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Converting Between ASCII Characters and Values

Problem

You want to get the number representing the ASCII value of a character, or you want to display the characters associated with a particular ASCII numerical value.

Solution

The built in function char->integer converts a character to an extended ASCII value (i.e., a number in the range of 0 to 255), while integer->char takes an extended ASCII value and returns the corresponding character.

> (char->integer #\C)
67
> (integer->char 68)
#\D

The procedures char->latin-1-integer and latin-1-integer->char support conversions between characters in the platform-specific character set and platform-independent Latin-1 (ISO 8859-1) values:

> (char->latin-1-integer #\U)
85
> (latin-1-integer->char 241)
#\ñ

Discussion

Character values are "extended ASCII" values from 0 to 255 (where the ASCII extensions are platform-specific). If integer->char is given an integer that is not in 0 to 255 inclusive, the exn:application:type exception is raised.

char->latin-1-integer returns the integer in 0 to 255 inclusive corresponding to the Latin-1 value for char, or #f if char (in the platform-specific character set) has no corresponding character in Latin-1.

latin-1-integer->char returns the character corresponding to the Latin-1 mapping of its input, or #f if the platform-specific character set does not support the corresponding Latin-1 character. If the input is not in 0 to 255 inclusive, the exn:application:type exception is raised.

For Unix and Mac OS, char->latin-1-integer and latin-1-integer->char are the same as char->integer and integer->char. For Windows, the platform-specific set and Latin-1 match except for the range #x80 to #x9F (which are unprintable control characters in Latin-1).