Why can't I change the values in a range of ty

2019-03-06 13:37发布

This is my first post so please "Go" easy on me. :) ... I am quite familiar with many traditional programming languages but I am new to Go and having trouble understanding the use of slices and ranges. The program code and comments below illustrate my consternation. Thank you!

package main

import (
    "fmt"
    "time"
)

type myStruct struct {
    Name  string
    Count int
}

Wrote my own Mod function because I could not find on in the Go libraries.

func modMe(mod int, value int) int {
    var m int
    var ret int

    m = value / mod
    ret = value - m*mod
    return ret
}

func main() {
    mod := 4
    cnt := 16
    fmt.Printf("Record mod is %d\r\n", mod)

Declare a structure array and add some records

    chartRecords := []myStruct{}

    for i := 0; i<=cnt ;i++ {
        n := myStruct{Count: i, Name: fmt.Sprintf("Joe%2d", i)}                         //Load some data
        chartRecords = append(chartRecords,n)
    }

Loading the data produces the output that I expect

    fmt.Printf("======ORIGINAL LOADED VALUES========\r\n")                                  
    i := 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out original values in the range
        i = i + 1
    }

Now I modify the values and print them to see that they appear modified. This looks as expected.

    fmt.Printf("======MODIFIED VALUES EXPECTED========\r\n")                                    
    i = 0
    for _, elem := range chartRecords {                                                 //looping thru the range of the data records
        mm := modMe(mod, i)                                                             //modMe is my function to return the Mod of a number based on moduls 'mod'
        elem.Count = mm                                                                 //assigning the new mod value to Count
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1                                                                       
    }

Now I simply loop through the same range again and print the same thing out. But the output shows the original values. I don't understand why this is happening. I'm guessing that it has something to do with slices and adding values rather than replacing the values.

    fmt.Printf("======CHECK AGAIN AND VALUES ARE BACK TO ORIGINAL========\r\n")                                         //Now lets loop through the same range
    i = 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1
    }                                                                                   //But this output shows the original values  WHY??
    time.Sleep(60 * time.Second)
}

The output looks like this... Screenshot Output

Thanks in advance for your advice.

2条回答
趁早两清
2楼-- · 2019-03-06 14:03

Adding these lines from a little help from my friend Russ.

Quote: Hi Victor, If I remember this correctly, when you for…range through a collection, the object returned is a copy of the original held in the collection (“value semantics”). So, the elem variable is a copy that you are assigning the count to. It’s valid code, but you aren’t updating the collection like you’re expecting. Consider adjusting your for…range loop to this:

    fmt.Printf("======NOW MODIFY VALUES THIS WAY========\r\n")                                  
    i = 0
    for idx := range chartRecords { 
        mm := modMe(mod, i)  
        chartRecords[idx].Count = mm  
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, chartRecords[idx].Count, chartRecords[idx].Name)  //Print out this elem.Count element in the range
        i = i + 1    
    } 

    fmt.Printf("======CHECK AGAIN AND VALUES ARE AS DESIRED========\r\n")                                       //Now lets loop through the same range
    i = 0
    for _, elem := range chartRecords {
        fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)    //Print out this elem.Count element in the range
        i = i + 1
    } 

Now I understand better and I hope this helps other newbies like me...

Have a great day!

Victor

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对你真心纯属浪费
3楼-- · 2019-03-06 14:10

The Go Programming Language Specification

For statements with range clause

A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block.


The Go Programming Language Specification is an easy read.

Put the updated elem iteration variable back in the chartRecords slice:

for i, elem := range chartRecords {
    elem.Count = modMe(mod, i)
    chartRecords[i] = elem
    fmt.Printf("No: %2d | Count: %2d | Name = %s\r\n", i, elem.Count, elem.Name)
}
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