toUpper

toupper is a standard library function that converts lowercase letters to their uppercase equivalents. It is defined in the C standard library header <ctype.h> and is also available in C++ via <cctype> in the std namespace.

The function prototype is int toupper(int ch). If ch is representable as an unsigned char and corresponds to a lowercase letter in the current locale, toupper returns the uppercase mapping of that character; otherwise it returns ch unchanged. The mapping is locale-dependent, so in the default C locale only ASCII a–z are mapped to A–Z, while other locales may map additional characters.

Usage notes: the argument to toupper should be either EOF or a value representable as unsigned char. To avoid undefined behavior with plain char values that may be negative, it is common to cast to unsigned char before calling the function, for example toupper((unsigned char)ch). If there is no lowercase mapping in the current locale, toupper simply returns the input value unchanged.

Related functions include tolower, which performs the inverse operation, and the locale-aware variants such as toupper_l in POSIX environments or the corresponding C++ facilities in <cctype>. In practice, toupper is used to normalize text for case-insensitive comparisons or formatting tasks, and its behavior can vary with the active locale. Examples: toupper('a') yields 'A'; toupper('1') yields '1'.