Index, rindex

Suppose we wish to search for a substring within a string. An iterator could be used, but the index() and rindex() methods are simpler.

Index() searches from the left (or start) of the string. And rindex() searches from the end of the string—the right part first.

First example

Consider this example Ruby code. The test string contains 3 "a" characters. So the starting location of our search matters.

test = "aaa"

# Part 1: use index() to search from start.
left = test.index("a")
puts left

# Part 2: use rindex() to search from end.
right = test.rindex("a")
puts right
0
2

No match

If no matching substring is found within the string, we get the special nil value. This can be tested in an if-statement.

input = "test"

# Part 1: test index() for nil result.
result1 = input.index("x")
puts "NOT FOUND 1" if result1 == nil

# Part 2: test rindex() for nil result.
result2 = input.rindex("x")
puts "NOT FOUND 2" if result2 == nil
NOT FOUND 1
NOT FOUND 2

Between, before, after

In developing more complex Ruby methods, like a between method, we must use index() and rindex to search a string. And we should be careful with a nil result.

Searching a string is commonly done in Ruby programs. And with index and rindex we can specify the direction, and starting position, of our search.