int isalpha(int c); returns true if c is an alphabetical letter and 0 otherwise.
int islower(int c);
int isupper(int c);
int isdigit(int c); returns a true value if c is digit and 0 (false) otherwise.
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ctype.h
int tolower(int c);
If c is an upper-case letter, it returns the corresponding lower-case letter; otherwise it returns c
int toupper(int c);
int isspace(int c); returns non-zero if it is space; 0, otherwise;
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stdlib.h
int atoi(const char *ptr);
double atof(const char *ptr);
long atol(const char *ptr);
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stdio.h
int getchar(void);
int putchar(int c);
It returns the character written, or EOF for an error.
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stdio.h
int sprintf(char *s, const char *format, …);
Same as printf except that the output is written into the string s, terminated with ‘\0’. The return value is the number of characters that are written into s.
int sscanf(char *s, const char *format, …);
Same as scanf except that the input characters are taken from string s. It returns number of items (variables) that are read, or EOF if an error occurs.