How can I use conditional assignment in ruby

Question

I want to set the value of a equal to the index found unless that index is zero, in which case I want to set it to some number minus that value. I am wondering if it is possible to perform this action (taken from python) in Ruby:

a='/-123456789X'.find(y)or 99-x

Does anyone know of a good way to do this?

Sometimes the best solution is the most straightforward: idx = str.index(substr); idx = 99 if idx == 0. Note that 99 minus zero is 99 and str.index(substr) #=> nil if str does not contain the substring substr. — Cary Swoveland
– Cary Swoveland, Commented Jul 13, 2018 at 2:57

Ursus · Accepted Answer · 2018-07-12 21:24:07Z

2

Try this one. Given x and y

a = "/-123456789X".index(y) || 99 - x

answered Jul 12, 2018 at 21:24

Ursus

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1 Comment

Schwern Over a year ago

Here's the docs for String#index. And an important difference between Ruby and Python (and many other languages) is that 0 is not false. Only false and nil are false. In Python 0 or 99 is 99. In Ruby 0 || 99 is 0.

engineersmnky · Accepted Answer · 2018-07-21 03:16:06Z

In python string.find(other) will return the index of other or -1 if other does not exist in string.

In ruby string.index(other) will return the index of other or nil if other does not exist in string.

"Truthy" and "Falsey" values:

ruby acknowledges nil as "falsey" and 0 as "truthy"; but
python acknowledges 0 as "falsey" and -1 as "truthy"

So your current python code has 3 possible return values:

-1 (non-existent sub-string)
99 - x (existent sub-string starting with '/')
n (index of existent sub-string that does not start with '/')

In order to achieve an equivalent result in ruby your code could look like this:

str = '/-123456789X'
a = if y.start_with?('/') && str.index(y)
  99 - x
else
  str.index(y) || -1
end

Other alternatives include:

# Ruby >= 2.5 using `String#match? 
str.match?(/\A#{y}/) ? 99 - x : str.index(y) || -1

That being said your actual request "I want to set the value of a equal to the index found unless that index is zero, in which case I want to set it to some number minus that value" seems a little different and I am not sure if this means that x is "that value" and what x should represent in that case.

Should x be the begining index?
Should x be the ending index?

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