If I do the following:
table = Array.new(
3,
Array.new(
3,
nil
)
)
# =>
[
[nil, nil, nil],
[nil, nil, nil],
[nil, nil, nil]
]
Now I would like to modify the value at index 2 in the second array, so I would do:
table[1][2] = 2.343
I would now expect to see:
# =>
[
[nil, nil, nil],
[nil, nil, 2.343],
[nil, nil, nil]
]
However what I'm getting is this:
[
[nil, nil, 2.343],
[nil, nil, 2.343],
[nil, nil, 2.343]
]
What am I not getting here?
PS: Running ruby 2.3
For fix with behavior, try next:
empty_table = Array.new(3) { Array.new(3) }
From array manual:
Note that the second argument populates the array with references to the same object. Therefore, it is only recommended in cases when you need to instantiate arrays with natively immutable objects such as Symbols, numbers, true or false.
you are essentially saying create an array with three elements, and put this element (the new array) in each space. The element you are putting in the first array, is just created once. The only way I know to do what you want is using a for loop to push as many new arrays into the first array as you need. something like this:
table = Array.new(1, Array.new(3, 0))
0..1.each do |i|
table.push(Array.new(3, 0)) #add two more arrays to the first dimension
end
Related
I have an integer with a value
I want to create an array with nil values representing this integer
For example:
i = 5
# The result i want is
[nil, nil, nil, nil, nil]
What is the most Ruby way of doing this? Aka, the shortest way. Without need of looping et c.
This is very basic question:
i = 5
Array.new(i) #=> [nil, nil, nil, nil, nil]
i = 5
[nil]*i #=> [nil, nil, nil, nil, nil]
I have built a presenter class whose only job is to convert a given array into a string. I am test driving the solution, so I started with "[nil, nil, nil]" but each nil will eventually be replaced with letter. That functionality is being handled by another class.
I am trying now to build an interface whose only job is to convert that string back to an array. So I will need to convert e.g. "[\"a\", \"b\", nil]" back to ["a", "b", nil]. But I am stuck.
For example, I'd like to convert
"[nil, nil, nil]"
to
[nil, nil, nil]
How could I do it?
Just use:
eval(string)
eval("[nil, nil, nil]")
Caution:
This is a very insecure method and you have to use it only if you are completely sure that the string contains a SAFE array..
I am guessing that you are producing the string yourself on one side, for example:
arr = [nil, nil, nil]
str = arr.inspect
#=> "[nil, nil, nil]"
Whereas I would advise you to serialize the array using a format such as JSON, YAML or Ruby's built in Marshalling library.
JSON
require 'json'
arr = [nil, nil, nil]
str = JSON.dump(arr)
#=> "[null,null,null]"
JSON.load(str)
#=> [nil, nil, nil]
YAML
require 'yaml'
arr = [nil, nil, nil]
str = YAML.dump(arr)
#=> "---\n- \n- \n- \n"
YAML.load(str)
#=> [nil, nil, nil]
Marshal
arr = [nil, nil, nil]
str = Marshal.dump(arr)
#=> "\x04\b[\b000"
Marshal.load(str)
#=> [nil, nil, nil]
I have a two-dimensional array of a bunch of strings, including some empty strings. I want to replace the empty strings with nil. How do I do this in Ruby?
Sample array:
[['cat','','dog',''],['','','fish','','horse']]
Desired output:
[['cat',nil,'dog',nil],[nil,nil,'fish',nil,'horse']]
[['cat','','dog',''],['','','fish','','horse']].map do |arr|
arr.map { |s| s unless s.empty? }
end
# => [["cat", nil, "dog", nil], [nil, nil, "fish", nil, "horse"]]
I'm experiencing the following. I expect only the first sub-element of the first sub array to be assigned "x", not the first element of each sub array. Can anyone explain this behaviour, and perhaps how to work around it? (Note that this may well be expected behaviour, but if it is, it contradicts my expectations.)
x = Array.new(3, Array.new(5))
# => [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
x[0][0] # => nil
x[0][0] = "x"
x
# => [["x", nil, nil, nil, nil], ["x", nil, nil, nil, nil], ["x", nil, nil, nil, nil]]
workaround is :
x = Array.new(3) { Array.new(5) }
x[0][0] = 'a'
x # => [["a", nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
new(size) {|index| block }
Here an array of the given size is created. Each element in this array is created by passing the element’s index to the given block and storing the return value.
Read also Common gotchas
When sending the second parameter, the same object will be used as the value for all the array elements. Since all the Array elements store the same array Array.new(5), changes to one of them will affect them all.
If multiple copies are what you want, you should use the block version which uses the result of that block each time an element of the array needs to be initialized, as I did above.
2d_array = Array.new(rows) { Array.new(columns) }
Here's the code:
# a = Array.new(3, Array.new(3))
a = [[nil,nil,nil],[nil,nil,nil]]
a[0][0] = 1
a.each {|line| p line}
With the output:
[1, nil, nil]
[nil, nil, nil]
but using the commented line:
[1, nil, nil]
[1, nil, nil]
[1, nil, nil]
So why is that?
The commented line is assigning three of the same reference to the array, so a change to one array will propagate across the other references to it.
As for the 2 arrays vs 3, that's simply a matter of the first line specifying 3 as its first parameter and only specifying 2 array literals in the second line.
To create the nested arrays without having any shared references:
a = Array.new(3) {Array.new(3)}
When passed a block ({...} or do ... end), Array.new will call the block to obtain the value of each element of the array.