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March 23, 2015

Groovy Goodness: Combine Elements Iterable with Index

Since Groovy 2.4.0 we can get the indices from the elements in a collection with the indices method. In addition to this method we can also use the withIndex to combine an Iterable with the indices directly. The output is a List of tuples where the first item is the value of the Iterable and the second the index value. We can pass an optional argument to the withIndex which is the starting point for the index values.
Another alternative is the indexed method. The indexed method returns a Map, where the key of the entry is the index value and the entry value is the Iterable value.

In the following example we use the withIndex method. The sample of the alphabet is the same as in the blog post about indices, but rewritten with the withIndex method:

def list = [3, 20, 10, 2, 1]
assert list.withIndex() == [[3, 0], [20, 1], [10, 2], [2, 3], [1, 4]]


def alphabet = 'a'..'z'

// Combine letters in alphabet
// with position and start at 1.
def alphabetIndices = alphabet.withIndex(1)

// alphabetIndices = [['a', 1], ['b', 2], ...]
assert alphabetIndices[0..2] == [['a', 1], ['b', 2], ['c', 3]]

// Find position of each letter
// from 'groovy' in alphabet.
def positionInAlphabet = 'groovy'.inject([]) { result, value ->
    result << alphabetIndices.find { it[0] == value }[1]
    result
}

assert positionInAlphabet == [7, 18, 15, 15, 22, 25]

In the next example we use the indexed method:

def list = [3, 20, 10, 2, 1]
assert list.indexed() == [0: 3, 1: 20, 2: 10, 3: 2, 4: 1]


def alphabet = 'a'..'z'

// Combine letters in alphabet
// with position and start at 1.
def alphabetIndices = alphabet.indexed(1)

// alphabetIndices = [1: 'a', 2: 'b', ...]
assert alphabetIndices.findAll { key, value -> key < 4} == [1: 'a', 2: 'b', 3: 'c']

// Find position of each letter
// from 'groovy' in alphabet.
def positionInAlphabet = 'groovy'.inject([]) { result, value ->
    result << alphabetIndices.find { it.value == value }.key
    result
}

assert positionInAlphabet == [7, 18, 15, 15, 22, 25]

Written with Groovy 2.4.1.