英文:
Go lang : search x digits from sets of numbers, why takes very long time to execute?
问题
我尝试编写了一些小程序，用于从一组数字中找到 x 位数，例如：我想要从 1 到 1000000000 中找到第 89 位的数字。
以下是我的代码：
package main
import (
"fmt"
"strconv"
)
var bucket string
func main() {
findDigits(89, 1000000000)
}
func findDigits(digits int, length int) {
for i := 1; i <= length; i++ {
bucket += strconv.Itoa(i)
}
fmt.Println("The", digits, "th digit from 1 -", length, "is:", string([]rune(bucket)[digits-1]))
}
有人知道我犯了什么错误吗？我需要一些建议来改进这段代码。
谢谢 
英文:
I tried to make small programs that find x digits from sets of numbers, for example : i want to find 89th digits from 1 - 1000000000.
Here is my code : https://play.golang.org/p/93yh_urX16
package main
import (
"fmt"
"strconv"
)
var bucket string
func main() {
findDigits( 89, 1000000000 )
}
func findDigits( digits int, length int) {
for i := 1; i <= length; i++ {
bucket += strconv.Itoa(i)
}
fmt.Println( "The", digits, "th digit from 1", "-", length, "is :", string ( [] rune ( bucket )[digits - 1] ) )
}
Does anyone knows, what mistakes i've made ? i need some advice to me for improving this code.
Thanks
答案1
得分: 1
你的程序非常非常低效。user1431317的程序也非常低效。
只需简单计算数值即可。即使对于像9,223,372,036,854,775,807这样的数字索引（在我的计算机上只需95.6纳秒和2次内存分配），它只需要几纳秒的CPU时间和几次内存分配。例如，
package main
import (
"fmt"
"math"
"strconv"
)
// digit返回从连接的非负整数序列中的第i个数字。
// 数字序列是01234567891011121314151617181920...
func digit(i int64) string {
// 有9个一位数的正整数，90个两位数，
// 900个三位数，依此类推。
if i <= 0 {
return "0"
}
j := int64(1)
w := 1
for {
t := j + 9*int64(math.Pow10(w-1))*int64(w)
if t < 0 || t > i {
break
}
j = t
}
k := i - j
n := k / int64(w)
m := k % int64(w)
d := strconv.FormatInt(int64(math.Pow10(w-1))+n, 10)[m]
return string(d)
}
func main() {
tests := []int64{
0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13,
88, 89,
188, 189, 190, 191, 192,
math.MaxInt32, math.MaxInt64,
}
for _, n := range tests {
fmt.Println(n, digit(n))
}
}
输出：
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 1
11 0
12 1
13 1
88 4
89 9
188 9
189 9
190 1
191 0
192 0
2147483647 2
9223372036854775807 9
英文:
Your program is very, very inefficient. user1431317's program is very inefficient.
Simply calculate the value. It will only take nanoseconds of CPU time and a few memory allocations, even for a digit index as large as 9,223,372,036,854,775,807 (95.6 nanoseconds and 2 allocations on my computer). For example,
package main
import (
"fmt"
"math"
"strconv"
)
// digit returns the ith digit from the sequence of
// concatenated non-negative integers.
// The sequence of digits is 01234567891011121314151617181920...
func digit(i int64) string {
// There are 9 one digit positive integers, 90 two digit,
// 900 three digit, and so on.
if i <= 0 {
return "0"
}
j := int64(1)
w := 1
for ; ; w++ {
t := j + 9*int64(math.Pow10(w-1))*int64(w)
if 0 > t || t > i {
break
}
j = t
}
k := i - j
n := k / int64(w)
m := k % int64(w)
d := strconv.FormatInt(int64(math.Pow10(w-1))+n, 10)[m]
return string(d)
}
func main() {
tests := []int64{
0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 11, 12, 13,
88, 89,
188, 189, 190, 191, 192,
math.MaxInt32, math.MaxInt64,
}
for _, n := range tests {
fmt.Println(n, digit(n))
}
}
Output:
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 1
11 0
12 1
13 1
88 4
89 9
188 9
189 9
190 1
191 0
192 0
2147483647 2
9223372036854775807 9
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