strconv package - strconv - Go Packages (original) (raw)

Package strconv implements conversions to and from string representations of basic data types.

Numeric Conversions

The most common numeric conversions are Atoi (string to int) and Itoa (int to string).

i, err := strconv.Atoi("-42") s := strconv.Itoa(-42)

These assume decimal and the Go int type.

ParseBool, ParseFloat, ParseInt, and ParseUint convert strings to values:

b, err := strconv.ParseBool("true") f, err := strconv.ParseFloat("3.1415", 64) i, err := strconv.ParseInt("-42", 10, 64) u, err := strconv.ParseUint("42", 10, 64)

The parse functions return the widest type (float64, int64, and uint64), but if the size argument specifies a narrower width the result can be converted to that narrower type without data loss:

s := "2147483647" // biggest int32 i64, err := strconv.ParseInt(s, 10, 32) ... i := int32(i64)

FormatBool, FormatFloat, FormatInt, and FormatUint convert values to strings:

s := strconv.FormatBool(true) s := strconv.FormatFloat(3.1415, 'E', -1, 64) s := strconv.FormatInt(-42, 16) s := strconv.FormatUint(42, 16)

AppendBool, AppendFloat, AppendInt, and AppendUint are similar but append the formatted value to a destination slice.

String Conversions

Quote and QuoteToASCII convert strings to quoted Go string literals. The latter guarantees that the result is an ASCII string, by escaping any non-ASCII Unicode with \u:

q := strconv.Quote("Hello, 世界") q := strconv.QuoteToASCII("Hello, 世界")

QuoteRune and QuoteRuneToASCII are similar but accept runes and return quoted Go rune literals.

Unquote and UnquoteChar unquote Go string and rune literals.

IntSize is the size in bits of an int or uint value.

ErrRange indicates that a value is out of range for the target type.

ErrSyntax indicates that a value does not have the right syntax for the target type.

AppendBool appends "true" or "false", according to the value of b, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b := []byte("bool:") b = strconv.AppendBool(b, true) fmt.Println(string(b))

}

Output:

bool:true

AppendFloat appends the string form of the floating-point number f, as generated by FormatFloat, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b32 := []byte("float32:") b32 = strconv.AppendFloat(b32, 3.1415926535, 'E', -1, 32) fmt.Println(string(b32))

b64 := []byte("float64:")
b64 = strconv.AppendFloat(b64, 3.1415926535, 'E', -1, 64)
fmt.Println(string(b64))

}

Output:

float32:3.1415927E+00 float64:3.1415926535E+00

AppendInt appends the string form of the integer i, as generated by FormatInt, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b10 := []byte("int (base 10):") b10 = strconv.AppendInt(b10, -42, 10) fmt.Println(string(b10))

b16 := []byte("int (base 16):")
b16 = strconv.AppendInt(b16, -42, 16)
fmt.Println(string(b16))

}

Output:

int (base 10):-42 int (base 16):-2a

AppendQuote appends a double-quoted Go string literal representing s, as generated by Quote, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b := []byte("quote:") b = strconv.AppendQuote(b, "Fran & Freddie's Diner") fmt.Println(string(b))

}

Output:

quote:""Fran & Freddie's Diner""

AppendQuoteRune appends a single-quoted Go character literal representing the rune, as generated by QuoteRune, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b := []byte("rune:") b = strconv.AppendQuoteRune(b, '☺') fmt.Println(string(b))

}

Output:

rune:'☺'

AppendQuoteRuneToASCII appends a single-quoted Go character literal representing the rune, as generated by QuoteRuneToASCII, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b := []byte("rune (ascii):") b = strconv.AppendQuoteRuneToASCII(b, '☺') fmt.Println(string(b))

}

Output:

rune (ascii):'\u263a'

AppendQuoteRuneToGraphic appends a single-quoted Go character literal representing the rune, as generated by QuoteRuneToGraphic, to dst and returns the extended buffer.

AppendQuoteToASCII appends a double-quoted Go string literal representing s, as generated by QuoteToASCII, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b := []byte("quote (ascii):") b = strconv.AppendQuoteToASCII(b, "Fran & Freddie's Diner") fmt.Println(string(b))

}

Output:

quote (ascii):""Fran & Freddie's Diner""

AppendQuoteToGraphic appends a double-quoted Go string literal representing s, as generated by QuoteToGraphic, to dst and returns the extended buffer.

