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- procedure:
**number->string***number [radix]* `Radix`must be an exact integer, either 2, 8, 10, or 16. If omitted,`radix`defaults to 10. 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

`number`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; 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

`number`is not a complex number or is a complex number with an non-rational real or imaginary part.Rationale: If

`number`is an inexact number represented using flonums, 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 non-flonum representations.

- variable:
**flonum-parser-fast?** This variable controls the behavior of

`string->number`

when parsing inexact numbers. Specifically, it allows the user to trade off accuracy against speed.When set to its default value,

`#f`

, the parser provides maximal accuracy, as required by the Scheme standard. If set to`#t`

, the parser uses faster algorithms that will sometimes introduce small errors in the result. The errors affect a few of the least-significant bits of the result, and consequently can be tolerated by many applications.

- variable:
**flonum-unparser-cutoff** This variable is

**deprecated**; use`param:flonum-printer-cutoff`

instead.

- parameter:
**param:flonum-printer-cutoff** This parameter controls the action of

`number->string`

when`number`is a flonum (and consequently controls all printing of flonums). This parameter may be called with an argument to set its value.The value of this parameter is normally a list of three items:

`rounding-type`One of the following symbols:

`normal`

,`relative`

, or`absolute`

. The symbol`normal`

means that the number should be printed with full precision. The symbol`relative`

means that the number should be rounded to a specific number of digits. The symbol`absolute`

means that the number should be rounded so that there are a specific number of digits to the right of the decimal point.`precision`An exact integer. If

`rounding-type`is`normal`

,`precision`is ignored. If`rounding-type`is`relative`

,`precision`must be positive, and it specifies the number of digits to which the printed representation will be rounded. If`rounding-type`is`absolute`

, the printed representation will be rounded`precision`digits to the right of the decimal point; if`precision`is negative, the representation is rounded`(-`

digits to the left of the decimal point.`precision`)`format-type`One of the symbols:

`normal`

,`scientific`

, or`engineering`

. This specifies the format in which the number will be printed.

`scientific`

specifies that the number will be printed using scientific notation:

. In other words, the number is printed as a significand between zero inclusive and ten exclusive, and an exponent.`x`.`xxx`e`yyy``engineering`

is like`scientific`

, except that the exponent is always a power of three, and the significand is constrained to be between zero inclusive and 1000 exclusive. If`normal`

is specified, the number will be printed in positional notation if it is “small enough”, otherwise it is printed in scientific notation. A number is “small enough” when the number of digits that would be printed using positional notation does not exceed the number of digits of precision in the underlying floating-point number representation; IEEE 754-2008 binary64 floating-point numbers have 17 digits of precision.

This three-element list may be abbreviated in two ways. First, the symbol

`normal`

may be used, which is equivalent to the list`(normal 0 normal)`

. Second, the third element of the list,`format-type`, may be omitted, in which case it defaults to`normal`

.The default value for

`param:flonum-printer-cutoff`

is`normal`

. If it is bound to a value different from those described here,`number->string`

issues a warning and acts as though the value had been`normal`

.

Some examples of `param:flonum-printer-cutoff`

:

(number->string (* 4 (atan 1 1))) ⇒ "3.141592653589793" (parameterize ((param:flonum-printer-cutoff '(relative 5))) (number->string (* 4 (atan 1 1)))) ⇒ "3.1416" (parameterize ((param:flonum-printer-cutoff '(relative 5))) (number->string (* 4000 (atan 1 1)))) ⇒ "3141.6" (parameterize ((param:flonum-printer-cutoff '(relative 5 scientific))) (number->string (* 4000 (atan 1 1)))) ⇒ "3.1416e3" (parameterize ((param:flonum-printer-cutoff '(relative 5 scientific))) (number->string (* 40000 (atan 1 1)))) ⇒ "3.1416e4" (parameterize ((param:flonum-printer-cutoff '(relative 5 engineering))) (number->string (* 40000 (atan 1 1)))) ⇒ "31.416e3" (parameterize ((param:flonum-printer-cutoff '(absolute 5))) (number->string (* 4 (atan 1 1)))) ⇒ "3.14159" (parameterize ((param:flonum-printer-cutoff '(absolute 5))) (number->string (* 4000 (atan 1 1)))) ⇒ "3141.59265" (parameterize ((param:flonum-printer-cutoff '(absolute -4))) (number->string (* 4e10 (atan 1 1)))) ⇒ "31415930000." (parameterize ((param:flonum-printer-cutoff '(absolute -4 scientific))) (number->string (* 4e10 (atan 1 1)))) ⇒ "3.141593e10" (parameterize ((param:flonum-printer-cutoff '(absolute -4 engineering))) (number->string (* 4e10 (atan 1 1)))) ⇒ "31.41593e9" (parameterize ((param:flonum-printer-cutoff '(absolute -5))) (number->string (* 4e10 (atan 1 1)))) ⇒ "31415900000."

- procedure:
**string->number***string [radix]* Returns a number of the maximally precise representation expressed by the given

`string`.`Radix`must be an exact integer, either 2, 8, 10, or 16. If supplied,`radix`is a default radix that may be overridden by an explicit radix prefix 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, then`string->number`

returns`#f`

.(string->number "100") ⇒ 100 (string->number "100" 16) ⇒ 256 (string->number "1e2") ⇒ 100.0 (string->number "15##") ⇒ 1500.0

Note that a numeric representation using a decimal point or an exponent marker is not recognized unless

`radix`is`10`

.

Next: Fixnum and Flonum Operations, Previous: Numerical operations, Up: Numbers [Contents][Index]