[Golang] 初探之 strconv.Atoi

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# 描述

strconv 包實現了字元串與數字（整數、浮點數等）之間的互相轉換。

# Examples

package mainimport ( "fmt" "strconv")func main() { // ParseUint v := "42" if s, err := strconv.ParseUint(v, 10, 32); err == nil { fmt.Printf("%T, %v
", s, s) } if s, err := strconv.ParseUint(v, 10, 64); err == nil { fmt.Printf("%T, %v
", s, s) } /* Output: uint64, 42 uint64, 42 */ // ParseInt v32 := "-354634382" if s, err := strconv.ParseInt(v32, 10, 32); err == nil { fmt.Printf("%T, %v
", s, s) } if s, err := strconv.ParseInt(v32, 16, 32); err == nil { fmt.Printf("%T, %v
", s, s) } v64 := "-3546343826724305832" if s, err := strconv.ParseInt(v64, 10, 64); err == nil { fmt.Printf("%T, %v
", s, s) } if s, err := strconv.ParseInt(v64, 16, 64); err == nil { fmt.Printf("%T, %v
", s, s) } /* Output: int64, -354634382 int64, -3546343826724305832 */ // Atoi v := "10" if s, err := strconv.Atoi(v); err == nil { fmt.Printf("%T, %v", s, s) } /* Output: int, 10 */}

# 源碼

// uint 位數const intSize = 32 << (^uint(0) >> 63)// IntSize is the size in bits of an int or uint value.const IntSize = intSize// uint64 最大數值const maxUint64 = (1<<64 - 1)// 無符號整型// @params s: 待轉化的數字字元串// @params base: 進位// @params bitSize: 位數// ParseUint is like ParseInt but for unsigned numbers.func ParseUint(s string, base int, bitSize int) (uint64, error) { const fnParseUint = "ParseUint" if len(s) == 0 { return 0, syntaxError(fnParseUint, s) } s0 := s switch { case 2 <= base && base <= 36: // 允許傳入的進位數 case base == 0: // 根據字元串前綴確認進位 switch { case s[0] == 0 && len(s) > 1 && (s[1] == x || s[1] == X): // 0x123 或 0X123 為16進位 if len(s) < 3 { return 0, syntaxError(fnParseUint, s0) } base = 16 s = s[2:] case s[0] == 0: // 0123 為8進位 base = 8 s = s[1:] default: // 其他為10進位 base = 10 } default: // 錯誤的進位數 return 0, baseError(fnParseUint, s0, base) } // 位數默認0 為系統判定 if bitSize == 0 { bitSize = int(IntSize) } else if bitSize < 0 || bitSize > 64 { return 0, bitSizeError(fnParseUint, s0, bitSize) } // Cutoff is the smallest number such that cutoff*base > maxUint64. // Use compile-time constants for common cases. var cutoff uint64 switch base { case 10: cutoff = maxUint64/10 + 1 case 16: cutoff = maxUint64/16 + 1 default: cutoff = maxUint64/uint64(base) + 1 } // 指定位數 bitSite 內允許的最大數值 maxVal := uint64(1)<<uint(bitSize) - 1 var n uint64 for _, c := range []byte(s) { var d byte switch { case 0 <= c && c <= 9: d = c - 0 case a <= c && c <= z: d = c - a + 10 case A <= c && c <= Z: d = c - A + 10 default: return 0, syntaxError(fnParseUint, s0) } // 判斷孰否超出進位範圍 if d >= byte(base) { return 0, syntaxError(fnParseUint, s0) } // 判斷下一步後是否越界 if n >= cutoff { // n*base overflows return maxVal, rangeError(fnParseUint, s0) } // 進位&加值 n *= uint64(base) n1 := n + uint64(d) // 判斷是否越界 if n1 < n || n1 > maxVal { // n+v overflows return maxVal, rangeError(fnParseUint, s0) } n = n1 } return n, nil}// ParseInt 根據給定的進位(0, 2 to 36)和位數大小(0 to 64)將字元串轉化後返回相應的值//// if base == 0:// prefix -> "0x"; base = 16// prefix -> "0" : base = 8// if base <2 || base > 36:// return err //// 參數 bitSize 給出轉換結果所允許的大小// 0 -> int// 8 -> int8// 16 -> int16// 32 -> int32// 64 -> int64// x < 0 || x >64 -> err//func ParseInt(s string, base int, bitSize int) (i int64, err error) { const fnParseInt = "ParseInt" // Empty string bad. if len(s) == 0 { return 0, syntaxError(fnParseInt, s) } // 判斷正負 s0 := s neg := false if s[0] == + { s = s[1:] } else if s[0] == - { neg = true s = s[1:] } // 轉換 uint 並且檢查溢出. var un uint64 un, err = ParseUint(s, base, bitSize) if err != nil && err.(*NumError).Err != ErrRange { err.(*NumError).Func = fnParseInt err.(*NumError).Num = s0 return 0, err } if bitSize == 0 { bitSize = int(IntSize) } // 檢查正負數是否 // +溢出 cutoff := uint64(1 << uint(bitSize-1)) if !neg && un >= cutoff { // + return int64(cutoff - 1), rangeError(fnParseInt, s0) } if neg && un > cutoff { // - return -int64(cutoff), rangeError(fnParseInt, s0) } n := int64(un) if neg { n = -n } return n, nil}// Atoi 和調用 ParseInt(s, 10, 0) 形同，只是返回 int 類型// Atoi returns the result of ParseInt(s, 10, 0) converted to type int.func Atoi(s string) (int, error) { const fnAtoi = "Atoi" sLen := len(s) if intSize == 32 && (0 < sLen && sLen < 10) || intSize == 64 && (0 < sLen && sLen < 19) { // 快速轉化路徑：適合int類型的小整數. s0 := s if s[0] == - || s[0] == + { s = s[1:] if len(s) < 1 { return 0, &NumError{fnAtoi, s0, ErrSyntax} } } n := 0 for _, ch := range []byte(s) { ch -= 0 if ch > 9 { return 0, &NumError{fnAtoi, s0, ErrSyntax} } n = n*10 + int(ch) } if s0[0] == - { n = -n } return n, nil } // 大整數. i64, err := ParseInt(s, 10, 0) if nerr, ok := err.(*NumError); ok { nerr.Func = fnAtoi } return int(i64), err}

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