How to convert a statement in a string into a format that can be evaluated (Ruby)

Question

I am writing a quiz program where I need to store boolean statements as strings in an array and output them to the terminal as part of a question. I then want to evaluate the contents of these strings and return the value so that I can test whether I answered the question correctly. Here is what I'm trying to do:

questions = ["!true", "!false", "true || true", "true && false"...]
puts "Answer true or false"
puts questions[0]
answer = gets.chomp
# evaluate value of questions[0] and compare to answer
...

Storing just the statements doesn't work the way I need it to:

questions = [!true, !false, true || true, true && false...]
puts questions[3].to_s

It returns the evaluated statement, ie "false", not "true && false". Any ideas of how to approach this?

I don't think there is a built in way, but check out stackoverflow.com/questions/8119970/… — jmccure
– jmccure, Commented Feb 20, 2015 at 7:49
I think without eval it hard to implement. You can write some parsing algorithm. — Farkhat Mikhalko
– Farkhat Mikhalko, Commented Feb 20, 2015 at 7:51
this can help to you stackoverflow.com/questions/12167613/… — Farkhat Mikhalko
– Farkhat Mikhalko, Commented Feb 20, 2015 at 7:52

shivam · Accepted Answer · 2015-02-20 07:53:30Z

2

You are looking for eval. Here:

a = "true && false"
eval a
# => false

a = "true && true"
eval a
# => true

eval will let you "convert a boolean statement stored in a string into a format that can be evaluated". You will need to change your logic accordingly to use it.

edited Feb 20, 2015 at 7:53

answered Feb 20, 2015 at 7:47

shivam

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2 Comments

Marisa Over a year ago

Embarrassingly simple! Thanks, this was exactly what I needed.

Ajedi32 Over a year ago

Standard disclaimer: This can do way more than just evaluate booleans. It can evaluate any Ruby statement (including exec 'rm -rf /'). Exercise caution accordingly. (E.g. User input should definitely not passed into this function.)

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