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Comments on What is the meaning of "short circuit" operators?
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What is the meaning of "short circuit" operators?
When reading about various operators used by programming languages, the term "short circuit behavior" is often used. For example in this C code:
int a = 0;
a && b++
Someone explained that b++ is never executed because the logical AND operator "short circuits". What do they even mean with this?
Assuming I'm a layman at electronics (but not necessarily at programming), the association I get when hearing "short circuit" is something like connecting + directly to - on a battery, resulting in a spectacular failure such as cables burning up. And that doesn't seem like something I would want to happen to my program...
Why is it called "short circuit behavior"? What's the analogy and how is it helpful in understanding how certain operators work?
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It means the program can give up early if checking the rest of a boolean expression is pointless.
For example, naively to evaluate p and q you must check the value of both p and q, and then do the and operation.
However, if you are a bit more clever, you'll see that when p is False, the result cannot be True no matter what q is. It's doomed already, so there's no point checking q. So we say p has short-circuited q.
A very common use is when people write things like if foo() and bar():. Both operands are executing some non-trivial code. If bar() happens to be a very computationally expensive function, it's useful to avoid running it when not necessary.
Besides performance, short circuits can reduce nesting. Another common idiom is: if x is not Null and x.baz == "something":. Without short circuit you would need two nested ifs, because the second expression would error out if x is indeed null. But with short circuits, if it is null, the program won't even attempt to check the second part so you can get away with this statement.
In electrical circuits, a short circuit is when electricity is provided a new (often unintended) path that it can use to bypass the real load (such as a lightbulb). Since it takes the path of least resistance, if you give it a shortcut, it will go through the shortcut without bothering to power your device. This is analogous to how the program skips the remainder of your expression when provided an easier way of completing evaluation.
Also, there are electronic versions of boolean expressions, such as AND gates. These use current yes/no as True/False. When building electronic circuits that evaluate booleans, you can have a literal short circuit mechanism.
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