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[[Guides:C/C Crash Course|Back to TOC]]

<metadesc content="Converting text to numbers, and numbers to text (atoi, atol, atoll, atof functions). Quick overview of the C syntax. Part of a online crash course for C/C++ programming language with the aim to be understandable by adults, kids, and teens ages 12 and up, "/>
==Converting text to numbers==

Every now and then, we may want to convert a text string to a number. C has a few very useful functions for doing this. They are pretty straight forward, and, you should know enough about C now to know how to use the from their function prototypes:
<blockquote>
<code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/atoi()|int atoi(char *)]]</code>
<br><code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/atol()|long atol(char *)]]</code>
<br><code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/atoll()|long long atoll(char *)]]</code> for C99 compliant compilers.
<br><code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/atof()|double atof(char *)]]</code>
</blockquote>

There are other ways, but these are the easiest, and most useful functions to use. the <code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/strtol()|strtol()]]</code>, <code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/strtod()|strtod()]]</code>, <code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/strtoll()|strtoll()]]</code>, <code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/strtold()|strtold()]]</code>, <code>[[Reference:C Functions and Keywords/ANSI-ISO Standard C/strtof()|strtof()]]</code> and other like-named functions, allow for more options. They take the form of:

<source lang=c>
float strtof(const char *nptr, char **endptr);
long int strtol(const char *nptr, char **endptr, int base); 
</source>

Some of these functions are described by the ANSI C standard, others, including stroll() and atoll() are defined by the ISO C99 and POSIX.1 (1996) standards. In order to use these functions, you need to also include the stdlib.h header file.