C - Format char array like printf

Question

I want to format a c string like printf does. For example:

char string[] = "Your Number:%i";
int number = 33;
// String should now be "Your Number:33"

Is there any library or a good way I could do this?

Georg Plaz · Accepted Answer · 2021-02-02 10:02:05Z

22

Use sprintf(char* out, const char* format, ... ); like so:

int main() {
  char str[] = "Your Number:%d";
  char str2[1000];
  int number = 33;
  sprintf(str2,str,number);
  printf("%s\n",str2);
  return 0;
}

Output:

---------- Capture Output ----------
> "c:\windows\system32\cmd.exe" /c c:\temp\temp.exe
Your Number:33

> Terminated with exit code 0.

edited Feb 2, 2021 at 10:02

Georg Plaz

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answered May 30, 2012 at 16:32

dcp

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3 Comments

user529758 Over a year ago

What if you called sprintf() with a 1001 character long string? (answer: a nice segfault. Use snprintf() instead.)

Felix Scheinost Over a year ago

Thank you, I already thought I remembered that there exists such an function but couldn't remember how it was called.

dcp Over a year ago

@H2C03 - Hmmm, I don't seem to have that function available with my compiler (MS Visual Studio 2008 C++). Good idea though, but I'm not sure it's available on all compilers.

Martin Beckett · Accepted Answer · 2012-05-30 16:30:01Z

1

sprintf - http://linux.die.net/man/3/sprintf

answered May 30, 2012 at 16:30

Martin Beckett

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Comments

Community · Accepted Answer · 2017-04-13 12:50:09Z

For debugging my Arduino sketches, I habitually use this solution, taken from Madivad's answer to How do I print multiple variables in a string?.

You add this function to your sketch, that will allow you to use printf(), although it falls over for float, %f.

// Function that printf and related will use to print
int serial_putchar(char c, FILE* f) {
    if (c == '\n') serial_putchar('\r', f);
    return Serial.write(c) == 1? 0 : 1;
}

Add the global

FILE serial_stdout;

and put this is setup()

fdev_setup_stream(&serial_stdout, serial_putchar, NULL, _FDEV_SETUP_WRITE);
stdout = &serial_stdout;

An example sketch would be:

// Function that printf and related will use to print
int serial_putchar(char c, FILE* f) {
    if (c == '\n') serial_putchar('\r', f);
    return Serial.write(c) == 1? 0 : 1;
}

FILE serial_stdout;


void setup(){
    Serial.begin(9600);

    // Set up stdout
    fdev_setup_stream(&serial_stdout, serial_putchar, NULL, _FDEV_SETUP_WRITE);
    stdout = &serial_stdout;

    printf("My favorite number is %6d!\n", 12);
}

void loop() {
  static long counter = 0;
  if (millis()%300==0){
    printf("millis(): %ld\tcounter: %ld (%02X)\n", millis(), counter, counter++);
    delay(1);    
  }
}

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