AppendUint appends the string form of the unsigned integer i, as generated by FormatUint, to dst and returns the extended buffer.

package main

import ( "fmt" "strconv" )

func main() { b10 := []byte("uint (base 10):") b10 = strconv.AppendUint(b10, 42, 10) fmt.Println(string(b10))

b16 := []byte("uint (base 16):")
b16 = strconv.AppendUint(b16, 42, 16)
fmt.Println(string(b16))

}

Output:

uint (base 10):42 uint (base 16):2a

Atoi is equivalent to ParseInt(s, 10, 0), converted to type int.

package main

import ( "fmt" "strconv" )

func main() { v := "10" if s, err := strconv.Atoi(v); err == nil { fmt.Printf("%T, %v", s, s) }

}

Output:

int, 10

CanBackquote reports whether the string s can be represented unchanged as a single-line backquoted string without control characters other than tab.

package main

import ( "fmt" "strconv" )

func main() { fmt.Println(strconv.CanBackquote("Fran & Freddie's Diner ☺")) fmt.Println(strconv.CanBackquote("can't backquote this"))

}

Output:

true false

FormatBool returns "true" or "false" according to the value of b.

package main

import ( "fmt" "strconv" )

func main() { v := true s := strconv.FormatBool(v) fmt.Printf("%T, %v\n", s, s)

}

Output:

string, true

FormatComplex converts the complex number c to a string of the form (a+bi) where a and b are the real and imaginary parts, formatted according to the format fmt and precision prec.

The format fmt and precision prec have the same meaning as in FormatFloat. It rounds the result assuming that the original was obtained from a complex value of bitSize bits, which must be 64 for complex64 and 128 for complex128.

FormatFloat converts the floating-point number f to a string, according to the format fmt and precision prec. It rounds the result assuming that the original was obtained from a floating-point value of bitSize bits (32 for float32, 64 for float64).

The format fmt is one of

The precision prec controls the number of digits (excluding the exponent) printed by the 'e', 'E', 'f', 'g', 'G', 'x', and 'X' formats. For 'e', 'E', 'f', 'x', and 'X', it is the number of digits after the decimal point. For 'g' and 'G' it is the maximum number of significant digits (trailing zeros are removed). The special precision -1 uses the smallest number of digits necessary such that ParseFloat will return f exactly. The exponent is written as a decimal integer; for all formats other than 'b', it will be at least two digits.

package main

import ( "fmt" "strconv" )

func main() { v := 3.1415926535

s32 := strconv.FormatFloat(v, 'E', -1, 32)
fmt.Printf("%T, %v\n", s32, s32)

s64 := strconv.FormatFloat(v, 'E', -1, 64)
fmt.Printf("%T, %v\n", s64, s64)

// fmt.Println uses these arguments to print floats
fmt64 := strconv.FormatFloat(v, 'g', -1, 64)
fmt.Printf("%T, %v\n", fmt64, fmt64)

}

Output:

string, 3.1415927E+00 string, 3.1415926535E+00 string, 3.1415926535

FormatInt returns the string representation of i in the given base, for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z' for digit values >= 10.

package main

import ( "fmt" "strconv" )

func main() { v := int64(-42)

s10 := strconv.FormatInt(v, 10)
fmt.Printf("%T, %v\n", s10, s10)

s16 := strconv.FormatInt(v, 16)
fmt.Printf("%T, %v\n", s16, s16)

}

Output:

string, -42 string, -2a

FormatUint returns the string representation of i in the given base, for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z' for digit values >= 10.

package main

import ( "fmt" "strconv" )

func main() { v := uint64(42)

s10 := strconv.FormatUint(v, 10)
fmt.Printf("%T, %v\n", s10, s10)

s16 := strconv.FormatUint(v, 16)
fmt.Printf("%T, %v\n", s16, s16)

