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Module std.algorithm.iteration

This is a submodule of std.algorithm. It contains generic iteration algorithms.

Cheat Sheet
Function Name Description
cache Eagerly evaluates and caches another range's front.
cacheBidirectional As above, but also provides back and popBack.
chunkBy chunkBy!((a,b) => a[1] == b[1])([[1, 1], [1, 2], [2, 2], [2, 1]]) returns a range containing 3 subranges: the first with just [1, 1]; the second with the elements [1, 2] and [2, 2]; and the third with just [2, 1].
cumulativeFold cumulativeFold!((a, b) => a + b)([1, 2, 3, 4]) returns a lazily-evaluated range containing the successive reduced values 1, 3, 6, 10.
each each!writeln([1, 2, 3]) eagerly prints the numbers 1, 2 and 3 on their own lines.
filter filter!(a => a > 0)([1, -1, 2, 0, -3]) iterates over elements 1 and 2.
filterBidirectional Similar to filter, but also provides back and popBack at a small increase in cost.
fold fold!((a, b) => a + b)([1, 2, 3, 4]) returns 10.
group group([5, 2, 2, 3, 3]) returns a range containing the tuples tuple(5, 1), tuple(2, 2), and tuple(3, 2).
joiner joiner(["hello", "world!"], "; ") returns a range that iterates over the characters "hello; world!". No new string is created - the existing inputs are iterated.
map map!(a => a * 2)([1, 2, 3]) lazily returns a range with the numbers 2, 4, 6.
mean Colloquially known as the average, mean([1, 2, 3]) returns 2.
permutations Lazily computes all permutations using Heap's algorithm.
reduce reduce!((a, b) => a + b)([1, 2, 3, 4]) returns 10. This is the old implementation of fold.
splitter Lazily splits a range by a separator.
substitute [1, 2].substitute(1, 0.1) returns [0.1, 2].
sum Same as fold, but specialized for accurate summation.
uniq Iterates over the unique elements in a range, which is assumed sorted.

Functions

NameDescription
cache(range) cache eagerly evaluates front of range on each construction or call to popFront, to store the result in a cache. The result is then directly returned when front is called, rather than re-evaluated.
cacheBidirectional(range) cache eagerly evaluates front of range on each construction or call to popFront, to store the result in a cache. The result is then directly returned when front is called, rather than re-evaluated.
chunkBy(r) Chunks an input range into subranges of equivalent adjacent elements. In other languages this is often called partitionBy, groupBy or sliceWhen.
group(r) Groups consecutively equivalent elements into a single tuple of the element and the number of its repetitions.
joiner(r, sep) Lazily joins a range of ranges with a separator. The separator itself is a range. If a separator is not provided, then the ranges are joined directly without anything in between them (often called flatten in other languages).
mean(r) Finds the mean (colloquially known as the average) of a range.
permutations(r) Lazily computes all permutations of r using Heap's algorithm.
splitter(r, s) Lazily splits a range using an element or range as a separator. Separator ranges can be any narrow string type or sliceable range type.
splitter(s) Lazily splits the string s into words, using whitespace as the delimiter.
substitute(r, substs) Returns a range with all occurrences of substs in r. replaced with their substitution.
sum(r) Sums elements of r, which must be a finite input range. Although conceptually sum(r) is equivalent to fold!((a, b) => a + b)(r, 0), sum uses specialized algorithms to maximize accuracy, as follows.
uniq(r) Lazily iterates unique consecutive elements of the given range (functionality akin to the uniq system utility). Equivalence of elements is assessed by using the predicate pred, by default "a == b". The predicate is passed to binaryFun, and can either accept a string, or any callable that can be executed via pred(element, element). If the given range is bidirectional, uniq also yields a bidirectional range.

Structs

NameDescription
Group Groups consecutively equivalent elements into a single tuple of the element and the number of its repetitions.
Permutations Lazily computes all permutations of r using Heap's algorithm.

Templates

NameDescription
cumulativeFold Similar to fold, but returns a range containing the successive reduced values. The call cumulativeFold!(fun)(range, seed) first assigns seed to an internal variable result, also called the accumulator. The returned range contains the values result = fun(result, x) lazily evaluated for each element x in range. Finally, the last element has the same value as fold!(fun)(seed, range). The one-argument version cumulativeFold!(fun)(range) works similarly, but it returns the first element unchanged and uses it as seed for the next elements. This function is also known as partial_sum, accumulate, scan, Cumulative Sum.
each Eagerly iterates over r and calls fun over each element.
filter Implements the higher order filter function. The predicate is passed to unaryFun, and can either accept a string, or any callable that can be executed via pred(element).
filterBidirectional Similar to filter, except it defines a bidirectional range. There is a speed disadvantage - the constructor spends time finding the last element in the range that satisfies the filtering condition (in addition to finding the first one). The advantage is that the filtered range can be spanned from both directions. Also, std.range.retro can be applied against the filtered range.
fold Implements the homonym function (also known as accumulate, compress, inject, or foldl) present in various programming languages of functional flavor. The call fold!(fun)(range, seed) first assigns seed to an internal variable result, also called the accumulator. Then, for each element x in range, result = fun(result, x) gets evaluated. Finally, result is returned. The one-argument version fold!(fun)(range) works similarly, but it uses the first element of the range as the seed (the range must be non-empty).
map Implements the homonym function (also known as transform) present in many languages of functional flavor. The call map!(fun)(range) returns a range of which elements are obtained by applying fun(a) left to right for all elements a in range. The original ranges are not changed. Evaluation is done lazily.
reduce Implements the homonym function (also known as accumulate, compress, inject, or foldl) present in various programming languages of functional flavor. There is also fold which does the same thing but with the opposite parameter order. The call reduce!(fun)(seed, range) first assigns seed to an internal variable result, also called the accumulator. Then, for each element x in range, result = fun(result, x) gets evaluated. Finally, result is returned. The one-argument version reduce!(fun)(range) works similarly, but it uses the first element of the range as the seed (the range must be non-empty).
substitute Returns a range with all occurrences of substs in r. replaced with their substitution.

Authors

Andrei Alexandrescu

License

Boost License 1.0.