Module sequtils

Author: Alexander Mitchell-Robinson (Amrykid)

This module implements operations for the built-in seq type which were inspired by functional programming languages.

For functional style programming you may want to pass anonymous procs to procs like filter to reduce typing. Anonymous procs can use the special do notation which is more convenient in certain situations.

Procs

proc concat[T](seqs: varargs[seq[T]]): seq[T]

Takes several sequences' items and returns them inside a new sequence.

Example:

let
  s1 = @[1, 2, 3]
  s2 = @[4, 5]
  s3 = @[6, 7]
  total = concat(s1, s2, s3)
assert total == @[1, 2, 3, 4, 5, 6, 7]
  Source Edit
proc cycle[T](s: seq[T]; n: Natural): seq[T]

Returns a new sequence with the items of s repeated n times.

Example:

let
s = @[1, 2, 3] total = s.cycle(3)

assert total == @[1, 2, 3, 1, 2, 3, 1, 2, 3]

  Source Edit
proc repeat[T](x: T; n: Natural): seq[T]

Returns a new sequence with the item x repeated n times.

Example:

let
total = repeat(5, 3)

assert total == @[5, 5, 5]

  Source Edit
proc deduplicate[T](seq1: seq[T]): seq[T]
Returns a new sequence without duplicates.
let
  dup1 = @[1, 1, 3, 4, 2, 2, 8, 1, 4]
  dup2 = @["a", "a", "c", "d", "d"]
  unique1 = deduplicate(dup1)
  unique2 = deduplicate(dup2)
assert unique1 == @[1, 3, 4, 2, 8]
assert unique2 == @["a", "c", "d"]
  Source Edit
proc zip[S, T](seq1: seq[S]; seq2: seq[T]): seq[tuple[a: S, b: T]]

Returns a new sequence with a combination of the two input sequences.

For convenience you can access the returned tuples through the named fields a and b. If one sequence is shorter, the remaining items in the longer sequence are discarded. Example:

let
  short = @[1, 2, 3]
  long = @[6, 5, 4, 3, 2, 1]
  words = @["one", "two", "three"]
  zip1 = zip(short, long)
  zip2 = zip(short, words)
assert zip1 == @[(1, 6), (2, 5), (3, 4)]
assert zip2 == @[(1, "one"), (2, "two"), (3, "three")]
assert zip1[2].b == 4
assert zip2[2].b == "three"
  Source Edit
proc distribute[T](s: seq[T]; num: Positive; spread = true): seq[seq[T]]

Splits and distributes a sequence s into num sub sequences.

Returns a sequence of num sequences. For some input values this is the inverse of the concat proc. The proc will assert in debug builds if s is nil or num is less than one, and will likely crash on release builds. The input sequence s can be empty, which will produce num empty sequences.

If spread is false and the length of s is not a multiple of num, the proc will max out the first sub sequences with 1 + len(s) div num entries, leaving the remainder of elements to the last sequence.

On the other hand, if spread is true, the proc will distribute evenly the remainder of the division across all sequences, which makes the result more suited to multithreading where you are passing equal sized work units to a thread pool and want to maximize core usage.

Example:

let numbers = @[1, 2, 3, 4, 5, 6, 7]
assert numbers.distribute(3) == @[@[1, 2, 3], @[4, 5], @[6, 7]]
assert numbers.distribute(3, false)  == @[@[1, 2, 3], @[4, 5, 6], @[7]]
assert numbers.distribute(6)[0] == @[1, 2]
assert numbers.distribute(6)[5] == @[7]
  Source Edit
proc map[T, S](data: openArray[T]; op: proc (x: T): S {.
closure
.}): seq[S] {.
inline
.}

Returns a new sequence with the results of op applied to every item in data.

Since the input is not modified you can use this version of map to transform the type of the elements in the input sequence. Example:

let
  a = @[1, 2, 3, 4]
  b = map(a, proc(x: int): string = $x)
assert b == @["1", "2", "3", "4"]
  Source Edit
proc map[T](data: var openArray[T]; op: proc (x: var T) {.
closure
.}) {.
deprecated
.}

Applies op to every item in data modifying it directly.

Note that this version of map requires your input and output types to be the same, since they are modified in-place. Example:

var a = @["1", "2", "3", "4"]
echo repr(a)
# --> ["1", "2", "3", "4"]
map(a, proc(x: var string) = x &= "42")
echo repr(a)
# --> ["142", "242", "342", "442"]

Deprecated since version 0.12.0: Use the apply proc instead.

  Source Edit
proc apply[T](data: var seq[T]; op: proc (x: var T) {.
closure
.}) {.
inline
.}

Applies op to every item in data modifying it directly.

