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| There are several variants of the '''atoi''' function, '''atol''', ''']''' and '''atoll''' , which are used to convert a string into a <code>long</code>, <code>double</code>, or <code>long</code> <code>long</code> type, respectively. The '''atoll''' was formerly known as '''atoq''' and was included into ]. | There are several variants of the '''atoi''' function, '''atol''', ''']''' and '''atoll''' , which are used to convert a string into a <code>long</code>, <code>double</code>, or <code>long</code> <code>long</code> type, respectively. The '''atoll''' was formerly known as '''atoq''' and was included into ]. | ||
| ==Function Code== | ==Function Code== | ||
| /*atoi: convert s to integer*/ | /*atoi: convert s to integer*/ | ||
| int atoi(char s) | int atoi(char s) | ||
| { | { | ||
| Line 18: | Line 18: | ||
| n = 0; | n = 0; | ||
| for(i = 0; s >= '0' && s <= '9'; i++) | for(i = 0; s >= '0' && s <= '9'; i++) | ||
| n = 10 * n + (s - '0') | n = 10 * n + (s - '0'); | ||
| return n; | return n; | ||
| } | } | ||
Revision as of 19:00, 6 September 2011
atoi is a function in the C programming language that converts a string into an integer numerical representation. atoi stands for ASCII to integer. It is included in the C standard library header file stdlib.h. Its prototype is as follows:
int atoi(const char *str);
The str argument is a string, represented by an array of characters, containing the characters of a signed integer number. The string must be null-terminated. When atoi encounters a string with no numerical sequence, it returns zero (0).
There are several variants of the atoi function, atol, atof and atoll , which are used to convert a string into a long, double, or long long type, respectively. The atoll was formerly known as atoq and was included into C99.
Function Code
/*atoi: convert s to integer*/
int atoi(char s) {
int i, n;
n = 0;
for(i = 0; s >= '0' && s <= '9'; i++)
n = 10 * n + (s - '0');
return n;
}
Deficiencies
It is impossible to tell whether the string holds valid sequence of digits that represents the number 0 or invalid number as the function returns 0 in both cases. The newer function strtol does not have this deficiency.
atoi is neither thread-safe, nor async-cancel safe on some operating systems.
Also, atoi only converts base ten ascii values (this may also be a benefit depending on perspective). strtol and other functions support alternate bases such as hexadecimal and octal.
Standards conformance
The atoi, atof, and atol functions are a part of the ISO standard C library (C89), while the atoll function is added by C99.
However, because of the ambiguity in returning 0 and lack of thread-safety and async-cancel safety on some operating systems, atoi is considered to be deprecated by strtol.
References
The Version 7 Unix Manual Pages © 1979 by Bell Telephone Laboratories, Incorporated.
The Version 1 Unix Manual page for atoi written by Ken Thompson (November 1971).