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Given an array, how can I find all indices of elements those match a given condition?
For example, if I have:
arr = ['x', 'o', 'x', '.', '.', 'o', 'x'] To find all indices where the item is x, I could do:
arr.each_with_index.map { |a, i| a == 'x' ? i : nil }.compact # => [0, 2, 6] or
(0..arr.size-1).select { |i| arr[i] == 'x' } # => [0, 2, 6] Is there a nicer way to achieve this?
588
Use a list comprehension. Use enumerate() to create a list of (index, value) tuples from the input list a_list . Use a list comprehension on this list of tuples to select every index that corresponds to a value that meets the given condition, and return the selections as a list.
To find the index of an element in a list, you use the index() function. It returns 3 as expected. However, if you attempt to find an element that doesn't exist in the list using the index() function, you'll get an error. To fix this issue, you need to use the in operator.
The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.
To find the index of specified element in given Array in C programming, iterate over the elements of array, and during each iteration check if this element is equal to the element we are searching for.
Ruby 1.9:
arr = ['x', 'o', 'x', '.', '.', 'o', 'x'] p arr.each_index.select{|i| arr[i] == 'x'} # =>[0, 2, 6] Code
82
Another way:
arr.size.times.select {|i| arr[i] == 'x'} # => [0, 2, 6] EDIT:
Not sure if this is even needed, but here they are.
Benchmarks:
arr = 10000000.times.map{rand(1000)}; Benchmark.measure{arr.each_with_index.map { |a, i| a == 50 ? i : nil }.compact} 2.090000 0.120000 2.210000 ( 2.205431) Benchmark.measure{(0..arr.size-1).select { |i| arr[i] == 50 }} 1.600000 0.000000 1.600000 ( 1.604543) Benchmark.measure{arr.map.with_index {|a, i| a == 50 ? i : nil}.compact} 1.810000 0.020000 1.830000 ( 1.829151) Benchmark.measure{arr.each_index.select{|i| arr[i] == 50}} 1.590000 0.000000 1.590000 ( 1.584074) Benchmark.measure{arr.size.times.select {|i| arr[i] == 50}} 1.570000 0.000000 1.570000 ( 1.574474) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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