Name Rascal/Libraries/Prelude/List/head

Synopsis Get the first element(s) from a list.

Function

1. &T head(list[&T] lst) throws EmptyList
2. list[&T] head(list[&T] lst, int n) throws IndexOutOfBounds

Usage import List;

Description

1. Returns the first element of a list or throws EmptyList when the list is empty. This is identical to top.
2. Returns the first n elements of a list or throws IndexOutOfBounds when the list is too short. This is similar to take.

Examples

rascal>import List;
ok

Get the first element:

rascal>head([1, 2, 3]);
int: 1
rascal>head(["zebra", "elephant", "snake", "owl"]);
str: "zebra"

An exception is thrown when taking the head of an empty list:

rascal>head([]);
|rascal://List|(3636,1037,<170,0>,<209,51>): EmptyList()
	at *** somewhere ***(|rascal://List|(3636,1037,<170,0>,<209,51>))
	at head(|stdin:///|(5,2,<1,5>,<1,7>))

Get the first n elements:

rascal>head([1, 2, 3, 4], 2);
list[int]: [1,2]
rascal>head(["zebra", "elephant", "snake", "owl"], 2);
list[str]: ["zebra","elephant"]

An exception is thrown when the second argument exceeds the length of the list:

rascal>head([1, 2, 3, 5], 5);
|rascal://List|(4713,113,<212,0>,<213,71>): IndexOutOfBounds(4)
	at *** somewhere ***(|rascal://List|(4713,113,<212,0>,<213,71>))
	at head(|stdin:///|(19,1,<1,19>,<1,20>))

Questions

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