Why Lua’s 0 (zero) as a true value makes sense

When I started to learn about Lua, I found it a little odd that the numeric 0 (zero) value is treated as a true boolean value by the interpreter, which was different from all other programming languages I had worked with.

However, it turns out that it makes a lot of sense to have 0 as a true value, and that’s when you take short circuit expressions into account.

For those who don’t know, short-circuit expressions are used to conditionally select values without having to code that selection into an if-elseif-else structure. Short-circuit expressions are built using logical (and, or, not) operators. The and operator returns the value of the first false expression, or the last expression, in case all others were true. The or operator returns the value of the first true expression, or the last expression, in case all others were false. Here are a few examples in Lua:

Code:

word = 1 < 2 and "hello" or "world"
print(word)

Output:

hello

Code:

level = 10 > 9 and 2 or 4
print(level)

Output:

2

Now, consider the following Lua short-circuit expression:

bit = math.random() < 0.5 and 0 or 1

Running it many times will psuedo-randomly assign both 0 and 1 values to the bit variable:

Code:

for i=1,8 do
    bit = math.random() < 0.5 and 0 or 1
    print(bit)
end

Output:

0
1
1
1
0
0
1
0

Now, consider what happens when we try to do the same thing in Python 2.6:

Code:

import random
for i in range(8):
    bit = random.random() < 0.5 and 0 or 1
    print(bit)

Output:

1
1
1
1
1
1
1
1

The first operand of the and expression is always false, because 0 is considered to be a false boolean value by the Python interpreter.

Even though one could argue that it’s just a matter of knowing how to rewrite such logical expressions, I believe it’s good not having to worry about undesired behavior caused by 0’s in short-circuit expressions. It allows me to write expressions just as they come to my mind, which makes me more productive than if I had to re-arrange them.

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