Editing J (programming language) (section)

==Verbs and Modifiers==
A program or routine – something that takes data as input and produces data as output – is called a ''verb''.  J has a rich set of predefined verbs, all of which work on multiple data types automatically: for example, the verb {{mono|i.}} searches within arrays of any size to find matches:

<syntaxhighlight lang="j">
   3 1 4 1 5 9 i. 3 1  NB. find the index of the first occurrence of 3, and of 1
0 1
   3 1 4 1 5 9 i: 3 1  NB. find the index of the last occurrence of 3, and of 1
0 3
</syntaxhighlight>

User programs can be named and used wherever primitives are allowed.

The power of J comes largely from its ''modifiers'': symbols that take nouns '''and verbs''' as operands and apply the operands in a specified way.  For example, the modifier {{mono|/}} takes one operand, a verb to its left, and produces a verb that applies that verb between each item of its argument.  That is, {{mono|+/}} is a verb, defined as 'apply {{mono|+}} between the items of your argument'  Thus, the sentence

<syntaxhighlight lang="j">
   +/ 1 2 3 4 5
</syntaxhighlight>

produces the effect of

<syntaxhighlight lang="j">
   1 + 2 + 3 + 4 + 5

   +/ 1 2 3 4 5
15
</syntaxhighlight>

J has roughly two dozen of these modifiers.  All of them can apply to any verb, even a user-written verb, and users may write their own modifiers.  While modifiers are powerful individually, allowing
* repeated execution, i. e. ''do-while''
* conditional execution, i. e. ''if''
* execution of regular or irregular subsets of arguments
some of the modifiers control the order in which components are executed, allowing modifiers to be combined in any order to produce the unlimited variety of operations needed for practical programming.