REBOL 3 Functions: foreach

Description

The foreach function repeats the evaluation of a block for each element of a series. It is used often in programs.

Example:

values: [11 22 33]
foreach value values [print value]
11
22
33

Another example that prints each word in a block along with its value:

colors: [red green blue]
foreach color colors [print [color get color]]
red 255.0.0
green 0.255.0
blue 0.0.255

If the series is a string, each character will be fetched:

string: "REBOL"
foreach char string [print char]
R
E
B
O
L

This example will print each filename from a directory block:

files: read %.
foreach file files [
    if find file ".t" [print file]
]
file.txt
file2.txt
newfile.txt
output.txt

Local Variables

The variables used to hold the foreach values are local to the block. Their value are only set within the block that is being repeated. Once the loop has exited, the variables return to their previously set values.

Multiple Elements

When a block contains groups of values that are related, foreach function can fetch all elements at the same time. For example, here is a block that contains a time, string, and price. By providing the foreach function with a block of words for the group, each of their values can be fetched and printed.

movies: [
     8:30 "Contact"      $4.95
    10:15 "Ghostbusters" $3.25
    12:45 "Matrix"       $4.25
]

foreach [time title price] movies [
    print ["watch" title "at" time "for" price]
]
watch Contact at 8:30 for $4.95
watch Ghostbusters at 10:15 for $3.25
watch Matrix at 12:45 for $4.25

In the above example, the foreach value block:

[time title price]

specifies that three values are to be fetched from movies for each evaluation of the block.

Series Reference

To reference the series itself during foreach you can use a set-word! within the variable block. This operation is similar to the forall and forskip functions.

Example:

foreach [v1: v2] [1 2 3] [?? [v1 v2]]
v1: [1 2 3] v2: 1
v1: [2 3] v2: 2
v1: [3] v2: 3

Notice that the v1 set-word does not affect the index position.

If you are using this option to remove values, please see the remove-each function which is many times faster for large series.

Foreach of Objects and Maps

The foreach function can also be used with object! and map! datatypes.

When using a single word argument, foreach will obtain the object field name or map key.

fruits: make object! [apple: 10 orange: 12 banana: 30]
foreach field fruits [print field]
apple
orange
banana

Note that each word is bound back to the object, and can be used to access the field value with get and set.

If a second word argument is provided, it will obtain the value of each entry:

foreach [field value] fruits [print [field value]]
apple 10
orange 12
banana 30

The same behavior applies to the map! datatype, except that empty keys (those set to none) will be skipped.

When a set-word! is used in the variables block, it will obtain the object value itself.