after the end of the loop. – vasanth89 Apr 02 '14 at 08:25

Convert integer to be used in strcat

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