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Why do some versions of toupper act strangely if given an upper-case letter?
C Interview Questions and Answers
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Why do some versions of toupper act strangely if given an upper-case letter?
Q; Why do some versions of toupper act strangely if given an upper-case letter?
Why does some code call islower before toupper?
A: In earlier times, toupper was a function-like preprocessor macro and was defined to work only on lower-case letters; it misbehaved if applied to digits, punctuation, or letters which were already upper-case. Similarly, tolower worked only on upper-case letters. Therefore, old code (or code written for wide portability) tends to call islower before toupper, and isupper before tolower.
The C Standard, however, says that toupper and tolower must work correctly on all characters, i.e. characters which don't need changing are left alone.
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