Array copies and nested state

The idea

An Array stores references to its elements. Assigning it to another variable shares the array, while dup creates a new outer array containing the same element references. Replacing an outer position in the duplicate is independent, but mutating a shared nested array or string can affect both views. Choose a copying strategy from the structure you actually need to isolate.

Filtering into a new result often communicates intent more clearly than deleting while traversing indices. Removing an element shifts later positions, which can make a forward loop skip adjacent matches. If mutation is required, use an appropriate collection operation or a carefully designed reverse traversal. A stable filter keeps the relative order and repeated occurrences of retained values; converting through a set would change that contract.

a=[[1],[2]]
b=a.dup
b[0] << 9
b[1]=[8]
p a
p b

[[1, 9], [2]]
[[1, 9], [8]]

The first nested array remains shared, but replacing b’s second position changes only the copied outer array.