libgo/go/math/big/doc.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 /*
   6 Package big implements arbitrary-precision arithmetic (big numbers).
   7 The following numeric types are supported:
   8
   9         Int    signed integers
  10         Rat    rational numbers
  11         Float  floating-point numbers
  12
  13 The zero value for an Int, Rat, or Float correspond to 0. Thus, new
  14 values can be declared in the usual ways and denote 0 without further
  15 initialization:
  16
  17         var x Int        // &x is an *Int of value 0
  18         var r = &Rat{}   // r is a *Rat of value 0
  19         y := new(Float)  // y is a *Float of value 0
  20
  21 Alternatively, new values can be allocated and initialized with factory
  22 functions of the form:
  23
  24         func NewT(v V) *T
  25
  26 For instance, NewInt(x) returns an *Int set to the value of the int64
  27 argument x, NewRat(a, b) returns a *Rat set to the fraction a/b where
  28 a and b are int64 values, and NewFloat(f) returns a *Float initialized
  29 to the float64 argument f. More flexibility is provided with explicit
  30 setters, for instance:
  31
  32         var z1 Int
  33         z1.SetUint64(123)                 // z1 := 123
  34         z2 := new(Rat).SetFloat64(1.25)   // z2 := 5/4
  35         z3 := new(Float).SetInt(z1)       // z3 := 123.0
  36
  37 Setters, numeric operations and predicates are represented as methods of
  38 the form:
  39
  40         func (z *T) SetV(v V) *T          // z = v
  41         func (z *T) Unary(x *T) *T        // z = unary x
  42         func (z *T) Binary(x, y *T) *T    // z = x binary y
  43         func (x *T) Pred() P              // p = pred(x)
  44
  45 with T one of Int, Rat, or Float. For unary and binary operations, the
  46 result is the receiver (usually named z in that case; see below); if it
  47 is one of the operands x or y it may be safely overwritten (and its memory
  48 reused).
  49
  50 Arithmetic expressions are typically written as a sequence of individual
  51 method calls, with each call corresponding to an operation. The receiver
  52 denotes the result and the method arguments are the operation's operands.
  53 For instance, given three *Int values a, b and c, the invocation
  54
  55         c.Add(a, b)
  56
  57 computes the sum a + b and stores the result in c, overwriting whatever
  58 value was held in c before. Unless specified otherwise, operations permit
  59 aliasing of parameters, so it is perfectly ok to write
  60
  61         sum.Add(sum, x)
  62
  63 to accumulate values x in a sum.
  64
  65 (By always passing in a result value via the receiver, memory use can be
  66 much better controlled. Instead of having to allocate new memory for each
  67 result, an operation can reuse the space allocated for the result value,
  68 and overwrite that value with the new result in the process.)
  69
  70 Notational convention: Incoming method parameters (including the receiver)
  71 are named consistently in the API to clarify their use. Incoming operands
  72 are usually named x, y, a, b, and so on, but never z. A parameter specifying
  73 the result is named z (typically the receiver).
  74
  75 For instance, the arguments for (*Int).Add are named x and y, and because
  76 the receiver specifies the result destination, it is called z:
  77
  78         func (z *Int) Add(x, y *Int) *Int
  79
  80 Methods of this form typically return the incoming receiver as well, to
  81 enable simple call chaining.
  82
  83 Methods which don't require a result value to be passed in (for instance,
  84 Int.Sign), simply return the result. In this case, the receiver is typically
  85 the first operand, named x:
  86
  87         func (x *Int) Sign() int
  88
  89 Various methods support conversions between strings and corresponding
  90 numeric values, and vice versa: *Int, *Rat, and *Float values implement
  91 the Stringer interface for a (default) string representation of the value,
  92 but also provide SetString methods to initialize a value from a string in
  93 a variety of supported formats (see the respective SetString documentation).
  94
  95 Finally, *Int, *Rat, and *Float satisfy the fmt package's Scanner interface
  96 for scanning and (except for *Rat) the Formatter interface for formatted
  97 printing.
  98 */
  99 package big