}

Output:

string, 42 string, 2a

IsGraphic reports whether the rune is defined as a Graphic by Unicode. Such characters include letters, marks, numbers, punctuation, symbols, and spaces, from categories L, M, N, P, S, and Zs.

package main

import ( "fmt" "strconv" )

func main() { shamrock := strconv.IsGraphic('☘') fmt.Println(shamrock)

a := strconv.IsGraphic('a')
fmt.Println(a)

bel := strconv.IsGraphic('\007')
fmt.Println(bel)

}

Output:

true true false

IsPrint reports whether the rune is defined as printable by Go, with the same definition as unicode.IsPrint: letters, numbers, punctuation, symbols and ASCII space.

package main

import ( "fmt" "strconv" )

func main() { c := strconv.IsPrint('\u263a') fmt.Println(c)

bel := strconv.IsPrint('\007')
fmt.Println(bel)

}

Output:

true false

Itoa is equivalent to FormatInt(int64(i), 10).

package main

import ( "fmt" "strconv" )

func main() { i := 10 s := strconv.Itoa(i) fmt.Printf("%T, %v\n", s, s)

}

Output:

string, 10

ParseBool returns the boolean value represented by the string. It accepts 1, t, T, TRUE, true, True, 0, f, F, FALSE, false, False. Any other value returns an error.

package main

import ( "fmt" "strconv" )

func main() { v := "true" if s, err := strconv.ParseBool(v); err == nil { fmt.Printf("%T, %v\n", s, s) }

}

Output:

bool, true

ParseComplex converts the string s to a complex number with the precision specified by bitSize: 64 for complex64, or 128 for complex128. When bitSize=64, the result still has type complex128, but it will be convertible to complex64 without changing its value.

The number represented by s must be of the form N, Ni, or N±Ni, where N stands for a floating-point number as recognized by ParseFloat, and i is the imaginary component. If the second N is unsigned, a + sign is required between the two components as indicated by the ±. If the second N is NaN, only a + sign is accepted. The form may be parenthesized and cannot contain any spaces. The resulting complex number consists of the two components converted by ParseFloat.

The errors that ParseComplex returns have concrete type *NumErrorand include err.Num = s.

If s is not syntactically well-formed, ParseComplex returns err.Err = ErrSyntax.

If s is syntactically well-formed but either component is more than 1/2 ULP away from the largest floating point number of the given component's size, ParseComplex returns err.Err = ErrRange and c = ±Inf for the respective component.

ParseFloat converts the string s to a floating-point number with the precision specified by bitSize: 32 for float32, or 64 for float64. When bitSize=32, the result still has type float64, but it will be convertible to float32 without changing its value.

ParseFloat accepts decimal and hexadecimal floating-point numbers as defined by the Go syntax for floating-point literals. If s is well-formed and near a valid floating-point number, ParseFloat returns the nearest floating-point number rounded using IEEE754 unbiased rounding. (Parsing a hexadecimal floating-point value only rounds when there are more bits in the hexadecimal representation than will fit in the mantissa.)

The errors that ParseFloat returns have concrete type *NumError and include err.Num = s.

If s is not syntactically well-formed, ParseFloat returns err.Err = ErrSyntax.

If s is syntactically well-formed but is more than 1/2 ULP away from the largest floating point number of the given size, ParseFloat returns f = ±Inf, err.Err = ErrRange.

ParseFloat recognizes the string "NaN", and the (possibly signed) strings "Inf" and "Infinity" as their respective special floating point values. It ignores case when matching.

package main

import ( "fmt" "strconv" )

func main() { v := "3.1415926535" if s, err := strconv.ParseFloat(v, 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat(v, 64); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("NaN", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } // ParseFloat is case insensitive if s, err := strconv.ParseFloat("nan", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("inf", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("+Inf", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("-Inf", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("-0", 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseFloat("+0", 32); err == nil { fmt.Printf("%T, %v\n", s, s) }

}

Output:

float64, 3.1415927410125732 float64, 3.1415926535 float64, NaN float64, NaN float64, +Inf float64, +Inf float64, -Inf float64, -0 float64, 0

ParseInt interprets a string s in the given base (0, 2 to 36) and bit size (0 to 64) and returns the corresponding value i.

The string may begin with a leading sign: "+" or "-".

