Convert int to bytes array in c

Question

I would like to convert an int to a char[4] where each byte in the char[4] contains a decimal value of 2. So in the following example:

int p = 2999999;

Convert p to the array that is identical to k where k is constructed via:

char k[4];
k[0] = 2;
k[1] = 99;
k[2] = 99;
k[3] = 99;

How can I do this in C?

Thanks in advance!

There is no direct conversion. You will have to write code for this. — Rishikesh Raje, Jan 03 '18 at 10:46
An idea (extremely inefficient) would be to convert the number to string, then split the string into 4 (string) parts, and then convert back each part. — CristiFati, Jan 03 '18 at 10:51
first convert `int` to `string` and then parse it as per your req. https://stackoverflow.com/questions/8257714/how-to-convert-an-int-to-string-in-c — roottraveller, Jan 03 '18 at 10:53
"I would like to convert an int to a char[4] where each byte in the char[4] contains a decimal value of 2" char k[4] = {2,2,2,2}; ?? Please can you clarify what you mean? — Code Gorilla, Jan 03 '18 at 10:59
What should happen with negative values like `int p = -2999999;`? — chux - Reinstate Monica, Jan 03 '18 at 15:09

gonczor · Accepted Answer · 2018-01-03T11:01:27.907

5

It's strange problem for me but OK. here's what I would have done:

Copy value of p so that it can be used later (I assume that this conversion doesn't intend to change its value).
Use modulo operation to get 2 last digits
cut down the copy of p to be able to read next two digits.

So complete answer is:

#include <stdio.h>

int main()
{
    int p = 2999999;
    char k[4];
    int p_copy = p;
    int i;
    for(i = 3; i >=0; i--)
    {
            k[i] = p_copy % 100;
            p_copy /= 100;
    }
    for(i = 0; i < 4; i++)
            printf("k[%d]: %d\n",i, k[i]);
    return 0;
}

And for sanity the output:

gonczor@wiktor-papu:~/tmp$ gcc test.c -o test
gonczor@wiktor-papu:~/tmp$ ./test
k[0]: 2
k[1]: 99
k[2]: 99
k[3]: 99

edited Jan 03 '18 at 11:01

answered Jan 03 '18 at 10:57

gonczor

3,994
1
21
46

I've changed looping order. Is there anything else? – gonczor Jan 03 '18 at 10:59
Oh, yes. I've included them in my test code but not here. Thanks for pointing out – gonczor Jan 03 '18 at 11:03
It's not a strange operation at all. This is representation is called ["Binary Coded Decimal"](https://en.m.wikipedia.org/wiki/Binary-coded_decimal). – Jongware Jan 03 '18 at 12:52
I haven't though about this. OK. makes sense. – gonczor Jan 03 '18 at 12:53

Mischa · Answer 2 · 2018-01-03T11:01:17.517

1

for (i =4; i--; k[i] = p % 100, p /= 100);

edited Jan 03 '18 at 11:01

answered Jan 03 '18 at 10:50

Mischa

2,240
20
18

Could you be specific? – Mischa Jan 03 '18 at 10:55
Considering `int k[5];` (one extra (last) element), this does the trick (after all the corrections). +1. – CristiFati Jan 03 '18 at 11:03
Sorry; typed on phone. The syntax is otherwise fine. Does not handle p < 0. Does not handle p > 99999999. Does not declare (i). Is that your problem? – Mischa Jan 03 '18 at 11:04

Convert int to bytes array in c

2 Answers2