Create a function that takes two inputs: idx (an array of integers) and str (a string). The function should return another string with the letters of str at each index in idx in order.

Examples

index_filter([2, 3, 8, 11], "Autumn in New York") ➞ "tune"

index_filter([0, 1, 5, 7, 4, 2], "Cry me a river") ➞ "creamy"

index_filter([9, -9, 2, 27, 36, 6, 5, 13, -1, 2, 0, 30, 2], 
  "That's life, I've got you under my skin") ➞ "frank sinatra"

Notes

• Indexes may not be in order or may be negative (see example #2 and #3).
• The output string must always be lowercase, but the input str may not be (see examples).