Why Does Appending to a Slice Found in a Go Map Require Reassignment?

Appending Directly to Slice Found in a Go Map

Directly appending to a slice retrieved from a map can be a tricky task to understand. Let's dive into the issue and its solution.

In the provided code, an attempt was made to append to a slice stored in a map:

<code class="go">mappedAminoAcid, ok := aminoAcidsToCodons[aminoAcid]
if ok {
    // NOT WORKING: mappedAminoAcid = append(mappedAminoAcid, codon)
    aminoAcidsToCodons[aminoAcid] = append(mappedAminoAcid, codon)
}</code>

However, this failed because append returns a new slice if the underlying array needs to grow. Therefore, it's necessary to assign the new slice back to the map entry.

Here's why directly assigning doesn't work:

When a slice is returned by a function or accessed from a map, it's a copy of the original slice, not a pointer. Modifications made to the copy are not reflected in the original.

For example, consider a string:

<code class="go">x := "foo"
y := x
y = "bar"
fmt.Println(x) // "foo" (unchanged)</code>

Since a nil slice is acceptable as an initial argument to append, the code can be simplified:

<code class="go">aminoAcidsToCodons := map[rune][]string{}
for codon, aminoAcid := range utils.CodonsToAminoAcid {
    aminoAcidsToCodons[aminoAcid] = append(aminoAcidsToCodons[aminoAcid], codon)
}</code>

In summary, when appending to a slice retrieved from a map, it's crucial to remember that the returned slice is a copy, and to assign the new slice back to the map entry.

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