Note that this requires your input and output types to be the same, since they are modified in-place. The parameter function takes a var T type parameter. Example:

var a = @["1", "2", "3", "4"]
echo repr(a)
# --> ["1", "2", "3", "4"]
apply(a, proc(x: var string) = x &= "42")
echo repr(a)
# --> ["142", "242", "342", "442"]
  Source Edit
proc apply[T](data: var seq[T]; op: proc (x: T): T {.
closure
.}) {.
inline
.}

Applies op to every item in data modifying it directly.

Note that this requires your input and output types to be the same, since they are modified in-place. The parameter function takes and returns a T type variable. Example:

var a = @["1", "2", "3", "4"]
echo repr(a)
# --> ["1", "2", "3", "4"]
apply(a, proc(x: string): string = x & "42")
echo repr(a)
# --> ["142", "242", "342", "442"]
  Source Edit
proc filter[T](seq1: seq[T]; pred: proc (item: T): bool {.
closure
.}): seq[T] {.
inline
.}

Returns a new sequence with all the items that fulfilled the predicate.

Example:

let
  colors = @["red", "yellow", "black"]
  f1 = filter(colors, proc(x: string): bool = x.len < 6)
  f2 = filter(colors) do (x: string) -> bool : x.len > 5
assert f1 == @["red", "black"]
assert f2 == @["yellow"]
  Source Edit
proc keepIf[T](seq1: var seq[T]; pred: proc (item: T): bool {.
closure
.}) {.
inline
.}

Keeps the items in the passed sequence if they fulfilled the predicate. Same as the filter proc, but modifies the sequence directly.

Example:

var floats = @[13.0, 12.5, 5.8, 2.0, 6.1, 9.9, 10.1]
keepIf(floats, proc(x: float): bool = x > 10)
assert floats == @[13.0, 12.5, 10.1]
  Source Edit
proc delete[T](s: var seq[T]; first, last: Natural)

Deletes in s the items at position first .. last. This modifies s itself, it does not return a copy.

Example:

let outcome = @[1,1,1,1,1,1,1,1]
var dest = @[1,1,1,2,2,2,2,2,2,1,1,1,1,1]
dest.delete(3, 8)
assert outcome == dest
  Source Edit
proc insert[T](dest: var seq[T]; src: openArray[T]; pos = 0)

Inserts items from src into dest at position pos. This modifies dest itself, it does not return a copy.

Example:

var dest = @[1,1,1,1,1,1,1,1]
let
  src = @[2,2,2,2,2,2]
  outcome = @[1,1,1,2,2,2,2,2,2,1,1,1,1,1]
dest.insert(src, 3)
assert dest == outcome
  Source Edit
proc all[T](seq1: seq[T]; pred: proc (item: T): bool {.
closure
.}): bool

Iterates through a sequence and checks if every item fulfills the predicate.

Example:

let numbers = @[1, 4, 5, 8, 9, 7, 4]
assert all(numbers, proc (x: int): bool = return x < 10) == true
assert all(numbers, proc (x: int): bool = return x < 9) == false
  Source Edit
proc any[T](seq1: seq[T]; pred: proc (item: T): bool {.
closure
.}): bool

Iterates through a sequence and checks if some item fulfills the predicate.

Example:

let numbers = @[1, 4, 5, 8, 9, 7, 4]
assert any(numbers, proc (x: int): bool = return x > 8) == true
assert any(numbers, proc (x: int): bool = return x > 9) == false
  Source Edit

Iterators

iterator filter[T](seq1: seq[T]; pred: proc (item: T): bool {.
closure
.}): T

Iterates through a sequence and yields every item that fulfills the predicate.

Example:

let numbers = @[1, 4, 5, 8, 9, 7, 4]
for n in filter(numbers, proc (x: int): bool = x mod 2 == 0):
  echo($n)
# echoes 4, 8, 4 in separate lines
  Source Edit

Templates

template filterIt(seq1, pred: untyped): untyped

Returns a new sequence with all the items that fulfilled the predicate.

Unlike the proc version, the predicate needs to be an expression using the it variable for testing, like: filterIt("abcxyz", it == 'x'). Example:

let
  temperatures = @[-272.15, -2.0, 24.5, 44.31, 99.9, -113.44]
  acceptable = filterIt(temperatures, it < 50 and it > -10)
  notAcceptable = filterIt(temperatures, it > 50 or it < -10)
assert acceptable == @[-2.0, 24.5, 44.31]
assert notAcceptable == @[-272.15, 99.9, -113.44]
  Source Edit
template keepItIf(varSeq: seq; pred: untyped)

Convenience template around the keepIf proc to reduce typing.

Unlike the proc version, the predicate needs to be an expression using the it variable for testing, like: keepItIf("abcxyz", it == 'x'). Example:

var candidates = @["foo", "bar", "baz", "foobar"]
keepItIf(candidates, it.len == 3 and it[0] == 'b')
assert candidates == @["bar", "baz"]
  Source Edit
template allIt(seq1, pred: untyped): bool

Checks if every item fulfills the predicate.

