C in a Nutshell

Name

atoi

Synopsis

Converts a string to an integer

#include <stdlib.h>
intatoi( const char *s );
long atol( const char *s );
long long atoll( const char *s );        (C99)

The atoi() function converts a string of characters representing a numeral into a number of int. Similarly, atol() returns a long integer, and in C99, the atoll() function converts a string into an integer of type long long.

The conversion ignores any leading whitespace characters (spaces, tabs, newlines). A leading plus sign is permissible; a minus sign makes the return value negative. Any character that cannot be interpreted as part of an integer, such as a decimal point or exponent sign, has the effect of terminating the numeral input, so that atoi() converts only the partial string to the left of that character. If under these conditions the string still does not appear to represent a numeral, then atoi() returns 0.

Example

char *s = " -135792468.00 Balance on Dec. 31";
printf("\"%s\" becomes %ld\n", s,atol(s));

These statements produce the output:

" -135792468.00 Balance on Dec. 31" becomes -135792468

See Also

strtol() and strtoll(); atof() and strtod()