Rubyists: Just use double-quoted strings.

Lawson Kurtz, Former Senior Developer

Article Category: #Code

Posted on


Added an additional test and test results to specifically address questions around parsing performance. (Thanks for the suggestion @JEG2.)


If you've written Ruby, you've heard it before: Use single quoted strings unless you need string interpolation.

It makes sense, right? When I instantiate strings using double quotes, the Ruby intepreter has to do extra work to figure out if it needs to perform interpolation. Since extra work means reduced performance, it seems reasonable to avoid double-quoted instantiation unless it's a necessity.

Prove It

As a huge fan of single quotes, I set out to prove just how naughty overuse of double-quoted strings really was. I built a test harness to compare the performance of code blocks, and started running tests.

The Tests

1: Fizzbuzz

A basic fizzbuzz program seemed like a good first test. So using double then single-quoted strings, I ran 100 rounds of fizzbuzzing up to n = 100,000, and compared the user CPU time of the runs with a T-test.


Double-quoted string mean 0.0872s
Single-quoted string mean 0.0866s
T-test p-value 0.4647

Statistically insignificant difference.

2: Instantiating a Small String

Dumbfounded by the results of the fizzbuzz tests, I thought that testing string instantiation exclusively would yield results more consistent with my original hypothesis. So I built the simplest test possible: 100 separate runs of a program that instantiates a single character string one million times.


Double-quoted string mean 0.1196s
Single-quoted string mean 0.1202s
T-test p-value 0.9087

Statistically insignificant difference.

3: Instantiating a Big String

Well maybe interpolation matching causes a bigger performance hit with bigger strings?


Double-quoted string mean 0.1228s
Single-quoted string mean 0.1237s
T-test p-value 0.2240

Statistically insignificant difference.

4: Instantiating an ENORMOUS String

Okay, so what if the string was A LOT bigger? Like, "the entire script of The Sandlot"-big?


Double-quoted string mean 0.1301s
Single-quoted string mean 0.1297s
T-test p-value 0.4746

Statistically insignificant difference.

5: Explicitly Testing the Parsing Process

Okay, so maybe we don't only care about runtime performance. What if we explicitly test parsing performance as well?


Double-quoted string mean 0.8939s
Single-quoted string mean 0.8964s
T-test p-value 0.3401

Statistically insignificant difference.

6: Testing the Test Harness

Maybe the test harness has poor sensitivity? I'll create a test comparing the instantiation of a string and a slightly bigger string and reject the harness if there's no detectable difference.


String mean 0.1214s
Longer string mean 0.1257s
T-test p-value 0.0000

Statistically significant difference.

Just use double-quoted strings

So if there's no meaningful performance difference between the two methods of Ruby string instantiation, shouldn't I just pick my favorite? Well, no. The absence of difference makes one better.

Since there is no measurable performance advantage for either, any time (however marginal) spent thinking or talking about a choice between the two is wasted. When you prefer single quoted strings, you have to think when you need interpolation. When you use double quoted strings, you never have to think. (I'd also add, anecdotally, that apostrophes are more prevalent in strings than double quotes, which again means less thinking when using double-quoted strings.)

Therefore always using double-quoted strings results in the least possible wasted time and effort.


If you're a pragmatic Ruby programmer, just use double-quoted strings. And if you're a theoretical performance devotee, well, you're probably not writing Ruby anyway.

Check out the tests and test harness, and contribute your own string tests here:

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