Scala: Append to Array within a Map

Question

I am learning immutable types in Scala and am struggling to get this elementary task done. I simply need to append to an array of doubles that is within a map. I do not want to use ArrayBuffer.

My ultimate goal is to make an adjacency matrix. When I append a new item to the map (an (Int, double) tuple) I want to increase the size of each array within the map--essentially increasing the dimension of the matrix.

var map = Map[Int, Array[Double]]()
map += (0 -> new Array[Double](5))

// HOW TO DO THIS
map(0) = map(0) :+ 0.01

for ((i, a) <- map) {
    print(i + ": ")
    for (d <- a) print(d + ", ")
}

What I have written above does not compile. However map(0) :+ 0.01 alone will but it does not achieve my goal of appending to an immutable array within a map.

can you please clarify the question a bit more.

Chaitanya
– Chaitanya

2018-07-13 13:00:10 +00:00
Commented Jul 13, 2018 at 13:00 — Chaitanya
– Chaitanya, Commented Jul 13, 2018 at 13:00

Duelist · Accepted Answer · 2018-07-13 13:13:00Z

2

Because it is immutable Map, you can't change the value in place, as you've tried to do using map(0) = map(0) :+ 0.01. The one of the possible solutions is using updated method, which returns updated map (all methods like add, remove, modify in immutable data structures return new data struture):

map = map.updated(0, map(0) :+ 0.01)

Some examples to prove:

var map = Map[Int, Array[Double]]()
map += (0 -> new Array[Double](5))
map = map.updated(0, map(0) :+ 0.01)
map(0) // res1: Array[Double] = Array(0.0, 0.0, 0.0, 0.0, 0.0, 0.01)
map = map.updated(0, map(0) :+ 0.02)
map(0) // res2: Array[Double] = Array(0.0, 0.0, 0.0, 0.0, 0.0, 0.01, 0.02)

edited Jul 13, 2018 at 13:13

answered Jul 13, 2018 at 13:03

Duelist

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2 Comments

Duelist Over a year ago

It can't. I'll give some examples soon

bcr Over a year ago

Okay sorry to give you the run around but I can confirm that your solution does work and does not drop entries in the map. Thanks for your help.

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