Why were Logical Operators created?

Question

Almost all programming languages are having the concept of logical operator I am having a query why logical operators were created. I googled and found its created for condition based operation, but that's a kind of usage i think.

I am interested in the answer that what are the challenges people faced without this operator. Please explain with example if possible.

This is kind of vague. You want to know why they were created, even though you found what their usage is? — Carcigenicate, Sep 28 '18 at 11:46
Assembly / cpu hardware *doesn't* have logical operators, so [tag:cpu-architecture] doesn't really apply. This is a high-level language design question. (I'm assuming you're talking about short-circuit evaluation operators like C's `a && b` which treats both sides as boolean, not bitwise-and `a & b`.) Can you explain what the CPU-architecture aspect of this question is, and why you added that tag? — Peter Cordes, Sep 28 '18 at 12:00
There are no challenges to face. They are for convenience only. Alan Turing showed they are not needed with his single instruction: `copy, minus, branch on negative`. — shawnhcorey, Oct 03 '18 at 12:45

score 1 · Accepted Answer · answered Sep 28 '18 at 12:15

I am interested in the answer that what are the challenges people faced without this operator.

Super-verbose deeply nested if() conditions, and especially loop conditions.

while (a && b) {
    a = something;
    b = something_else;
}

written without logical operators becomes:

while (a) {
    if (!b) break;   // or if(b){} else break;  if you want to avoid logical ! as well
    a = something;
    b = something_else;
}

Of if you don't want a loop, do you want to write this?

if (c >= 'a') {
    if (c <= 'z') {
        stuff;
    }
}

No, of course you don't because it's horrible compared to if (c >= 'a' && c <= 'z'), especially if there's an else, or this is inside another nesting. Especially if your coding-style rules require 8-space indentation for each level of nesting, or the { on its own line making each level of nesting eat up even more vertical space.

Note that a&b is not equivalent to a&&b: even apart from short-circuit evaluation. (Where b isn't even evaluated if a is false.) e.g. 2 & 1 is false, because their integer bit patterns don't have any of the same bits set.

Short-circuit evaluation allows loop conditions like while(p && p->data != 0) to check for a NULL pointer and then conditionally do something only on non-NULL.

Compact expressions were a big deal when computers were programmed over slow serial lines using paper teletypes.

Also note that these are purely high-level language-design considerations. CPU hardware doesn't have anything like logical operators; it usually takes multiple instructions to implement a ! on an integer (into a 0/1 integer, not when used as an if condition).

if (a && b) typically compiles to two test/branch instructions in a row.

Why were Logical Operators created?

1 Answers1