Ruby: How to count the number of times a string appears in another string?

Question

I'm trying to count the number of times a string appears in another string.

I know you can count the number of times a letter appears in a string:

string = "aabbccddbb"
string.count('a')
=> 2

But if I search for how many times 'aa' appears in this string, I also get two.

string.count('aa')
=> 2

I don't understand this. I put the value in quotation marks, so I'm searching for the number of times the exact string appears, not just the letters.

Please clarify (with an edit): does 'aa' appear once or twice in the string 'aaa'. — Cary Swoveland
It should probably be twice in that case. Positions 0 and 1 && Positions 1 and 2 — Johnson
Certainly, you are an excellent poster. I rewarded you Cary Swoveland. — Johnson

Cary Swoveland Cary Swoveland · Accepted Answer · 2014-09-19T16:51:43

Here are a couple of ways to count the numbers of times a given substring appears in a string (the first being my preference). Note (as confirmed by the OP) the substring 'aa' appears twice in the string 'aaa', and therefore five times in:

string="aaabbccaaaaddbb"

#1

Use String#scan with a regex that contains a positive lookahead that looks for the substring:

def count_em(string, substring)
  string.scan(/(?=#{substring})/).count
end

count_em(string,"aa")
 #=> 5

Note:

"aaabbccaaaaddbb".scan(/(?=aa)/)
  #=> ["", "", "", "", ""]

A positive lookbehind produces the same result:

"aaabbccaaaaddbb".scan(/(?<=aa)/)
  #=> ["", "", "", "", ""]

As well, String#scan can be replaced with String#gsub.

#2

Convert to an array, apply Enumerable#each_cons, then join and count:

def count_em(string, substring)
  string.each_char.each_cons(substring.size).map(&:join).count(substring)
end

count_em(string,"aa")
  #=> 5

We have:

enum0 = "aaabbccaaaaddbb".each_char
  #=> #<Enumerator: "aaabbccaaaaddbb":each_char>

We can see the elements that will generated by this enumerator by converting it to an array:

enum0.to_a
  #=> ["a", "a", "a", "b", "b", "c", "c", "a", "a", "a",
  #    "a", "d", "d", "b", "b"]

enum1 = enum0.each_cons("aa".size)
  #=> #<Enumerator: #<Enumerator: "aaabbccaaaaddbb":each_char>:each_cons(2)>

Convert enum1 to an array to see what values the enumerator will pass on to map:

enum1.to_a
  #=> [["a", "a"], ["a", "a"], ["a", "b"], ["b", "b"], ["b", "c"],
  #    ["c", "c"], ["c", "a"], ["a", "a"], ["a", "a"], ["a", "a"], 
  #    ["a", "d"], ["d", "d"], ["d", "b"], ["b", "b"]]

c = enum1.map(&:join)
  #=> ["aa", "aa", "ab", "bb", "bc", "cc", "ca",
  #    "aa", "aa", "aa", "ad", "dd", "db", "bb"]
c.count("aa")
  #=> 5

Ruby: How to count the number of times a string appears in another string?

3 Answers