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  1. #include <stdio.h>
  2. #include <string.h>
  3. #include <stdlib.h>
  4. int get_val(char hex_digit) {
  5.   if (hex_digit >= '0' && hex_digit <= '9') {
  6.     return hex_digit - '0';
  7.   } else {
  8.     return hex_digit - 'A' + 10;
  9.   }
  10. }
  11. void convert_to_oct(const char* hex, char** res) {
  12.   int hex_len = strlen(hex);
  13.   int oct_len = (hex_len/3) * 4;
  14.   int i;
  15.  
  16.   // One hex digit left that is 4 bits or 2 oct digits.
  17.   if (hex_len%3 == 1) {
  18.     oct_len += 2;
  19.   } else if (hex_len%3 == 2) { // 2 hex digits map to 3 oct digits
  20.     oct_len += 3;
  21.   }
  22.  
  23.   (*res) = malloc((oct_len+1) * sizeof(char));
  24.   (*res)[oct_len] = 0; // don't forget the terminating char.
  25.  
  26.   int oct_index = oct_len - 1; // position we are changing in the oct representation.
  27.   for (i = hex_len - 1; i - 3 >= 0; i -= 3) {
  28.     (*res)[oct_index] = get_val(hex[i]) % 8 + '0';
  29.     (*res)[oct_index - 1] = (get_val(hex[i])/8+ (get_val(hex[i-1])%4) * 2) + '0';
  30.     (*res)[oct_index - 2] = get_val(hex[i-1])/4 + (get_val(hex[i-2])%2)*4 + '0';
  31.     (*res)[oct_index - 3] = get_val(hex[i-2])/2 + '0'; 
  32.     oct_index -= 4;
  33.   }
  34.  
  35.   // if hex_len is not divisible by 4 we have to take care of the extra digits:
  36.   if (hex_len%3 == 1) {
  37.      (*res)[oct_index] = get_val(hex[0])%8 + '0';
  38.      (*res)[oct_index - 1] = get_val(hex[0])/8 + '0';
  39.   } else if (hex_len%3 == 2) {
  40.      (*res)[oct_index] = get_val(hex[1])%8 + '0';
  41.      (*res)[oct_index - 1] = get_val(hex[1])/8 + (get_val(hex[0])%4)*4 + '0';
  42.      (*res)[oct_index - 2] = get_val(hex[0])/4 + '0';
  43.   }
  44. }
  45.  
  46. int main() {
  47.   const char* hex_val = "A111";
  48.   char * res;
  49.   convert_to_oct(hex_val, &res);
  50.   printf("%s", res);
  51.   free(res);
  52.   return 0;
  53. }
Success #stdin #stdout 0.01s 1852KB
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https://ideone.com/67lN2c
language:
C (gcc 8.3)
created:
11 years ago
visibility:
public

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