How can I concatenate multiple chars into an int?

Question

I'm using C++ to parse binary data from files. Each byte gets stored in a char array, at least when working from cplusplus.com's example. The issue here is that my data source uses 24-bit values, so bytes have to be combined.

Here is a very simple example. The desired output is AABB but the actual output is 165 due to it doing addition.

#include <iostream>
using namespace std;

int main() {
    unsigned char one = 0xAA;
    unsigned char two = 0xBB;
    unsigned int sum = one + two;
    cout << hex << sum << "\n";
    return 0;
}

"`AABB`" is not an `unsigned int`. Do you want the number `0xAABB`, or a string of four characters? — aschepler, Jul 23 '19 at 01:43
Open your C++ book to the chapter that explains bitwise operators, and read it. You will need how to learn to use several of them, such as the shift-left operator, and the logical or operator. And also understand the need for appropriate casting, to appropriate integer types, as an integral part of calculations. — Sam Varshavchik, Jul 23 '19 at 01:43
@aschepler The number. So while it's less obvious visually, `one` is `170`, `two` is `187` and `sum` should be `43707`. — spacer GIF, Jul 23 '19 at 01:47
@spacerGIF No need to ask. Use a calculator. They come in hex flavours too. — Ted Lyngmo, Jul 23 '19 at 01:49
Thank you to Henri Menke, robthebloke and mattloulou. All these solutions work a treat! — spacer GIF, Jul 23 '19 at 02:03

score 3 · Accepted Answer · answered Jul 23 '19 at 01:50

3

uint32_t bytesTo24Bit(uint8_t hi, uint8_t mid, uint8_t low)
{
  return (uint32_t(hi) << 16) | (uint32_t(mid) << 8) | low;
}

answered Jul 23 '19 at 01:50

robthebloke

9,331
9
12

Side note: This [shouldn't depend on endianness](https://stackoverflow.com/a/7184905/10957435), which is good news. Although this is talking about C. I'd imagine it'd be the same, though. – Jul 23 '19 at 02:10
Using a 24 bit bitfield in a struct based on `int_fast32_t` instead of `uint32_t` saves up to 25% space. – Jul 23 '19 at 02:57

score 1 · Answer 2 · answered Jul 23 '19 at 01:59

1

A very simple solution to this would be to use the 'left shift equals' <<= operator.

an example of what your code could do would be:

int main() {
    unsigned char one = 0xAA;
    unsigned char two = 0xBB;
    unsigned int sum = one;
    sum <<= 8; //This shifts the value of AA 8 bits to the left
    sum += two;

    cout << hex << sum << "\n";
    return 0;
}

answered Jul 23 '19 at 01:59

mattloulou

37
5

`unsigned int`'s size is implementation defined and may be as low as 16 bits. OP asked for 24 bit int, which is not guaranteed to fit in `unsigned int`. – Jul 23 '19 at 02:52

How can I concatenate multiple chars into an int?

2 Answers2