Returning the other argument of 2 possible arguments without using conditions

Question

For example, if I have a function that is guaranteed to receive either 5 or 7 as an argument, I want the function to return 5 if received 7 and 7 if received 5 without using any conditions.

I was asked this in an interview and was pretty stumped, thanks.

While I think this is a good interview problem, as it gauges the interviewees ability to quick solve simple problems, I don't think this is a great question for this site - it is both trivial and useless. I'm voting to close as 'too localized'. — BlueRaja - Danny Pflughoeft, Feb 25 '13 at 09:37
I just realized it's only a rephrased "swap without temp" question http://stackoverflow.com/questions/804706/swap-two-variables-without-using-a-temp-variable?lq=1 — Martheen, Feb 25 '13 at 10:00
It should have been posted on http://codegolf.stackexchange.com/ — B Faley, Feb 25 '13 at 10:45
`Stumped` is the correct answer, as they want to employ someone who writes code that is plain and obvious-to-the-reader, rather than peppering it with im-smarter-than-you snares at every return. — John Mee, Feb 25 '13 at 13:05

score 96 · Accepted Answer · answered Feb 24 '13 at 20:39

96

Simple arithmetic:

return 7 - input + 5;

(which can be simplified as return 12 - input;)

Let's say the input is 7:

return 7 - 7 + 5 --> return 5

Or if the input is 5:

return 7 - 5 + 5 --> return 7

answered Feb 24 '13 at 20:39

Simon Forsberg

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+1, Even `return 12 - input` – Ofiris Feb 24 '13 at 20:40
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That's incredible! Could you share the thought process behind this? – davegri Feb 24 '13 at 20:41
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Nope, just math ;) – Femaref Feb 24 '13 at 20:41
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@PremGenError It's not amazing, it's mathematics! Which... of course can be quite amazing sometimes :) – Simon Forsberg Feb 24 '13 at 20:42
@SimonAndréForsberg true... :) – PermGenError Feb 24 '13 at 20:43
What if the input isn't numeric? I read the question as "two arguments of any type". What if they're strings or dates instead? – Ryan Stewart Feb 24 '13 at 20:58
@RyanStewart Considering it's guaranteed to be either 5 or 7, I just assumed it was integer. Otherwise, you could just convert it to integer -- which, isn't using any conditions and thus following the rules. – Simon Forsberg Feb 24 '13 at 21:01
@RyanStewart But of course, if you at compile time did not know which type of input is sent, then I think you'd be pretty much screwed if you want it to work for **any** type. And I can not think of any general solution for the parameters `Object a, Object b` without using conditions. – Simon Forsberg Feb 24 '13 at 21:02
@SimonAndréForsberg: Not at all (on the "screwed" bit). See [my answer](http://stackoverflow.com/a/15056754/839646). – Ryan Stewart Feb 24 '13 at 21:16
Great for those occasions when the "i" and "f" and '=' keys aren't functioning properly :) – Marc Feb 25 '13 at 10:46

score 73 · Answer 2 · edited Feb 24 '13 at 21:32

73

You can use any simple commutative calculation that can be reversed:

addition: f(x)=7+5-x
xor: f(x)=7^5^x
multiplication: f(x)=7*5/x

edited Feb 24 '13 at 21:32

Pshemo

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answered Feb 24 '13 at 20:43

aditsu quit because SE is EVIL

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How does the xor one work? – davegri Feb 24 '13 at 20:48
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The reverse of xor is also xor. In each example, x will cancel one of the terms, leaving the other one as the result. – aditsu quit because SE is EVIL Feb 24 '13 at 20:49
how would the xor be written in java code? simple return 7^5^x ? – davegri Feb 24 '13 at 20:50
Yes that's right. You can also replace 7^5 with its result (2). – aditsu quit because SE is EVIL Feb 24 '13 at 20:51
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Beware that you can generalize the multiplication approach if and only if _a_ and _b_ (instead of _5_ and _7_) both are nonzero. – Lumen Feb 24 '13 at 22:03
Reminds me of how to swap 2 numbers without a temporary! Use XOR. – QuentinUK Feb 25 '13 at 03:11
+1 For realising you don't need a line. You could also use .. a cubic polynomial, trigonometric functions, any function you can dream up which passes through 2 points (5,7) and (7,5) – wim Feb 25 '13 at 06:51

score 34 · Answer 3 · edited Feb 25 '13 at 08:03

34

public int f(int x) {
    return x ^ 2;
}

In binary:

7 = 111
5 = 101
2 = 010

XOR (^ in java) flips the 2 bit on if it's off and off if it's on.

edited Feb 25 '13 at 08:03

Grijesh Chauhan

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answered Feb 24 '13 at 20:41

Skip Head

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an explanation would be great! thanks – davegri Feb 24 '13 at 20:48
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This is basically the same as my 2nd example, because 2 = 7 ^ 5 – aditsu quit because SE is EVIL Feb 24 '13 at 20:50
@user2105338 `^` = XOR operator. Look up that and you might learn more. – Simon Forsberg Feb 24 '13 at 20:51
@aditsu, thanks. I was thinking I had to worry about the higher order bits for some reason. – Skip Head Feb 24 '13 at 20:52
+1 This is the best solution in my opinion, and was probably what the interviewer was looking for. – arshajii Feb 24 '13 at 21:30

score 15 · Answer 4 · answered Feb 25 '13 at 09:22

15

How about:

public int q(int in)
{
    static final int[] ret = {0, 0, 0, 0, 0, 7, 0, 5};
    return ret[in];
}

answered Feb 25 '13 at 09:22

キキジキ

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+1 for un-regarding space complexity .. :) – URL87 Feb 25 '13 at 09:39
this is a great answer! loving it – davegri Feb 25 '13 at 11:22
+1 Good answer and simple. – Grijesh Chauhan Feb 25 '13 at 11:27

Ryan Stewart · Answer 5 · 2013-02-25T01:26:30.130

10

If I had been the one interviewing and you solved it only for numeric input, my next question would have been, "How would you solve this problem for non-numeric input?" because I wouldn't be looking for mathematical cleverness. Instead, how about this?

