Converting int to char* in C when sprintf is too costly to use

Question

I need to convert an int value to a char string, I used the following function where score is of type int.

void updateScore(){
   char str[5] = "0";
   sprintf(str, "%d", score);
   drawString(str);
}

void drawString5x7(char *string){
   while (*string) 
      drawChar5x7(*string++);
}

This seems to work, except this snippet of code is part of a much larger system where I am coding a video game on a microcontroller (MSP430) with limited memory space available. I am able to update the score one time before my game freezes and crashes.

For some reason, whenever I add the sprintf() function (or other similar functions like snprintf, or asprintf), it eats up a lot of the available memory on the MCU.

With the sprintf functions it compiles at 11407 bytes. If I comment that single line out, my code compiles at 4714 bytes

I'm not too familiar with converting int to char strings, however, I don't think it's supposed to consume that much memory. Any guidance will be much appreciated.

Answer 1

Here is my quick implementation of int to string function:

// buf - string buffer, e.g. 16 bytes
// len - size of buf
// value - integer value
char * i2s( char * buf, int len, int value )
{
    char m = 0;

    if ( value < 0 )
    {
        // if ( value == -2147483648 ) return "-2147483648"; // fix for INT_MIN
        value = -value;
        m = '-'; // minus for negative values
    }
    buf[len - 1] = 0; // 0 for end of string
    for ( int i = len - 2; i > 0; i-- ) // loop from right (last digits) to left
    {
        buf[i] = '0' + ( value % 10 );
        value /= 10;
        if ( value == 0 )
            break;
    }
    if ( m ) buf[--i] = m;
    return buf + i;
}

Usage:

char my_buf[16];

char * s = i2s( my_buf, sizeof my_buf, 10850 ); // 10850 is the int value

Answer 2

There already is a general solution posted. In your case, where the upper and lower limits are known, you can use the following code:

void updateScore() {
   char str[5] = "0";     /* Can hold up to 4 digits. */
   int i, div = 1000; /* 10^(length of str - 2). */
   int sc = score;
   /* Fast-forward to the first non-zero digit */
   while (div > sc) {
     div /= 10;
   }
   for (i = 0; div; i++) {
     str[i] = ´0´ + sc / div;
     sc %= div;
     div /= 10;
   }
   drawString(str);
}

If you want to 0-pad the score you don't need to fast-forward to the first non-zero digit:

void updateScore() {
   char str[5] = "0";     /* Can hold up to 4 digits. */
   int i, div = 1000; /* 10^(length of str - 2). */
   int sc = score;
   for (i = 0; div; i++) {
     str[i] = ´0´ + sc / div;
     sc %= div;
     div /= 10;
   }
   drawString(str);
}

Answer 3

You can print integers manually, like

#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>

void drawChar ( char c )
{
    printf("%c", c);
}

void drawScore(uint16_t score)
{
    bool startFound = false;
    uint16_t devider = 1000;

    while (devider != 0)
    {
        uint8_t digit = (score / devider);

        if (( digit == 0) && (startFound == false))
        {
            // draw space
            drawChar(0x20);
        }
        else
        {
            // first digit of score found
            startFound = true;

            // draw space
            drawChar(digit+'0');
        }

        // move to the next digit
        score -= digit * devider;
        devider /= 10;
    }
}

int main(void)
{
    drawScore(1234);
    printf("\n");
    drawScore(234);
    printf("\n");
    drawScore(34);
    printf("\n");
    drawScore(4);
    printf("\n");
}

OUTPUT on a linux terminal.

1234
 234
  34
   4

Answer 4

Five characters is a not much - to cover each value of an int, you need at least 10 digits + sign + terminating 0 character, i. e. 12. I'd create an array of size of power of two then, so 16:

char str[16];

Then you can encode from the backwards simply:

str[sizeof(str)] = 0;
char* pos = str + (sizeof(str) - 1);
int n = score < 0 ? -score : score;
do // do while handles n == 0 correctly!
{
    // possible, if all digits are placed one after another
    // in ascending order, such as in ASCII (and those based on):
    *--pos = '0' + n % 10;
    // alternative, portable over all encodings:
    //*--pos = "0123456789"[n % 10];
    n /= 10;
}
while(n);
if(score < 0)
    *--pos = '-';

drawString(pos);

Edit (looking at the other answers (and comments) posted while writing mine):

Above solution, too, will fail for INT_MIN!

You could catch this separately (score == INT_MIN) – or simply rely on this value never being reached anyway (because not reasonable in your game).

Additionally: my output is left-aligned! If you want right-aligned output, you'd could simply place additional spaces in a for loop:

for(char* align = str + 4; pos > align;)
    *--pos = ' ';

Be aware that you cannot compare arbitrary pointers with <, >, <=, >=! Here, it is possible, because both pointers point to the same array!