Shulou(Shulou.com)06/03 Report--

Big data, a good programmer, continues to share the collective operation functions of the Scala series.

4.6 important functions of sets

4.6.1sum/max/min/count

Finding the maximum or minimum value in a sequence is a very common requirement, as follows:

Val numbers = Seq (11,2,5,1,6,3,9)

Numbers.max / / 11

Numbers.min / / 1

A more advanced example that contains a sequence of books

Case class Book (title: String, pages: Int)

Val books = Seq (Book ("Future of Scala developers", 85)

Book (Parallel algorithms, 240)

Book (Object Oriented Programming, 130)

Book (Mobile Development, 495))

/ / Book (Mobile Development,495)

Books.maxBy (book = > book.pages)

/ / Book (Future of Scala developers,85)

Books.minBy (book = > book.pages)

As shown above, the minBy & maxBy method solves the problem of complex data. You only need to choose the attribute that determines the maximum or minimum of the data.

4.6.2 Filterin

Filter a numeric List to get only odd elements.

Val numbers = Seq (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) numbers.filter (n = > n% 2 = 0)

Val books = Seq (Book ("Future of Scala developers", 85)

Book (Parallel algorithms, 240)

Book (Object Oriented Programming, 130)

Book (Mobile Development, 495))

Books.filter (book = > book.pages > = 120)

4.6.3 Flatten

Val abcd = Seq ('averse,' baked, 'clocked,' d')

Val efgj = Seq ('estranged,' faded, 'grubbed,' h')

Val ijkl = Seq ('iTunes,' jacks, 'kicks,' l')

Val mnop = Seq ('masked,' nicked, 'oiled,' p')

Val qrst = Seq ('Q,'r,'s,'t')

Val uvwx = Seq ('upright,' vicious, 'wicked,' x')

Val yz = Seq ('yearly,' z')

Val alphabet = Seq (abcd, efgj, ijkl, mnop, qrst, uvwx, yz)

/ /

/ List (a, b, c, d, e, f, g, h, I, j, k, l, m, n, o, p, Q, r, s, t

/ / u, v, w, x, y, z)

Alphabet.flatten

Flatten is used when you have a collection of collections and you want to manipulate all the elements of those collections.

4.6.4 Operations between collections

Difference, intersection and union

Val num1 = Seq (1,2,3,4,5,6)

Val num2 = Seq (4,5,6,7,8,9)

/ / List (1,2,3)

Num1.diff (num2)

/ / List (4,5,6)

Num1.intersect (num2)

List (1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9)

Num1.union (num2)

The union in the above example retains the duplicate elements. What if we don't need to repeat it? You can use the distinct function at this point

/ List (1, 2, 3, 4, 5, 6, 7, 8, 9)

Num1.union (num2). Distinct

The following is an illustration of the above functions:

4.6.5 map (Mapping) list elements

Map is one of the most commonly used functions in the Scala collection. It is very powerful:

Val numbers = Seq (1, 2, 3, 4, 5, 6)

/ List (2, 4, 6, 8, 10, 12)

Numbers.map (n = > n * 2)

Val chars = Seq ('averse,' baked, 'clocked,' d')

/ / List (A, B, C, D)

Chars.map (ch = > ch.toUpper)

The logic of the map function is to traverse the elements in the collection and call the function on each element.

4.6.6 flatMap

FlatMap consists of the following two functions:

Map & flatten

Example:

Val abcd = Seq ('averse,' baked, 'clocked,' d')

/ / List (A, a, B, b, C, c, D, d)

Abcd.flatMap (ch = > List (ch.toUpper, ch))

4.6.7 conditional check on the entire collection

Val numbers = Seq (3,7,2,9,6,5,1,4,2) / ture numbers.forall (n = > n

N > 5)

The forall function is created to deal with such requirements.

4.6.8 grouping collections

For example, if you split a set into even and odd sets, the partition function can help us do this:

Val numbers = Seq (3,7,2,9,6,5,1,4,2)

/ (List (2,6,4,2), List (3,7,9,5,1))

Numbers.partition (n = > n% 2 = = 0)

4.6.9 Fold

Another popular operation is fold, and you can usually consider foldLeft and foldRight. They do the same job in different ways:

Val numbers = Seq (1,2,3,4,5)

/ / 15 numbers.foldLeft (0) ((res, n) = > res + n)

Val words = Seq ("apple", "dog", "table")

/ / 13 words.foldLeft (0) ((resultLength, word) = > resultLength + word.length)

FoldLeft, reduceRight, and foldRight

The method foldLeft works much like reduceLeft, but it lets you specify a value as the first element.

Scala > val a = Array (1,2,3)

A: Array [Int] = Array (1,2,3)

Scala > a.reduceLeft (+)

Res6: Int = 6

Scala > a.foldLeft (100)

Res7: Int = 106

Scala > a.foldLeft (200)

Res8: Int = 206,

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