Integer conversion to Hexadecimal String in C without (sprintf/printf libraries)

Question

I have these input variables:

 uint16 temperature = 0x1f12;
 uint8 array[8] = {0,0,0,...0}

And I want to have

array8[0] = '1';
array8[1] = 'f';
array8[2] = '1';
array8[3] = '2';
array8[4] = '\0';
array8[5] = '\0';
array8[6] = '\0';
array8[7] = '\0';

However, for memory problems (I'm working with microcontrollers!) I need to avoid functions such as sprintf, printf, puts, etc.

How should I do?

Best regards,

just shift & mask the values. Add `'0'` if between 0 and 9, else add `'A'-10`. — Jean-François Fabre, Jun 14 '17 at 15:53
Should `array[]` write leading `0` digits? Example: should `temperature = 0x123` --> `"123"` or `"0123"` or it makes no difference? — chux - Reinstate Monica, Jun 14 '17 at 19:22
"for memory problems (uC) I need to avoid functions such as sprintf" ? sprintf should not use a lot of memory. — Stargateur, Jun 14 '17 at 19:26
@Stargateur 'sprintf' consumes 800bytes in a 8KB flash memory in my microcontroller — menchopez, Jun 15 '17 at 08:45
@xvan I don't want to convert an integer to its hexadecimal . — menchopez, Jun 15 '17 at 08:47
Your edit breaks the existing answers. Also, `array8 = '1f12'` does not make any sense. Please rollback your question. — user694733, Jun 15 '17 at 11:44
array8[0] = '1'; array8[1] = 'f'; array8[2] = '1'; array8[3] = '2'; array8[4] = 0 ... array8[7] = 0 — menchopez, Jun 15 '17 at 13:07

chux - Reinstate Monica · Answer 1 · 2017-06-14T19:19:46.100

2

Use recursion to determine 1 hex digit at a time.

// print digit at the end and return the next address
static char *itohexa_helper(char *dest, unsigned x) {
  if (x >= 16) {
    dest = itohexa_helper(dest, x/16);
  }
  *dest++ = "0123456789abcdef"[x & 15];
  return dest;
}

char *itohexa(char *dest, unsigned x) {
  *itohexa_helper(dest, x) = '\0';
  return dest;
}

int main(void) {
  char array[8];
  uint16_t temperature = 0x1f11;
  puts(itohexa(array, temperature));
  puts(itohexa(array, 0));
  puts(itohexa(array, 0x1234567));
  puts(itohexa(array, UINT_MAX & 0xFFFFFFF));
}

Output

1f11
0
1234567
fffffff

edited Jun 14 '17 at 19:19

answered Jun 14 '17 at 16:05

chux - Reinstate Monica

143,097
13
135
256

Not my downvote, but It generates the number in the reversed order. – Jean-François Fabre Jun 14 '17 at 18:35
@Jean-FrançoisFabre True. I 1st wrote it reversed as that makes the most sense, but then mis-read OP's desired order. Hmmm `uint16 temperature = 0x89ab;` would have been clearer. Will reverse. – chux - Reinstate Monica Jun 14 '17 at 19:05
@Jean-FrançoisFabre Fixed. – chux - Reinstate Monica Jun 14 '17 at 19:21
Alternative: https://ideone.com/wcpl10 – Stargateur Jun 15 '17 at 10:22
@Stargateur Interesting. – chux - Reinstate Monica Jun 15 '17 at 13:10

nglee · Accepted Answer · 2017-06-15T14:25:14.977

1

This code uses only 8 additional bytes in stack(int i, j).

int i;
for (i = 0; i < 8; i++) {
    array[7 - i] = temperature % 16;
    temperature /= 16;

    if (temperature == 0)
        break;
}

if (i == 8)
    i--;

int j;
for (j = 0; j <= i; j++)
    array[j] = array[7 - i + j];
for (j = i + 1; j < 8; j++)
    array[j] = 0;

for (j = 0; j < 8; j++)
    if (array[j] < 10)
        array[j] += '0';
    else
        array[j] += 'a' - 10;

This code first converts temperature = 0x1f12 to array[8] = { 0, 0, 0, 0, 1, 15, 1, 2}.

Then shifts the elements of array so that it becomes array[8] = { 1, 15, 1, 2, 0, 0, 0, 0 }.

And then converts the numbers to corresponding characters: array[8] = { '1', 'f', '1', '2', '0', '0', '0', '0' }.

Note also that this if condition

    if (i == 8)
        i--;

is never met, since break condition always suffices in the first for loop, even for temperature >= 0x10000000. It's just there in the hope that it might help someone understand this code.

edited Jun 15 '17 at 14:25

answered Jun 15 '17 at 13:47

nglee

1,913
9
32

With `temperature = 0x123`, this answers forms array `"12300000"`, certainly not OP's goal. – chux - Reinstate Monica Jun 15 '17 at 15:42
@chux I think that's what OP wants.. – nglee Jun 15 '17 at 15:46
OP's rely to [this](https://stackoverflow.com/questions/44549385/integer-conversion-to-hexadecimal-string-in-c/44569241?noredirect=1#comment76096549_44549385) was ["123"](https://stackoverflow.com/questions/44549385/integer-conversion-to-hexadecimal-string-in-c/44569241?noredirect=1#comment76114556_44549385), although OP did accept your answer, hmmm. – chux - Reinstate Monica Jun 15 '17 at 15:47
Sorry for my C level... I wanted what @nglee posted. Thank you for help everyone. – menchopez Jun 16 '17 at 07:55

score 0 · Answer 3 · answered Jun 14 '17 at 16:34

int convert()
{
 uint16_t temperature = 0x1f11;
 uint8_t array[8] = {0,0,0,0,0,0,0,0};

 int i = 0;

 for(i = 0; i < 8; i++){
    array[i] = (0xf000 & temperature) >> 12 ;
    temperature <<= 4;
    printf("array[%d] = %x\n", i ,array[i]);
 }
 return(0);
}

Integer conversion to Hexadecimal String in C without (sprintf/printf libraries)

3 Answers3

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