Merging two index into one in array in C - c

Question

I have an unsigned char array which contains hex bytes like below:
unsigned char array[255];
array[0] = 'F';
array[1] = 'F';
array[2] = 'E';
array[3] = '2';
array[4] = 'A';
array[5] = 'A';
array[6] = 'C';
array[7] = 'C';
I want to merge them so that it becomes:
array[0] = "FF"
array[1] = "E2"
array[2] = "AA"
array[3] = "CC"
array[0] = '\xFF';
array[1] = '\xE2';
array[2] = '\xAA';
array[3] = '\xCC';
I have tried using sprintf but then I do not know how to specify index number in it. Any help.?

Answer 1

So, you want to convert your string, made of hexadecimal characters, into an array of bytes, right? Know your data.
sprintf() will not help you, since it produces strings. Instead, you will need to extract the 'value' of each hexa character and use it to calculate the value of your bytes.
So, let's create a helper function to convert a hexadecimal character to its integer value (or -1 if it is invalid). We will use the characters' ASCII values and the fact that character ranges are contiguous in the ASCII table
int char2hexa(unsigned char c)
{
if(c >= '0' && c <= '9') {
return (c - '0'); /* will return 0-9 */
} else if(c >= 'A' && c <= 'F') {
return (c - 'A') + 10; /* will return 10-15 */
} else if(c >= 'a' && c <= 'f') {
return (c - 'a') + 10; /* will return 10-15 */
} else {
return -1;
}
}
Now a byte will be constructed from two hexa values by using one as the upper nibble (multiplied by 16 or shifted left by 4) and the other as a lower nibble, so let's have a function for that:
unsigned char hexvals2byte(int upper, int lower)
{
return (upper * 16 + lower);
}
and put the pieces together. I will assume that:
you know the length of your input data
the length is even (you need two characters per byte)
you want to put the result in the same array
Here comes.
#include <stdio.h>
#include <stdlib.h>
unsigned char array[255];
array[0] = 'F';
array[1] = 'F';
array[2] = 'E';
array[3] = '2';
array[4] = 'A';
array[5] = 'A';
array[6] = 'C';
array[7] = 'C';
unsigned length = 8;
int upper, lower;
for(int i = 0; i < length; i+=2) {
upper = char2hexa(array[i]);
lower = char2hexa(array[i+1]);
if(upper < 0 || lower < 0) {
/* handle input data format error */
fprintf(stderr, "ERROR: Cannot decode hexa values '%c%c'\n", array[i], array[i+1]);
exit(EXIT_FAILURE);
}
array[i/2] = hexvals2byte(upper, lower);
}

Answer 2

So you need a result array of unsigned char result[128][3], then assign the part results, grouping 2 source elements into one result sub-element:
unsigned char result[128][3] = { 0 };
int i;
for (i = 0; i < 255; ++i)
{
result[i/2][i%2] = array[i];
}
The reason for size 3 is, that you need 2 characters and one zero-delimiter to form a string.

Answer 3

An easy way to convert a digit to number is to subtract '0' from it:
char digit = '3';
int number = digit - '0'; /* number = 3 */
This works only for digits (digit >= '0' && digit <= '9'), for hexadecimal digits ('A', 'B', etc.) you have to do a little more job:
unsigned char result[127];
int i;
unsigned char current;
unsigned char calc_diff(unsigned char digit) {
if(digit >= '0' && digit <= '9')
return '0';
else if(digit >= 'A' && digit <= 'F')
return 'A' - 10;
else if(digit >= 'a' && digit <= 'f')
return 'a' - 10;
else
return 0; // handle invalid digit
}
for(i = 0; i < 128; ++i) {
current = array[2 * i];
result[i] = (current - calc_diff(current)) << 4;
current = array[(2 * i) + 1];
result[i] |= current - calc_diff(current);
}

