The function "isalnum( )" returns a non-zero value if the character is alphanumeric and zero otherwise. What is header file to be used in the main program ?
ctype.h
isalnum() belongs to the character-classification family, and every member of that family is declared in ctype.h — option 3.
The name itself is the clue: the whole is… group tests what type of character it has been given, and ctype stands for character type.
| Header | Contains | Typical members |
|---|---|---|
| stdio.h | Input and output | printf, scanf, fopen, fgets |
| io.h | Low-level file handles (non-standard, DOS/Windows) | open, read, write, close |
| ctype.h | Character classification and conversion | isalnum, isalpha, isdigit, toupper |
| string.h | Operations on whole strings | strlen, strcpy, strcmp, strcat |
The natural confusion is with string.h, since both deal with text. The dividing line is the unit of work: ctype.h functions each take a single character and answer a question about it, while string.h functions operate on null-terminated arrays of characters. isalpha(s[i]) needs ctype.h; strlen(s) needs string.h.
The full classification set is worth knowing as a group, since exam questions rotate through it: isalpha (letter), isdigit (0–9), isalnum (either), isspace, ispunct, isupper, islower, isxdigit. The same header also declares the two conversions toupper and tolower.
A subtlety in the prototype :
int isalnum(int c);
The parameter is int, not char, so that the value EOF can be passed — it lies outside the range of char. That is why the return is also int: the standard guarantees only "non-zero" for true, not the value 1, so results must be tested as if (isalnum(c)) and never compared with == 1.
What happens if the header is omitted : older C compilers assume an undeclared function returns int and compile it anyway, so the mistake often escapes notice until the behaviour differs on another platform. C99 made the implicit declaration an error.
Hence, the header file required is ctype.h.
Similar Questions
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Int m;
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