isdigit
From cppreference.com
| Defined in header <ctype.h>
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| int isdigit( int ch ); |
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Checks if the given character is a numeric character (0123456789).
The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF.
Parameters
| ch | - | character to classify |
Return value
Non-zero value if the character is a numeric character, zero otherwise.
Notes
isdigit and isxdigit are the only standard narrow character classification functions that are not affected by the currently installed C locale, although some implementations (e.g. Microsoft in 1252 codepage) may classify additional single-byte characters as digits.