Appending to a slice of slices

Getting Help
1

I’m currently engaged in writing my second Go program - the first was a modified form of Hello World called Hello Pete - and I have encountered an aspect of slices which I am struggling to understand. I’m hoping that someone here will be kind enough to assist with an explanation.

I’m looking to read a csv file one record at a time, extract a subset of the fields and use them to populate a slice of string slices. My problem lies in the final part of the process.

I’ve tried to reproduce the behaviour I am seeing both here and in the playground.

package main

import (
	"fmt"
)

func main() {
	mySOS := make([][]string, 1)
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	row0 := []string{"Twas", "brillig", "and", "the", "slithy", "toves"}
	row1 := []string{"did", "gyre", "and", "gimble", "in", "the"}
	row2 := []string{"wabe", "all", "mimsey", "were", "the", "borogoves"}
	// row3 := []string{"and", "the", "mome", "raths", "outgrabe", "."}

	s := make([]string, 3)
	
	s[0] = row0[2]
	s[1] = row0[3]
	s[2] = row0[4]
	
	fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, s)
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	s[0] = row1[2]
	s[1] = row1[3]
	s[2] = row1[4]
	
	fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, s)
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	s[0] = row2[2]
	s[1] = row2[3]
	s[2] = row2[4]
	
	fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, s)
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n", mySOS, mySOS)
}

https://play.golang.org/p/o0ufVU3sBzR

My puzzlement is over the fact that mySOS always ends up being fully populated with the final slice from the test data. After spending a considerable amount of time staring at it I tried a slightly different approach which can be seen here.

package main

import (
	"fmt"
)

func main() {
	mySOS := make([][]string, 1)
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	row0 := []string{"Twas", "brillig", "and", "the", "slithy", "toves"}
	row1 := []string{"did", "gyre", "and", "gimble", "in", "the"}
	row2 := []string{"wabe", "all", "mimsey", "were", "the", "borogoves"}
	// row3 := []string{"and", "the", "mome", "raths", "outgrabe", "."}
	
	/*
	s := make([]string, 3)
	
	s[0] = row0[2]
	s[1] = row0[3]
	s[2] = row0[4]
	*/
	
	// fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, row1[2:5])
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	/*
	s[0] = row1[2]
	s[1] = row1[3]
	s[2] = row1[4]
	*/
	
	// fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, row0[2:5])
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n\n", mySOS, mySOS)
	
	/*
	s[0] = row2[2]
	s[1] = row2[3]
	s[2] = row2[4]
	*/
	
	// fmt.Printf("s is of type %T and has value %v\n", s, s)
	mySOS = append(mySOS, row2[2:5])
	fmt.Printf("mySliceOfSlices is of type %T and has value %v\n", mySOS, mySOS)
}

https://play.golang.org/p/nbWWEjHTzEX

This time mySOS contains the data I was expecting. However I still cannot see why the result of the initial code differs.

As previously stated I would be grateful if anyone can help clear up my obvious misunderstanding.

2

In your first example you are continuously redefining the slice s[:3] so that it ends up pointing to the last set of values. You need a new backing array for each “s” slice. See: https://play.golang.org/p/Vz1b8afT_eJ

3

Ah, OK. So would I be correct in thinking then that it is a reference to s - sticking with the first example - which is appended to mySOS, as opposed to the value?

4

A slice “value” includes a pointer to the “backing array.” (See “Slice Internals” at https://blog.golang.org/go-slices-usage-and-internals.)

So, yes, you’re appending references to the backing array that you’re continually updating through the slice reference.

5

Got it. Many thanks for both that, and also the link to the well written and lucid article.

6

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