What is the idea behind ^= 32, that converts lowercase letters to upper and vice versa?

Question

I was solving some problem on codeforces. Normally I first check if the character is upper or lower English letter then subtract or add 32 to convert it to the

Answer 1

Plenty of good answers here that describe how this works, but why it works this way is to improve performance. Bitwise operations are faster than most other operations within a processor. You can quickly do a case insensitive comparison by simply not looking at the bit that determines case or change case to upper/lower simply by flipping the bit (those guys that designed the ASCII table were pretty smart).

Obviously, this isn't nearly as big of a deal today as it was back in 1960 (when work first began on ASCII) due to faster processors and Unicode, but there are still some low-cost processors that this could make a significant difference as long as you can guarantee only ASCII characters.

https://en.wikipedia.org/wiki/Bitwise_operation

On simple low-cost processors, typically, bitwise operations are substantially faster than division, several times faster than multiplication, and sometimes significantly faster than addition.

NOTE: I would recommend using standard libraries for working with strings for a number of reasons (readability, correctness, portability, etc). Only use bit flipping if you have measured performance and this is your bottleneck.