If the base argument is 0, the true base is implied by the string's prefix following the sign (if present): 2 for "0b", 8 for "0" or "0o", 16 for "0x", and 10 otherwise. Also, for argument base 0 only, underscore characters are permitted as defined by the Go syntax forinteger literals.

The bitSize argument specifies the integer type that the result must fit into. Bit sizes 0, 8, 16, 32, and 64 correspond to int, int8, int16, int32, and int64. If bitSize is below 0 or above 64, an error is returned.

The errors that ParseInt returns have concrete type *NumErrorand include err.Num = s. If s is empty or contains invalid digits, err.Err = ErrSyntax and the returned value is 0; if the value corresponding to s cannot be represented by a signed integer of the given size, err.Err = ErrRange and the returned value is the maximum magnitude integer of the appropriate bitSize and sign.

package main

import ( "fmt" "strconv" )

func main() { v32 := "-354634382" if s, err := strconv.ParseInt(v32, 10, 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseInt(v32, 16, 32); err == nil { fmt.Printf("%T, %v\n", s, s) }

v64 := "-3546343826724305832"
if s, err := strconv.ParseInt(v64, 10, 64); err == nil {
    fmt.Printf("%T, %v\n", s, s)
}
if s, err := strconv.ParseInt(v64, 16, 64); err == nil {
    fmt.Printf("%T, %v\n", s, s)
}

}

Output:

int64, -354634382 int64, -3546343826724305832

ParseUint is like ParseInt but for unsigned numbers.

A sign prefix is not permitted.

package main

import ( "fmt" "strconv" )

func main() { v := "42" if s, err := strconv.ParseUint(v, 10, 32); err == nil { fmt.Printf("%T, %v\n", s, s) } if s, err := strconv.ParseUint(v, 10, 64); err == nil { fmt.Printf("%T, %v\n", s, s) }

}

Output:

uint64, 42 uint64, 42

Quote returns a double-quoted Go string literal representing s. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for control characters and non-printable characters as defined byIsPrint.

package main

import ( "fmt" "strconv" )

func main() { // This string literal contains a tab character. s := strconv.Quote("Fran & Freddie's Diner ☺") fmt.Println(s)

}

Output:

""Fran & Freddie's Diner\t☺""

QuoteRune returns a single-quoted Go character literal representing the rune. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for control characters and non-printable characters as defined by IsPrint. If r is not a valid Unicode code point, it is interpreted as the Unicode replacement character U+FFFD.

package main

import ( "fmt" "strconv" )

func main() { s := strconv.QuoteRune('☺') fmt.Println(s)

}

Output:

'☺'

QuoteRuneToASCII returns a single-quoted Go character literal representing the rune. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for non-ASCII characters and non-printable characters as defined by IsPrint. If r is not a valid Unicode code point, it is interpreted as the Unicode replacement character U+FFFD.

package main

import ( "fmt" "strconv" )

func main() { s := strconv.QuoteRuneToASCII('☺') fmt.Println(s)

}

Output:

'\u263a'

QuoteRuneToGraphic returns a single-quoted Go character literal representing the rune. If the rune is not a Unicode graphic character, as defined by IsGraphic, the returned string will use a Go escape sequence (\t, \n, \xFF, \u0100). If r is not a valid Unicode code point, it is interpreted as the Unicode replacement character U+FFFD.

package main

import ( "fmt" "strconv" )

func main() { s := strconv.QuoteRuneToGraphic('☺') fmt.Println(s)

s = strconv.QuoteRuneToGraphic('\u263a')
fmt.Println(s)

s = strconv.QuoteRuneToGraphic('\u000a')
fmt.Println(s)

s = strconv.QuoteRuneToGraphic('	') // tab character
fmt.Println(s)

}

Output:

'☺' '☺' '\n' '\t'

QuoteToASCII returns a double-quoted Go string literal representing s. The returned string uses Go escape sequences (\t, \n, \xFF, \u0100) for non-ASCII characters and non-printable characters as defined by IsPrint.

package main

import ( "fmt" "strconv" )

func main() { // This string literal contains a tab character. s := strconv.QuoteToASCII("Fran & Freddie's Diner ☺") fmt.Println(s)

