How to convert hex to binary?

Question

For Objective-C:

Hi everyone, I'm trying to convert a hex input into binary. For example, someone enters in :

A55

I want that to convert to

101001010101

I've tried looking through past posts and none seem to be working for me. Any help would be greatly appreciated.

[relevant](http://stackoverflow.com/questions/6175833/how-to-convert-nsstring-to-binary-and-that-binary-to-hexadecimal) — dudeofea, Mar 05 '13 at 01:43

score 3 · Answer 1 · answered Mar 05 '13 at 01:49

Use a lookup table: there are only 16 possible characters in a HEX representation, each corresponding to a four-character binary code group. Go through the HEX character-by-character, obtain a lookup, and put it in the resultant NSString.

Here is a copy of the lookup table for you.

There are multiple options as to how to do lookups. The simplest way would be making a 128-element array, and placing NSStrings at the elements corresponding to codes of the characters (i.e. at positions '0', '1', ..., 'E', 'F', with single quotes; these are very important).

score 1 · Accepted Answer · edited May 23 '17 at 11:51

1

I believe there is a built-in function for this. If not, you should at least be able to go hex->dec then dec->bin

You can write the conversion from scratch if you know the number of characters, bin to hex is common enough algorithmically.

edited May 23 '17 at 11:51

Community

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answered Mar 05 '13 at 01:49

Eric G

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score 0 · Answer 3 · answered Mar 05 '13 at 01:53

Build a lookup table (an array where you can supply a value between 0 and 15 to get the binary for that hex digit):

char *hex_to_bin[] = {
    "0000", "0001", "0010", "0011",
    /* ... */
    "1100", "1101", "1110", "1111"
};

There should be 16 elements in that table. The conversion process for multiple digits is to handle one digit at a time, appending the results onto the end of your result storage.

Use getchar() to read a char:

int c = getchar();
if (c < 0) { puts("Error: Invalid input or premature closure."); }

Use strchr() to determine which array index to retrieve:

char *digits = "00112233445566778899AaBbCcDdEeFf";
size_t digit = (strchr(digits, c) - digits) / 2;

Look up the corresponding binary values for digit:

printf("%s", hex_to_bin[digit]); // You'll want to use strcat here.

How to convert hex to binary?

3 Answers3