Copy array

An array is stored in a region of memory. When we modify it, we modify that region of memory. Two array variables can point to the same region.

Ruby syntax note

We can copy an array, with slice syntax, to create separate arrays. This can be helpful in recursive methods.

Example

First, this program slices an array with the square-bracket syntax. It specifies a slice on "values" from 0 to values.length. This is a total-array slice.

Info After the slice, "values" and "copy" point to separate memory regions. We test that our arrays are separate.

Detail We modify the copied array, and the original array is not changed. An array copy was made.

# Create an array.
values = ["cat", "dog", "mouse"]
print "VALUES: ", values, "\n"

# Copy the array using a total-array slice.
# ... Modify the new array.
copy = values[0 .. values.length]
copy[0] = "snail"
print "COPY:   ", copy, "\n"

# The original array was not modified.
print "VALUES: ", values, "\n"
VALUES: ["cat", "dog", "mouse"]
COPY:   ["snail", "dog", "mouse"]
VALUES: ["cat", "dog", "mouse"]

Example 2

This example is similar, but has one difference. It specifies the length of the slice with a final index of negative one. This means the last index.

Tip This syntax form is not better (or worse) than the previous one. It comes down to what syntax form you prefer.

values = [1, 10, 100]

# Copy the array with a different syntax.
copy = values[0 .. -1]
copy[0] = 1000

# Display the arrays.
print values, "\n"
print copy, "\n"[1, 10, 100]
[1000, 10, 100]

Slice

The slice method is an alias for the square-bracket slicing syntax. So calling slice with the same arguments yields an equivalent result.

values = [8, 9, 10]

# Use slice method to copy array.
copy = values.slice(0 .. -1)
copy[0] = 5

# Display two arrays.
print values, "\n"
print copy, "\n"[8, 9, 10]
[5, 9, 10]

Summary

An array copy is sometimes needed. If you do not copy an array, the original array will be modified. And once modified, you might not be able to retrieve the original values.