Do logical operators affect each other in a if statement?

Question

I have 4 statements, I only want to check statement 2-4 if statement 1 is true the following is the pseudo code of what I am trying to achieve

if (statement 1) {
  if (statement 2 or statement 3 or statement 4){
    do something()
  }
}

I was wondering whether the following code will do the same

if(s1 && s2 || s3 || s4) {
  doSomething();
}

or if I have to do it like

if (s1) {
  if (s2 || s3 || s4) {
     doSomething();
  }
}

Be careful with operator precedence. `&&` has higher precedence than `||`, so you want `if(s1 && (s2 || s3 || s4))`. — Blaze, Aug 21 '19 at 13:36
`s1 && s2 || s3 || s4`, due to rules of operator precedence, is equivalent to `(s1 && s2) || s3 || s4`, not to `s1 && (s2 || s3 || s4)`. Your second lot of code tests the latter. — Peter, Aug 21 '19 at 13:39

NathanOliver · Accepted Answer · 2019-08-21T13:41:30.853

8

Due to operator precedence

if(s1 && s2 || s3 || s4)

is the same as

if((s1 && s2) || s3 || s4)

since && has a higher precedence than ||. What you want is

if(s1 && (s2 || s3 || s4))

which will only be true if s1 is true and any of s2, s3, and s4 are true.

You are also guaranteed to have short circuiting on && as long as that is a built in operator and that means that if s1 is false, s2, s3, and s4 will not be evaluated. This also happens wth || for built in operators and as soon as you hit a true expression the rest won't be evaluated as true or anything is true.

edited Aug 21 '19 at 13:41

answered Aug 21 '19 at 13:37

NathanOliver

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@bolov I've added that. – NathanOliver Aug 21 '19 at 13:40
One more important thing. In `if (a && b)` if `a` is falve, `b` is guaranteed not to be evaluated (the same is true for `if (a || b)`, i.e. if `a` is true, `b` wont be evaluated). This is typically used in statements like `if (p_node && p_node->something)`. – Nestor Aug 21 '19 at 13:41
so for example ```if (mode = 0 (acc.balance < 0 || acc.balance < change)) return 0;```? – Ariel Hurdle Aug 21 '19 at 13:51
@ArielHurdle `if (mode == 0 && (acc.balance < 0 || acc.balance < change)) return 0;` – NathanOliver Aug 21 '19 at 13:53
@NathanOliver ah yes, typo on my behalf there :) and thanks for the great answer! – Ariel Hurdle Aug 21 '19 at 13:54
@ArielHurdle No problem. Hope it helps. – NathanOliver Aug 21 '19 at 13:54

score 1 · Answer 2 · answered Aug 21 '19 at 13:38

In C++ the logical operators && and || are short-circuiting - this means that the left operand of the operator is going to be evaluated first, and if the result won't be affected by doing further evaluations then it'll stop there. For example, x() && y() will indeed only evaluate y() if x() was true, and x() || y() will only evaluate y() if x() was false.

As a recommendation, you should take care to put parentheses around things to make it more clear which order of operations you intended though:

s1 && (s2 || s3 || s4)

rather than

s1 && s2 || s3 || s4

score 1 · Answer 3 · answered Aug 21 '19 at 13:42

You need to put parenthesis on your || statements. So:

if(s1 && (s2 || s3 || s4)) { // << note the parenthesis
  doSomething();
}

is equivalent with

if (s1) {
  if (s2 || s3 || s4) {
     doSomething();
  }
}

This works because of Short-circuit evaluation. Basically if s1 is false, the parenthesis won't be evaluated. You can read about it here.

Do logical operators affect each other in a if statement?

3 Answers3