Range.md

# Range Clauses

Spec: http://golang.org/doc/go_spec.html#For_statements

## Summary

A range clause provides a way to iterate over an array, slice, string, map, or channel.

## Example

```go
for k, v := range myMap {
	log.Printf("key=%v, value=%v", k, v)
}

for v := range myChannel {
	log.Printf("value=%v", v)
}

for i, v := range myArray {
	log.Printf("array value at [%d]=%v", i, v)
}
```

## Reference

If only one value is used on the left of a range expression, it is the 1st value in this table.

| Range expression | 1st value | 2nd value (optional) | notes |
|:-----------------|:----------|:---------------------|:------|
| array or slice  a  ` [n]E `, ` *[n]E `, or ` []E `  | index    ` i  int ` |  ` a[i] `       E    |
| string          s  string type          | index    ` i  int ` |   rune  ` int `      | range iterates over Unicode code points, not bytes |
| map             m  ` map[K]V `              | key      ` k  K ` | value  ` m[k] `       V |
| channel         c  chan E               | element  ` e  E ` | _none_               |

## Gotchas

When iterating over a slice or map of values, one might try this:

```go
items := make([]map[int]int, 10)
for _, item := range items {
	item = make(map[int]int, 1) // Oops! item is only a copy of the slice element.
	item[1] = 2                 // This 'item' will be lost on the next iteration.
}
```

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:

```go
items := make([]map[int]int, 10)
for i := range items {
	items[i] = make(map[int]int, 1)
	items[i][1] = 2
}
```