Complicated if else logic in Java

Question

I have three if statements:

if (!(a == 0 && b == 0)) {...}

if (!(a == 0 && b != 0)) {...}

if (!(a != 0 && b != 0)) {...}

I would like to combine them in one code block such as a method.

I don't want the other statements to run if one has run. There are workarounds if I want to avoid coming up with some good logic but I'd like to know if there is a beautiful way to write that.

Well then, use `||` to join the evaluation of all your `if`s. Apart of that, I won't say this is complicated `if-else` logic. — Luiggi Mendoza, Dec 31 '14 at 04:17
Well somebody doing electronics said there is a way to make this more compact. I thought such a way existed. — Sandah Aung, Dec 31 '14 at 04:21
There should be and you could do it after some boolean analysis. But more important: is this really necessary? Does this improves the code readability and performance or any other relevant factor for your app? If yes, then this is a great opportunity to improve this, probably with a code refactor. But if it is not, then just leave it as is. — Luiggi Mendoza, Dec 31 '14 at 04:22
"I don't want the other statements to run if one has run"--then you'll have to rethink your logic, because there's some overlap. For example, if `a` and `b` are both 0, the second and third statements will run in your example--but you say you only want one of them, so which one? — ajb, Dec 31 '14 at 04:26
Hang on - you want to omit the other cases if the first one runs? And to omit the third one if the second one runs? So, basically put an `else` before the second and third ones? In that case, the third one will never run! — Dawood ibn Kareem, Dec 31 '14 at 04:26
The first think I'd do is use DeMorgan's Law. Instead of `if (!(a == 0 && b == 0))`, write `if (a != 0 || b != 0)`. This will make it clearer what's going on, especially if you've written it wrong... this will be true unless **both** `a` and `b` are 0. — ajb, Dec 31 '14 at 04:29
If the logic executed in `{...}` is the same then the tests are completely redundant as the code will be executed if a == 0 or b == 0 or a != 0 or b != 0 — Steve C, Dec 31 '14 at 04:35
@ajb The problem is I came up with each of the if statements on their own and at one point I had to combine them in a way that I described in the question. — Sandah Aung, Dec 31 '14 at 04:37

jmj · Answer 1 · 2014-12-31T04:49:18.813

5

if (!(a == 0 && b b == 0)) {...}

truth table

a b r

z n T 
n n T 
z z F
n z T

for

if (!(a == 0 && b b != 0)) {...}

truth table

z n F
n n F 
z z T
n z T

for

if (!(a != 0 && b b != 0)) {...}

truth table

z n T
n n F
z z T
n z T

common case

n z T

so result that works on all 3 condition is

a != 0 && b == 0

note that: all 3 conditions are totally different, this will only work if you care to execute if

(first && second && third) is true

validate all case by yourself

_{(z = zero, n = non zero, a, b variables, r = result, T = true, F = false)}

edited Dec 31 '14 at 04:49

answered Dec 31 '14 at 04:23

jmj

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Nice explanation with truth tables :) – Thusitha Thilina Dayaratne Dec 31 '14 at 04:26
I think you've actually answered something quite different from what the OP intended to ask. Either you've misunderstood the question, or I have. In any case, I guess that's why it's been closed as unclear. – Dawood ibn Kareem Dec 31 '14 at 07:58
I think OP wants to make all three condition AND and derive a simpler condition – jmj Dec 31 '14 at 07:59

score 1 · Answer 2 · answered Dec 31 '14 at 04:29

1

Simplest way to express this:

public int foo(boolean a, boolean b) {
  int result;
  if (a) {
    if (b) { result = 1; }
    else { result = 2; }
  else {
    if (b) { result = 3; }
    else { result = 4; }
  return result;   
}

(notice that this evaluates a and b once each, which is the minimum you can expect here)

To keep @DavidWallace happy, here's a translation to the particular form of the original question:

public int foo(int a, int b) {
  int result;
  if (a==0) {
    if (b==0) { result = 1; }  // or whatever should happen, not specified in the question
    else { result = 2; }
  else {
    if (b==0) { result = 3; }
    else { result = 4; }
  return result;   
}

answered Dec 31 '14 at 04:29

Jon Kiparsky

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But in the question, `a` and `b` are both `int`, not `boolean`. – Dawood ibn Kareem Dec 31 '14 at 04:30
@DavidWallace Jon refers to `boolean a` and `boolean b` parameters. – Luiggi Mendoza Dec 31 '14 at 04:30
@LuiggiMendoza You what now? – Dawood ibn Kareem Dec 31 '14 at 04:31
@DavidWallace Yes, this is the general form of the solution - I imagine it's not too very difficult to translate it to any particular boolean expressions you might have in mind – Jon Kiparsky Dec 31 '14 at 04:31

score 1 · Answer 3 · answered Dec 31 '14 at 04:42

Just to re-word your question. You say that if one block runs, the others shouldn't. So when you wrote

if (!(a == 0 && b == 0)) {...}

if (!(a == 0 && b != 0)) {...}

if (!(a != 0 && b != 0)) {...}

You actually meant

if (!(a == 0 && b == 0)) { 
     // first ...
}
else if (!(a == 0 && b != 0)) {
     // second ...
}
else if (!(a != 0 && b != 0)) { 
     // third ...
}

But this is equivalent to

if (a != 0 || b != 0) { 
    // first ... 
}
else if (b == 0) { 
    // second ... 
}

Notice that the third ... can never run, as I stated in my comment.

Complicated if else logic in Java

3 Answers3