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Theoretical question

I'm trying to find new practical ways to convert integers into strings and the other way around. I only know the .to_s ; .to_i ; .to_f methods and would like to know if there are other ways to do to it without writing + to put together the variables. For example:

var1 = 16

puts 'I\'m ' + var1.to_s + ' years old.'

In longer codes is getting tiring writing all this to just convert a integer to a string.

By the way I also found this Timer program here on Stack and the #{ is an example of what I'm trying to do. Adding an integer to a string without + and .to_s But I don't know how it works.

30.downto(0) do |i|
  puts "00:00:#{'%02d' % i}"
  sleep 1
end

Thank you in advance for the suggestions!

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    The documentation for string literals is worth reading. It explains escape sequences, interpolation, the difference between '...' and "...", introduces %q(...) and %Q(...) and more. Commented Oct 2, 2020 at 8:21

2 Answers 2

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Ruby has a pretty powerful string interpolator feature using #{...} where that can contain fairly arbitrary Ruby code. The end result is always converted to a string using, effectively, to_s.

That is you can do this:

puts "00:00:#{'%02d' % i}"

Where that gets stringified and then interpolated.

This is roughly the same as:

i_str = '%02d' % i
puts "00:00:#{i_str}"

Where that is effectively:

i_str = '%02d' % i
puts "00:00:%s" % i_str

You could also combine that into a single operation:

puts "00:00:%02d" % i

Where you generally use interpolation or sprintf-style template strings, not both at the same time. It keeps your code cleaner since only one mechanism is in play.

The only reason .to_s is needed when doing concatenation is Ruby is very particular about "adding" together two things. x + y has a completely different outcome depending on what x and y are.

Consider:

# Integer + Integer (Integer#+)
1 + 2
# => 3

# Array + Array (Array#+)
[ 1 ] + [ 2 ]
# => [1,2]

# String + String (String#+)
"1" + "2"
# => "12"

Note that in each case it's actually a different method being called, and the general form of x + y is:

x.send(:+, y)

So it's actually a method call, and as such, each method may impose restrictions on what it can operate on by emitting exceptions if it can't or won't deal.

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5 Comments

Ohh, so the '%02d' has nothing to do with the string interpolation
To be technical, that's a form of interpolation, in effect, but not using the #{...} string interpolation mechanism. Ruby, like C, Python, Perl and others, supportsprintf notation for strings when using the String#% method. What it does is gives you more control over how that value is interpolated in terms of a template. The #{...} method requires you to do all the formatting yourself.
But the only thing I'm missing is what the d stands for?
If you follow that documentation link it links through to the full table of placeholders where d represents "argument as a decimal number".
Additionally we could coerce a String/Integer combo e.g. class String;def coerce(other); [other.to_s,self];end;end then 1 + "s" #=> "1s"(explicit coercion) alternatively class Integer; alias_method :to_str, :to_s;end then "s" + 1 #=> "s1" (implicit coercion)
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It's called string interpolation. For example:

puts "I\'m #{var1} years old."

The way it works is this:

  1. You have to enclose the string in double quotes, not single quotes.
  2. You put your variable inside this: #{}, e.g. "#{variable}".

This will always convert non-string variables into strings, and plug (i.e. interpolate) them into the surrounding string.

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3 Comments

So the double quotes are important, I didn't know that. Thank you!
You don't have to escape the single quote when using doubles quotes. (makes the string easier to read)
@Stefan Right, you can use single quotes inside double quotes, and also double quotes inside single quotes, without escaping them.

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