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

c++ bit-manipulation ascii
Devon picture Devon · Feb 5, 2019 · Viewed 16.9k times · Source

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 corresponding letter. But I found someone do ^= 32 to do the same thing. Here it is: 

char foo = 'a';
foo ^= 32;
char bar = 'A';
bar ^= 32;
cout << foo << ' ' << bar << '\n'; // foo is A, and bar is a

I have searched for an explanation for this and didn't find out. So why this works?

Answer

Hanjoung Lee picture Hanjoung Lee · Feb 5, 2019

Let's take a look at ASCII code table in binary.

A 1000001    a 1100001
B 1000010    b 1100010
C 1000011    c 1100011
...
Z 1011010    z 1111010

And 32 is 0100000 which is the only difference between lowercase and uppercase letters. So toggling that bit toggles the case of a letter.

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