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.
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:
Now I understand better and I hope this helps other newbies like me...
Have a great day!
Victor
The Go Programming Language Specification is an easy read.
Put the updated
elem
iteration variable back in thechartRecords
slice: