Check if file contains string

Question

So I found this question on here, but I'm having an issue with the output and how to handle it with an if statement. This is what I have, but it's always saying that it's true even if the word monitor does not exist in the file

if File.readlines("testfile.txt").grep(/monitor/)   do something end 

Should it be something like == "nil"? I'm quite new to ruby and not sure of what the outputs would be.

Answer 1

I would use:

if File.readlines("testfile.txt").grep(/monitor/).any? 

or

if File.readlines("testfile.txt").any?{ |l| l['monitor'] } 

---

Using readlines has scalability issues though as it reads the entire file into an array. Instead, using foreach will accomplish the same thing without the scalability problem:

if File.foreach("testfile.txt").grep(/monitor/).any? 

or

if File.foreach("testfile.txt").any?{ |l| l['monitor'] } 

See "Why is "slurping" a file not a good practice?" for more information about the scalability issues.

Answer 2

Enumerable#grep does not return a boolean; it returns an array (how would you have access to the matches without passing a block otherwise?).

If no matches are found it returns an empty array, and [] evaluates to true. You'll need to check the size of the array in the if statement, i.e.:

if File.readlines("testfile.txt").grep(/monitor/).size > 0   # do something end 

The documentation should be your first resource for questions like this.