Example:

let numbers = @[1, 4, 5, 8, 9, 7, 4]
assert allIt(numbers, it < 10) == true
assert allIt(numbers, it < 9) == false
  Source Edit
template anyIt(seq1, pred: untyped): bool

Checks if some item fulfills the predicate.

Example:

let numbers = @[1, 4, 5, 8, 9, 7, 4]
assert anyIt(numbers, it > 8) == true
assert anyIt(numbers, it > 9) == false
  Source Edit
template toSeq(iter: untyped): untyped {.
.}

Transforms any iterator into a sequence.

Example:

let
  numeric = @[1, 2, 3, 4, 5, 6, 7, 8, 9]
  odd_numbers = toSeq(filter(numeric) do (x: int) -> bool:
    if x mod 2 == 1:
      result = true)
assert odd_numbers == @[1, 3, 5, 7, 9]
  Source Edit
template foldl(sequence, operation: untyped): untyped

Template to fold a sequence from left to right, returning the accumulation.

The sequence is required to have at least a single element. Debug versions of your program will assert in this situation but release versions will happily go ahead. If the sequence has a single element it will be returned without applying operation.

The operation parameter should be an expression which uses the variables a and b for each step of the fold. Since this is a left fold, for non associative binary operations like subtraction think that the sequence of numbers 1, 2 and 3 will be parenthesized as (((1) - 2) - 3). Example:

let
  numbers = @[5, 9, 11]
  addition = foldl(numbers, a + b)
  subtraction = foldl(numbers, a - b)
  multiplication = foldl(numbers, a * b)
  words = @["nim", "is", "cool"]
  concatenation = foldl(words, a & b)
assert addition == 25, "Addition is (((5)+9)+11)"
assert subtraction == -15, "Subtraction is (((5)-9)-11)"
assert multiplication == 495, "Multiplication is (((5)*9)*11)"
assert concatenation == "nimiscool"
  Source Edit
template foldl(sequence, operation, first): untyped

Template to fold a sequence from left to right, returning the accumulation.

This version of foldl gets a starting parameter. This makes it possible to accumulate the sequence into a different type than the sequence elements.

The operation parameter should be an expression which uses the variables a and b for each step of the fold. The first parameter is the start value (the first a) and therefor defines the type of the result. Example:

let
  numbers = @[0, 8, 1, 5]
  digits = foldl(numbers, a & (chr(b + ord('0'))), "")
assert digits == "0815"
  Source Edit
template foldr(sequence, operation: untyped): untyped

Template to fold a sequence from right to left, returning the accumulation.

The sequence is required to have at least a single element. Debug versions of your program will assert in this situation but release versions will happily go ahead. If the sequence has a single element it will be returned without applying operation.

The operation parameter should be an expression which uses the variables a and b for each step of the fold. Since this is a right fold, for non associative binary operations like subtraction think that the sequence of numbers 1, 2 and 3 will be parenthesized as (1 - (2 - (3))). Example:

let
  numbers = @[5, 9, 11]
  addition = foldr(numbers, a + b)
  subtraction = foldr(numbers, a - b)
  multiplication = foldr(numbers, a * b)
  words = @["nim", "is", "cool"]
  concatenation = foldr(words, a & b)
assert addition == 25, "Addition is (5+(9+(11)))"
assert subtraction == 7, "Subtraction is (5-(9-(11)))"
assert multiplication == 495, "Multiplication is (5*(9*(11)))"
assert concatenation == "nimiscool"
  Source Edit
template mapIt(seq1, typ, op: untyped): untyped

Convenience template around the map proc to reduce typing.

The template injects the it variable which you can use directly in an expression. You also need to pass as typ the type of the expression, since the new returned sequence can have a different type than the original. Example:

let
  nums = @[1, 2, 3, 4]
  strings = nums.mapIt(string, $(4 * it))
assert strings == @["4", "8", "12", "16"]
Deprecated since version 0.12.0: Use the mapIt(seq1, op)
template instead.
  Source Edit
template mapIt(seq1, op: untyped): untyped

Convenience template around the map proc to reduce typing.

The template injects the it variable which you can use directly in an expression. Example:

let
  nums = @[1, 2, 3, 4]
  strings = nums.mapIt($(4 * it))
assert strings == @["4", "8", "12", "16"]
  Source Edit
template applyIt(varSeq, op: untyped)

Convenience template around the mutable apply proc to reduce typing.

The template injects the it variable which you can use directly in an expression. The expression has to return the same type as the sequence you are mutating. Example:

var nums = @[1, 2, 3, 4]
nums.applyIt(it * 3)
assert nums[0] + nums[3] == 15
  Source Edit
template newSeqWith(len: int; init: untyped): untyped
creates a new sequence, calling init to initialize each value. Example:
var seq2D = newSeqWith(20, newSeq[bool](10))
seq2D[0][0] = true
seq2D[1][0] = true
seq2D[0][1] = true

import random
var seqRand = newSeqWith(20, random(10))
echo seqRand
  Source Edit