List<String> options = new ArrayList<>(Arrays.asList("bob", "fred"));
options.remove("bob");
System.out.println(options.get(0));

That can obviously be easily adapted to any type, including Object, so long as the equality of the objects works out correctly, and as a bonus, it can be expressed much more concisely in other languages, such as Groovy:

println((["bob", "fred"] - "bob").first())

The output, in either case, is obviously "fred". If I were the one interviewing, this is the answer I'd be looking for.

edited Feb 25 '13 at 01:26

answered Feb 24 '13 at 21:15

Ryan Stewart

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I'm pretty sure remove() uses a condition, but I'm not sure if that would be considered breaking the rules or not – davegri Feb 24 '13 at 21:20
At least the Groovy version is breaking the rules, since at least in all programming languages I know of, `it != "bob"` is a condition. – Simon Forsberg Feb 24 '13 at 21:36
@Simon: True. I was thinking of "conditional" rather than "condition". But I didn't put what I meant to there, anyway. Fixed it. – Ryan Stewart Feb 24 '13 at 21:48
@user2105338: That's not what the question says. It says "without using conditions". There are no conditions in my code (except for the original Groovy snippet, which I corrected). – Ryan Stewart Feb 24 '13 at 21:49
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I like the answer, but my interpretation of "without using any conditions" does not allow usage of a method that uses conditions. – Simon Forsberg Feb 24 '13 at 22:05
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@user2105338: Basically, solving the problem this way shows that you can solve problems by manipulating common data structures, which is a useful thing to have in a developer. What does it show if you come up with a mathematical trick that only solves the problem for numbers? – Ryan Stewart Feb 24 '13 at 22:06
@Simon: To me, "without using any conditions" translates to "in a functional style", aka: "I want to see if your mind is stuck in a single paradigm." Isn't that a more useful thing to know about a job candidate than how his math skills are? – Ryan Stewart Feb 24 '13 at 22:09
using math is a way to avoid thinking in the "single paradigm" (that is: avoid the traditional if-else in this case). I believe there are many programmers (like the OP himself) that does not think about using math in a situation like this. Although they are probably even less that think of a solution like yours :) – Simon Forsberg Feb 24 '13 at 22:17
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@Simon: "Math" isn't a [Programming Paradigm](http://en.wikipedia.org/wiki/Programming_paradigm). I'm talking about functional vs. imperative. The phrasing sounds exactly like that kind of question. – Ryan Stewart Feb 24 '13 at 23:44
Ok, I see your point. I also noticed you got a downvote. Even though I don't think this was what the interviewer was looking for, I do think it is a helpful and informational answer and will give you an upvote for it. – Simon Forsberg Feb 25 '13 at 01:03
@Simon: Downvotes are part of life around here :) It doesn't bother me much, except that there was no accompanying comment. I'm not trying to be difficult. I learn from this place, so I try to contribute back alternate (and sometimes controversial!) points of view that will make people think :) Thanks! – Ryan Stewart Feb 25 '13 at 01:30
@RyanStewart I don't think the question was solely about mathematical cleverness. It's probably a question to see if the candidate knows about bitwise operation which is quite complex that beginners tend to avoid (even the experienced ones!) but if learned correctly can give you elegant solutions. It is also a good indicator of a candidate's analytical skills. Now, using objects as inputs will defeat that purpose. – Adrian M Feb 25 '13 at 10:19

score 8 · Answer 6 · answered Feb 25 '13 at 10:08

8

public int xyz(int x) {
    return 35 / x;
}

answered Feb 25 '13 at 10:08

SREEJITH

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While the accepted answer is good, I like this one more, because I would not think about it. I immediately thought about `7 MOD input + 5` :) – Michal B. Feb 25 '13 at 10:40
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public int xyz(int x) { return (12- x); } – SREEJITH Feb 25 '13 at 11:39

score 7 · Answer 7 · edited Jun 20 '20 at 09:12

How does the xor one work? [for case f(x) = 7^5^x ]

XOR (^) is Exclusive OR and works this way

a|b|a^b
-------
0|0| 0
0|1| 1
1|0| 1
1|1| 0

So XOR (^) can be used to change bits of some number. For example when we want to change last two bits of any number (like xxxx10 to xxxx01) we can do it with numbrer ^ 3 since 3 is binary 00011.

Here are few facts about XOR

XOR is symmetric -> a^b = b^a
XOR is associative -> (a^b)^c = a^(b^c)
a^a = 0 (ones in a will be replaced with zeros and zeros will be not changed)

example for a = 157 (binary 010011101)
```
  010011101
^ 010011101
-----------
  000000000
```
0^a = a (ones in a can only change zeros so they will change them to ones)
```
  000000000
^ 010011101
-----------
  010011101
```

so using facts (1) and (2) 7^5^x == x^7^5 == x^5^7

Lets try to check how x^7^5 will work for x=7.

(x^7)^5 = (7^7)^5 = 0^5 = 5

And same happens for x=5

(x^5)^7 = (5^5)^7 = 0^7 = 7

Returning the other argument of 2 possible arguments without using conditions

7 Answers7