}

Output:

""Fran & Freddie's Diner\t\u263a""

QuoteToGraphic returns a double-quoted Go string literal representing s. The returned string leaves Unicode graphic characters, as defined byIsGraphic, unchanged and uses Go escape sequences (\t, \n, \xFF, \u0100) for non-graphic characters.

package main

import ( "fmt" "strconv" )

func main() { s := strconv.QuoteToGraphic("☺") fmt.Println(s)

// This string literal contains a tab character.
s = strconv.QuoteToGraphic("This is a \u263a	\u000a")
fmt.Println(s)

s = strconv.QuoteToGraphic(`" This is a ☺ \n "`)
fmt.Println(s)

}

Output:

"☺" "This is a ☺\t\n" "" This is a ☺ \n ""

QuotedPrefix returns the quoted string (as understood by Unquote) at the prefix of s. If s does not start with a valid quoted string, QuotedPrefix returns an error.

package main

import ( "fmt" "strconv" )

func main() { s, err := strconv.QuotedPrefix("not a quoted string") fmt.Printf("%q, %v\n", s, err) s, err = strconv.QuotedPrefix(""double-quoted string" with trailing text") fmt.Printf("%q, %v\n", s, err) s, err = strconv.QuotedPrefix("or backquoted with more trailing text") fmt.Printf("%q, %v\n", s, err) s, err = strconv.QuotedPrefix("'\u263a' is also okay") fmt.Printf("%q, %v\n", s, err)

}

Output:

"", invalid syntax ""double-quoted string"", "or backquoted", "'☺'",

Unquote interprets s as a single-quoted, double-quoted, or backquoted Go string literal, returning the string value that s quotes. (If s is single-quoted, it would be a Go character literal; Unquote returns the corresponding one-character string. For an empty character literal Unquote returns the empty string.)

package main

import ( "fmt" "strconv" )

func main() { s, err := strconv.Unquote("You can't unquote a string without quotes") fmt.Printf("%q, %v\n", s, err) s, err = strconv.Unquote(""The string must be either double-quoted"") fmt.Printf("%q, %v\n", s, err) s, err = strconv.Unquote("or backquoted.") fmt.Printf("%q, %v\n", s, err) s, err = strconv.Unquote("'\u263a'") // single character only allowed in single quotes fmt.Printf("%q, %v\n", s, err) s, err = strconv.Unquote("'\u2639\u2639'") fmt.Printf("%q, %v\n", s, err)

}

Output:

"", invalid syntax "The string must be either double-quoted", "or backquoted.", "☺", "", invalid syntax

UnquoteChar decodes the first character or byte in the escaped string or character literal represented by the string s. It returns four values:

  1. value, the decoded Unicode code point or byte value;
  2. multibyte, a boolean indicating whether the decoded character requires a multibyte UTF-8 representation;
  3. tail, the remainder of the string after the character; and
  4. an error that will be nil if the character is syntactically valid.

The second argument, quote, specifies the type of literal being parsed and therefore which escaped quote character is permitted. If set to a single quote, it permits the sequence \' and disallows unescaped '. If set to a double quote, it permits \" and disallows unescaped ". If set to zero, it does not permit either escape and allows both quote characters to appear unescaped.

package main

import ( "fmt" "log" "strconv" )

func main() { v, mb, t, err := strconv.UnquoteChar(\"Fran & Freddie's Diner\", '"') if err != nil { log.Fatal(err) }

fmt.Println("value:", string(v))
fmt.Println("multibyte:", mb)
fmt.Println("tail:", t)

}

Output:

value: " multibyte: false tail: Fran & Freddie's Diner"

A NumError records a failed conversion.

package main

import ( "fmt" "strconv" )

func main() { str := "Not a number" if _, err := strconv.ParseFloat(str, 64); err != nil { e := err.(*strconv.NumError) fmt.Println("Func:", e.Func) fmt.Println("Num:", e.Num) fmt.Println("Err:", e.Err) fmt.Println(err) }

}

Output:

Func: ParseFloat Num: Not a number Err: invalid syntax strconv.ParseFloat: parsing "Not a number": invalid syntax