C language int num to char string

Question

How do I get from:

int i = 1023

to:

char string[4] = "1023"

Here's my attempted code:

Attempt 1

#include <stdio.h>

main(){

    int avg_adc = 1023;
    char myString[4];
    sprintf(myString, "%d", avg_adc); 
}  
//Output returned 4

Attempt 2

main(){

    int avg_adc = 1023;
    char myString[4];
    snprintf(myString, 4, "%d", avg_adc); 
}  
//Output returned 4

Does these outputs not make myString == char myString[4] = "1023"; ?

I'm just really confused how to get the output I need.

"1023" is a five symbol string. You are allocating only four on the stack. — Ari0nhh, Oct 27 '15 at 03:58
Yeah I realized that while troubleshooting. I ended up going another method but I appreciate the help. — Arch, Oct 28 '15 at 14:15
If your own solution solves this question, you are free to add it as an answer (and [even encouraged](http://stackoverflow.com/help/self-answer)). — Jongware, Oct 28 '15 at 14:32

score 9 · Answer 1 · edited Oct 27 '15 at 09:51

9

You need to allocate 5 elements for myString ('1','0','2','3','\0')

#include <stdio.h>

int main(void){
  int avg_adc = 1023;
  char myString[5];
  sprintf(myString, "%d", avg_adc);
  printf("%s\n", myString);
  return 0;
}

edited Oct 27 '15 at 09:51

amdixon

3,814
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answered Oct 27 '15 at 04:02

jh314

27,144
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Thank you, I ended up going with a different method but I appreciate the help. I knew I was close.. rip – Arch Oct 28 '15 at 14:18
@Arch Sure no problem. If it helps you should accept the answer. – jh314 Oct 28 '15 at 14:21

score 1 · Answer 2 · 2015-10-28T20:12:38.100

From ISO C11 draft n1570:

The sprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

In other words, the return value of

sprintf(myString, "%d", 1023);

is indeed 4, but it actually writes 5 characters to myString. More is revealed with snprintf, which limits the number of characters written to the specified value, and this number should account for the terminating null character:

Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than [the specified limit].

int avg_adc = 1023;
char myString[4];
int charsWritten;

charsWritten = snprintf(myString, sizeof(myString), "%d", avg_adc);
if (charsWritten >= 0 && charsWritten < sizeof(myString)) {
    printf("OK :-)\n");
} else {
    printf("TROUBLE :-(\n");
    printf("Returned: %d\n", charsWritten);
    printf("myString=%s, expected %d\n", myString, avg_adc);
}

Output on my system:

TROUBLE :-(
Returned: 4
myString=102, expected 1023

In other words, you need to check the return value of snprintf. If the test fails, then your result string was truncated, and it needs to be one character larger than the return value to fit all of the characters that you want it to store, assuming a negative value isn't returned. Declaring char myString[5]; results in the following output:

OK :-)

Ziezi · Answer 3 · 2015-10-28T14:51:24.327

0

The in-memory representation of myString would look like this:

            --- --- --- --- ---
myString:  | 1 | 0 | 2 | 3 |\0 |
            --- --- --- --- ---

i.e. char arrays are terminated by a null character '\0', for which you forgot to allocate memory.

In your code, if you want to store int avg_adc = 1023;, you should account for the one extra char and define the array as:

char myString[5];

edited Oct 28 '15 at 14:51

answered Oct 28 '15 at 14:46

Ziezi

6,375
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score -2 · Answer 4 · answered Oct 27 '15 at 07:12

-2

#include <stdio.h>

main()
{
        int avg_adc = 1023;
        char myString[4];
        sprintf(myString, "%d", avg_adc);
        printf("%s",myString); //you forgot this..
}

answered Oct 27 '15 at 07:12

lucifer

175
14

if this answer is useful, please accept this answer – lucifer Oct 27 '15 at 07:16
4

You're storing a 5-character string in a 4-character array. – Nisse Engström Oct 27 '15 at 07:46
However, the question asks why there is no output. The absence of output functions in the question _does_ explain that... – Lundin Oct 27 '15 at 10:31
@NisseEngström run it ,this program gives perfect output after tell anything .... – lucifer Oct 27 '15 at 13:13

C language int num to char string

Attempt 1

Attempt 2

4 Answers4