Vim & Vision

[ #124 ] Booleans and Boolean Expressions... Permalink

Perl, Python, Ruby Added less than a year ago

It's funny how the little, basic things that you re-examine when you are trying to teach someone else a programming language can surprise you. I thought I knew how these different languages handled this topic but I was wrong. Here we go:

I want to show you some differences between Perl, Python and Ruby (the three of which I consider to be the main "scripting" languages at the moment) with respect to their handling of booleans and boolean expressions. I was taken by surprise with some of this behaviour, as I tend to mainly use booleans as the results of conditional expressions - and fairly explicitly when I do.

Perl

No boolean type.

$ perl -e 'if (100 > 1) { print "true\n" } else { print "false\n" }'
true
$ perl -e 'if (100) { print "true\n" } else { print "false\n" }'
true
$ perl -e 'if (1) { print "true\n" } else { print "false\n" }'
true
$ perl -e 'if (0) { print "true\n" } else { print "false\n" }'
false
$ perl -e 'if (-1) { print "true\n" } else { print "false\n" }'
true
$ perl -e 'if ("") { print "true\n" } else { print "false\n" }'
false
$ perl -e 'if ("hello") { print "true\n" } else { print "false\n" }'
true

Python

Has a specific boolean type.

>>> x = False

$ python -c 'print ("true" if 100 > 1 else "false")'
true
$ python -c 'print ("true" if 100 else "false")'
true
$ python -c 'print ("true" if 1 else "false")'
true
$ python -c 'print ("true" if 0 else "false")'
false
$ python -c 'print ("true" if -1 else "false")'
true
$ python -c 'print ("true" if "" else "false")'
false
$ python -c 'print ("true" if "hello" else "false")'
true

... ok so it's the same as Perl at this point, but note these following boolean comparisons and compare to the upcoming Ruby code:

>>> 0 == False
True
>>> 1 == True
True
>>> 100 == True
False
>>> -1 == True
False

Ruby

$ ruby -e 'if (100 > 1) then puts "true"; else puts "false"; end'
true
$ ruby -e 'if (100) then puts "true"; else puts "false"; end'
true
$ ruby -e 'if (1) then puts "true"; else puts "false"; end'
true
$ ruby -e 'if (0) then puts "true"; else puts "false"; end'
true
$ ruby -e 'if (-1) then puts "true"; else puts "false"; end'
true
$ ruby -e 'a = ""; if (a) then puts "true"; else puts "false"; end'
true
$ ruby -e 'a = "hello"; if (a) then puts "true"; else puts "false"; end'
true

... ok ... that's fairly different. That's because Ruby only counts something as false if it is specifically equal to 'false' or 'nil'. Interesting. What about boolean comparisons that aren't wrapped in conditional expressions?

(Python uses 'True' and 'False', Ruby uses 'true' and 'false')

irb(main):001:0> 0 == false
=> false
irb(main):002:0> 1 == true
=> false
irb(main):003:0> 100 == true
=> false
irb(main):004:0> -1 == true
=> false

That is considerably different to Python, right?

What do we know?

Perl has no explicit boolean type, but Python and Ruby do
Python treats conditional expressions in a way that I would expect, every integer except 0 is True, and every string except "" is True
Python treats boolean comparisons to 0 and 1 a bit differently when they aren't in a conditional. E.g.: 0 == False and 1 == True, but any other number also == False
Ruby treats numbers completely differently, it handles expressions like 100 > 1 in a way I would expect, but a single/simple number in a boolean conditional is ALWAYS true because it isn't nil
Ruby treats boolean comparisons the same way as it treats conditionals, when comparing numbers to boolean values the result is always false

OK so this post wasn't exactly straightforward, but it shows how major languages can treat something as simple as booleans so differently.

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This is the blog of Brad Willis, a software engineer living in Brisbane.

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