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Slices, maps and strings · 3 MIN READ

Comma-ok is how you tell zero from missing

Report whether a key is present when its value can be zero.

The Tour's "Mutating Maps" page is the two-value read: elem, ok := m[key]. A missing key and a present zero look the same if you only take elem. Scores, counts, flags — anything whose zero is a legitimate stored value — need the ok.

delete(m, key) is the other half of that page. Deleting a missing key is a no-op, not an error. After delete, ok is false even if the old value was zero.

SEE THE IDEA

Presence is a second result

  1. 01

    Stored zero

    go
    m["known"] = 0

    value is 0; ok is true

  2. 02

    Absent entry

    go
    v, ok := m["absent"]

    value is 0; ok is false

  3. 03

    Decision

    go
    if !ok { ... }

    Handle absence explicitly

The boolean answers whether the key exists, independently of the value.

In practice

Use comma-ok when zero is a meaningful stored value. The same pattern appears in type assertions and channel receives, but the boolean answers a different presence question in each case. Give it a name such as found when that improves the branch.

Watch out

Testing v == 0 cannot distinguish an absent key from an entry explicitly containing zero.

Official reference ↗

Work through the exercise

go
package main

// Has reports whether key is in m, even when m[key] is 0.
func Has(m map[string]int, key string) bool {
	return m[key] != 0
}

func main() {}
Try it yourself

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