# Iteration


The primary means of iteration in q are 

-   [implicit](implicit-iteration.md) in its operators and keywords
-   the **map iterator** [Each](../ref/maps.md#each) and its variants distribute evaluation through data structures
-   the **accumulating iterators** Scan and Over control successive iterations, where the result of one evaluation becomes an argument to the next
-   the control words `do` and `while`


## Implicit iteration

Most operators and keywords have iteration [built into them](implicit-iteration.md).

> **Warning — A common beginner error is to specify iteration where it is already implicit**
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## Iterators

The [iterators](../ref/iterators.md) are unary operators. 
They take values as arguments and derive functions that apply them repeatedly.

> **Detail — Value**
>
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> An [applicable value](glossary.md#applicable-value) is a q object that can be indexed or applied to one or more arguments:
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> -   function: operator, keyword, lambda, or derived function
> -   list: vector, mixed list, matrix, or table
> -   dictionary
> -   file- or process handle
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The iterators can be applied postfix, and almost always are. 
For example, the Over iterator `/` applied to the Add operator `+` derives the function `+/`, which reduces a list by summing it.
```q
q)(+/)2 3 4 5
14
```

There are two groups of iterators: maps and accumulators.


### Maps

The [maps](../ref/maps.md) – Each, Each Left, Each Right, Each Prior, and Each Parallel – apply a map to each item of a list or dictionary.
```q
q)count "zero"                             / count the chars (items) in a string
4
q)(count')("The";"quick";"brown";"fox")    / count each string
3 5 5 3
```


### Accumulators

The [accumulators](../ref/accumulators.md) – Scan and Over – apply a value successively, first to the argument, then to the results of successive applications. 


## Control words

The control words [`do`, and `while`](control.md) also enable iteration, but are rarely required. 

> **Tip — Do as little as possible**
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> First see if the iteration you want is already implicit in the operators and keywords.
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> If not, use the map and accumulator iterators to specify the iteration you need.
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> If you find yourself using the `do` or `while` control words, you probably missed something.
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> > “I’ll say no more than necessary. If that.”

> — ‘Chili’ Palmer in _Get Shorty_.
>
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[Implicit iteration](implicit-iteration.md)