Why can't I change the values in a range of type structure?

Question

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.

Answer 1

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)
}

Answer 2

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