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6.2.7 Numerical input and output

procedure: number->string z ¶

procedure: number->string z radix ¶

It is an error if radix is not one of 2, 8, 10, or 16.

The procedure number->string takes a number and a radix and returns as a string an external representation of the given number in the given radix such that

(let ((number number)
      (radix radix))
  (eqv? number
        (string->number (number->string number
                                        radix)
                        radix)))

is true. It is an error if no possible result makes this expression true. If omitted, radix defaults to 10.

If z is inexact, the radix is 10, and the above expression can be satisfied by a result that contains a decimal point, then the result contains a decimal point and is expressed using the minimum number of digits (exclusive of exponent and trailing zeroes) needed to make the above expression true [howtoprint, howtoread]; otherwise the format of the result is unspecified.

The result returned by number->string never contains an explicit radix prefix.

Note: The error case can occur only when z is not a complex number or is a complex number with a non-rational real or imaginary part.

Rationale:

If z is an inexact number and the radix is 10, then the above expression is normally satisfied by a result containing a decimal point. The unspecified case allows for infinities, NaNs, and unusual representations.

procedure: string->number string ¶

procedure: string->number string radix ¶

Returns a number of the maximally precise representation expressed by the given string. It is an error if radix is not 2, 8, 10, or 16.

If supplied, radix is a default radix that will be overridden if an explicit radix prefix is present in string (e.g. "#o177"). If radix is not supplied, then the default radix is 10. If string is not a syntactically valid notation for a number, or would result in a number that the implementation cannot represent, then string->number returns #f. An error is never signaled due to the content of string.

(string->number "100")    ⇒ 100
(string->number "100" 16) ⇒ 256
(string->number "1e2")    ⇒ 100.0

Note: The domain of string->number may be restricted by implementations in the following ways. If all numbers supported by an implementation are real, then string->number is permitted to return #f whenever string uses the polar or rectangular notations for complex numbers. If all numbers are integers, then string->number may return #f whenever the fractional notation is used. If all numbers are exact, then string->number may return #f whenever an exponent marker or explicit exactness prefix is used. If all inexact numbers are integers, then string->number may return #f whenever a decimal point is used.

The rules used by a particular implementation for string->number must also be applied to read and to the routine that reads programs, in order to maintain consistency between internal numeric processing, I/O, and the processing of programs. As a consequence, the R⁵RS permission to return #f when string has an explicit radix prefix has been withdrawn.