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The for Loop

Iterating by Values

	for		value-pattern in	collection-expr	{
			body-expr
	}

E's for loop should be familiar at least to Python, Perl, and csh programmers. You can use the for loop to iterate over the values contained in anything E considers to be a collection. In each successive iteration, the value-pattern is matched against a successive value from the collection. The body-expr is evaluated once each time the match succeeds. Any variable names defined by the value-pattern are visible in the body-expr.

Typically, the value-pattern will simply be a single variable name; in this case the match always succeeds, and each iteration runs with this variable defined to hold each value from the collection in turn:

? pragma.syntax("0.8")

? def average(samples) :any {
>     var sum := 0
>     for value in samples {
>         sum += value
>     }
>     sum / samples.size()
> }
# value: <average>

? average([1, 3, 5, 100, 6])
# value: 23.0

In this example, the collection is a ConstList constructed by the square bracket expression in the call to average.

In E, a ConstList is an immutable list of values. E considers a variety of other objects to be collections as well, and all collections can be treated as containing key-value pairs. (In the case of a list, the keys are the indices of the elements, starting at zero and counting upward.) The unabbreviated form of the for loop has both a key-pattern and a value-pattern, as we shall see in the next section.