Answer 4

You want to convert characters to their hexadecimal value and combine them in pairs.
Here is a simple program to illustrate how you can do this:
#include <stdio.h>
#include <string.h>
static int xdigit(unsigned char c) {
/* this method is inefficient but works for all encodings */
static const char xdigits[] = "abcdef0123456789ABCDEF";
const char *p = memchr(xdigits, c, 22);
return p ? (p - xdigits + 10) & 15 : -1;
}
int main(void) {
unsigned char array[255];
while (scanf("%254s", array) == 1) {
int i, j, d, d2 = 0;
for (i = j = 0; array[i] != '\0'; i++) {
d = xdigit(array[i]);
if (d < 0) {
printf("invalid hexadecimal digit: %c\n", array[i]);
break;
}
d2 = (d2 << 4) | d;
if (i & 1) {
array[j++] = (unsigned char)d2;
d2 = 0;
}
}
array[j] = '\0';
printf("converted array: %s\n", array);
}
return 0;
}
Here is a more elaborate version, with an separate conversion function and more explicit output:
#include <ctype.h>
#include <stdio.h>
#include <string.h>
static int xdigit(unsigned char c) {
switch (c) {
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
return c - '0';
case 'A': case 'a':
return 10;
case 'B': case 'b':
return 11;
case 'C': case 'c':
return 12;
case 'D': case 'd':
return 13;
case 'E': case 'e':
return 14;
case 'F': case 'f':
return 15;
default:
return -1;
}
}
int xconvert(unsigned char *dest, const unsigned char *src, int len) {
int i, j, d, d2 = 0;
for (i = j = 0; i < len; i++) {
d = xdigit(src[i]);
if (d < 0) {
printf("invalid hexadecimal digit: %c\n", src[i]);
return -1;
}
d2 = (d2 << 4) | d;
if (i & 1) {
dest[j++] = (unsigned char)d2;
d2 = 0;
}
}
if (i & 1) {
printf("missing trailing digit\n");
return -1;
}
return j;
}
int main(void) {
unsigned char array[255];
int i, len, c;
while (scanf("%254s", array) == 1) {
len = xconvert(array, array, strlen((char *)array));
if (len >= 0) {
printf("converted array: \"");
for (i = 0; i < len; i++) {
c = array[i];
if (isprint(c)) {
putchar(c);
} else {
printf("\\x%02X", c);
}
}
printf("\"\n");
}
}
return 0;
}

Answer 5

my stab at it. Here you have to know the size of the array. in your case 255
//counters and array's
int first = 0;
int second = 0;
int count = 0;
char foo[8] = {'F', 'F', 'E', '2', 'A', 'A', 'C', 'C'};
//array half the size of the first one.
char *done[4];
//loop through first array
while (first <= 7)
{
//if its the first letter
if (second == 0)
{
//allocate enough mem to second arr
done[count] = (char *)malloc(sizeof(char *) * 3);
//assaign the first letter
done[count][0] = foo[first];
//indicate the next step for the second letter
second = 1;
}
//if its the second letter
else if (second == 1)
{
//assign second letter
done[count][1] = foo[first];
//null the string
done[count][2] = '\0';
//increase posistion index for the second arr
count++;
//indicate nexxt step is a the first letter of the next step
second = 0;
}
//increment the index for the first arr
first++;
}

Answer 6

Given that the data is in ASCII format and you want to merge it into a raw binary format, then:
for(size_t i=0; i<n; i++)
{
array[i] = to_int(array[i]);
}
where to_int() is your custom routine for converting from hexadecimal ASCII to integer. That is, if digit subtract with '0', else if upper-case letter subtract with 'A' and add 0xA.
Then after that, merge the items:
for(size_t i=0; i<n; i+=2)
{
array[i] = (unsigned int)array[i]<<4 | array[i+1];
}

Answer 7

I suppose you mix up an 8 bit value (e.g. 0x0F, which is 15 in decimal) with a character value like 'F' (which corresponds to 70 in ASCII format) with a string literal "FF" (which corresponds to a pointer to a sequence of the three character values {'F','F','\0'}.
From the context you present it seems that you mean 8 bit values, which are represented by data type unsigned char.
Given that, the code could look as follows:
unsigned char array[255] = { 0xF,0xE,0x2,0xA,0xA,0xC,0xC };
int target=0;
for (int i=0; i<254; i+=2) {
array[target] = (array[i] << 4) + array[i+1];
target++;
}

Answer 8

Maybe there are uncertain things but perhaps you want to do this.
#include <stdio.h>
int main(void){
unsigned char array[255] = {//or char -> hex byte
'\xF', '\xF', '\xE', '\x2',
'\xA', '\xA', '\xC', '\xC',
};
int len = 8;
for(int i = 0, j = 0; i < len; i += 2){
array[j++] = (array[i] << 4) | array[i+1];
}
len = len / 2;
for(int i = 0; i < len; i++){
printf("%02hhX", array[i]);//FFE2AACC
}
printf("\n");
}
When the data held first is a hex character
#include <stdio.h>
#include <ctype.h>
int main(void){
unsigned char array[255] = {
'F', 'F', 'E', '2',
'A', 'A', 'C', 'C',
};
int len = 8;
for(int i = 0, j = 0; i < len; i++){
if(isxdigit(array[i])){//maybe redundant
if(isdigit(array[i])){
array[i] -= '0';
} else {
array[i] -= isupper(array[i]) ? 'A' : 'a';
array[i] += 10;
}
} else {
fprintf(stderr, "invalid data %c\n", array[i]);
return -1;
}
}
for(int i = 0, j = 0; i < len; i += 2){
array[j++] = (array[i] << 4) | array[i+1];
}
len = len / 2;
for(int i = 0; i < len; i++){
printf("%02hhX", array[i]);
}
printf("\n");
}