56 lines
1.7 KiB
Markdown
56 lines
1.7 KiB
Markdown
# Range Clauses
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Spec: https://go.dev/ref/spec#For_statements
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## Summary
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A range clause provides a way to iterate over an array, slice, string, map, or channel.
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## Example
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```go
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for k, v := range myMap {
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log.Printf("key=%v, value=%v", k, v)
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}
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for v := range myChannel {
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log.Printf("value=%v", v)
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}
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for i, v := range myArray {
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log.Printf("array value at [%d]=%v", i, v)
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}
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```
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## Reference
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If only one value is used on the left of a range expression, it is the 1st value in this table.
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| Range expression | 1st value | 2nd value (optional) | notes |
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|:-----------------|:----------|:---------------------|:------|
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| array or slice a ` [n]E `, ` *[n]E `, or ` []E ` | index ` i int ` | ` a[i] ` E |
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| string s string type | index ` i int ` | rune ` int ` | range iterates over Unicode code points, not bytes |
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| map m ` map[K]V ` | key ` k K ` | value ` m[k] ` V |
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| channel c chan E | element ` e E ` | _none_ |
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## Gotchas
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When iterating over a slice or map of values, one might try this:
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```go
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items := make([]map[int]int, 10)
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for _, item := range items {
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item = make(map[int]int, 1) // Oops! item is only a copy of the slice element.
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item[1] = 2 // This 'item' will be lost on the next iteration.
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}
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```
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The ` make ` and assignment look like they might work, but the value property of ` range ` (stored here as ` item `) is a _copy_ of the value from ` items `, not a pointer to the value in ` items `. The following will work:
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```go
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items := make([]map[int]int, 10)
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for i := range items {
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items[i] = make(map[int]int, 1)
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items[i][1] = 2
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}
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``` |