Question regarding day 9

[ildede]

Hi there,
I'm reading the solution of day 9 part A and I do not understand this line

                sum += (cell - b'0') as usize + 1;

What is b'0'?

I'm pretty new to Rust, and in this case I'm not finding anything in documentation.
...or I don't know where to look 😭

Thanks a lot!

[GaurangTandon]

Hi @ildede , b is a byte constant prefix and specifies that the following value (char '0') should be interpreted as a u8/byte value (48). See more detailed discussion here.

[ildede]

OK!

And this is done because cell is a byte too, and not u8 or other number.
Right?

If so its clear, thanks @GaurangTandon !

[timvisee]

That's right.

I read the file as bytes, getting one byte for each character. A byte is a u8, which is of course, also a number. See the ASCII table for a list of corresponding decimal numbers for each character byte.

In Rust:

• '0' is a char
• b'0' is a u8
• '0' as u8 == b'0'
• So b'0' is a short literal for the 0 character byte.

Because bytes are numbers (u8) you can do arithmetic on them. In this case, I wanted to convert each character into it's numeric value. Using the bytes directly doesn't work because the numbers will be offset by 48 (0 is 48, 1 is 49, 2 is 50...), so I have to subtract 48 from them.

Rather than subtracting 48, I subtract b'0'. This is of course the same, but b'0' looks more logical than a random 48 number.

So, this converts a digit character into a numeric value: cell - b'0'

I could keep it an u8, but I cast it to an usize because that's the type I like to use in the rest of the code, hence: (cell - b'0) as usize

And this is done because cell is a byte too, and not u8 or other number.

In Rust a byte is the same as an u8. So, cell is a byte, which is also a number.