Strict Weak Ordering of "i" and "Love"-CodePudding

Assumption:: a < b returns true when a precedes b.

Code

std::string a = "i";
std::string b = "Love";
std::cout << "(a < b) = " <<  ((a < b) ? "true" : "false");

Output

(a < b) = false

We know "i" preceedes "Love" so why is the above code return false?
Is my initial assumption wrong?
Help is appreciated thanks.

CodePudding user response：

In ASCII uppercase come earlier than lowercase. You can print its values by std::cout << (int)'L' << ' ' << (int)'i';

CodePudding user response：

Lowercase “l” has a higher character code than uppercase “L” in the character encoding your C implementation uses. Most C implementations currently use ASCII for the characters A-Z and a-z. In ASCII, the code for “L” is 76, and the code for “i” is 105.

To see the codes your C implementation uses for characters, you can convert them to int and print them. Here is an example using a character constant:

std::cout << static_cast<int>('i') << '\n'; // Will print 105 in implementations that use ASCII.

For string literals and strings generally, you can select individual characters using subscripts:

std::cout << static_cast<int>("Love"[0]) << '\n'; // Will print 76 in implementations that use ASCII.

When you want to compare individual characters ignoring case, you can use toupper or tolower (declared in <cctype>) to convert both characters to the same case for comparison:

std::cout << (tolower(a[0]) < tolower(b[0]) ? "true" : "false") << '\n';

To do this for strings, you may need to write additional code; I am not aware of a case-insensitive string compare in the standard C library.

CodePudding user response：

Other users have already pointed out that in most common character encodings (i.e.: ASCII and Unicode), lower-case letter i does not precede upper-case letter L. Read more about ASCII and the difference between lower- and upper-case letters.

I'd like to add that if you want to perform a comparison without caring about letter case (so that i precedes L like you want), you can make use of specific functions like POSIX-compliant strcasecmp(). See also other methods to compare strings ignoring the case.