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Converting Between Octal and Hexadecimal

Problem

You want to convert a string (e.g., "0x55" or "0755") containing an octal or hexadecimal number to the correct number.

Solution

Use the string->number and number->string built in functions.

> (string->number "55" 16)
85
> (string->number "0755" 8)
493

Discussion

The string->number function takes an optional radix (which must be an exact integer, either 2, 8, 10, or 16). If not supplied, it defaults to decimal unless an explicit radix is included in the number string. For example, "#o177" is octal, "#xff" is hexadecimal.

> (string->number "#o177")
127
> (string->number "#XFF")
255

Note that the radix and numbers may be upper or lower case.

Here's an example that accepts a number in either decimal, octal, or hex, and prints that number in all three bases. It uses the oct function to convert from octal and hexadecimal if the input began with a 0. It then uses printf to convert back into hex, octal, and decimal as needed.

(define (convert str)
  (let ((num (string->number str)))
    (printf "decimal=~a, hex=~a, octal=~a"
            (number->string num)
            (number->string num 16)
            (number->string num 8))))

> (convert "#o17")
decimal=15, hex=f, octal=17
> (convert "17")
decimal=17, hex=11, octal=21
> (convert "#X17")
decimal=23, hex=17, octal=27

The following code converts Unix file permissions. They're always given in octal, so we assume base 8:

(define (fp-convert str)
  (let ((perm (string->number str 8)))
    (printf "The decimal value is ~a~n" (number->string perm))))

> (fp-convert "777")
The decimal value is 511

See Also