drop

This page is a reference guide for the drop family of operators. It describes:

• The behaviour of drop, dropWhile, dropThrough, dropLast and dropLastIf.

• How drop operators propagate chunks.

As the drop family of operators have normal error propagation and finalizer handling, these are not described.

Behaviour

drop

The drop operator pulls a given number of elements from an input stream and discards them, then pulls and outputs the remaining elements. It is analogous to drop on lists.

The following example drops a varying number of elements from a stream of characters.

Experiment with dropping different numbers of elements.

Notice that after discarding the given number of elements, drop behaves as an identity operation.

drop(1) is equivalent to tail.

dropWhile

The dropWhile operator discards elements that satisfy a given predicate. It is analogous to dropWhile on lists.

In the following example, dropWhile pulls a character from the input stream and discards it if it is not equal to b.

The first character a is discarded. The second character b tests false, so is outputted. The third character c is then pulled and outputted.

dropThrough

The dropThrough operator behaves similarly to dropWhile, however it also drops the element for which the predicate tests false.

In the following example, dropWhile pulls a character from the input stream and discards it if it is not equal to b.

The first character a is discarded. The second character b tests false, but is also discarded. The third character c is then pulled and outputted.

dropThrough(p) is equivalent to dropWhile(p).drop(1).

dropLast

The dropLast operator drops the last element of the input stream. It is analogous to init on lists.

In the following example, dropLast pulls from an input stream of characters. The last character c is discarded.

The last character is dropped.

When it is first pulled on, dropLast pulls two characters from the input stream. It outputs a after pulling the second character b. It must do so to verify that a is not the last character.

Note that dropLast has no effect if the input stream is infinite. It is only useful on finite streams.

dropLastIf

The dropLastIf operator behaves similarly to dropLast, but only discards the last element if it satisfies the given predicate.

In the following example, the dropLastIf operator discards the last element if it is not equal to b.

The input stream is constructed by taking a varying number of characters from a stream of a, b and c. Experiment with different numbers of characters to see how dropLastIf behaves.

If the number of characters taken is 1 or 3, the last element outputted by the input stream is a or c respectively. The characters satisfy the predicate of dropLastIf, so are not outputted.

If the number of characters taken is 2, the last element outputted by the input stream is b. The predicate is not satisfied and b is outputted.

Chunk propagation

drop

The drop operator preserves the chunks of its input stream. If the number of elements dropped falls between a chunk, the chunk is partitioned and the start of it is discarded.

In the following example, the input stream outputs two chunks of [a, b]. Experiment with varying the number of elements dropped.

Notice that if an odd number of elements is dropped, the chunk is partitioned into [a] and [b], and the last partition [b] is outputted.

The dropWhile and dropThrough operators propagate chunks similarly to drop.

dropLast

The dropLast operator also preserves the chunks of its input.

In the following example, the input stream outputs three chunks of [a, b].

Note that when dropLast is pulled on for the first time, it pulls two chunks from the input stream before outputting the first chunk.

The dropLastIf operator propagates chunks similarly to dropLast.