Why does this output occur for this if statement?

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Java

double j = 128.22, k = 129.22;
if ((j++) == k)
    j += 10;
k += 10;
System.out.println(j);
System.out.prinkln(k);

The output is 129.22
139.22

Why does it read the if statement as false? I'm teaching programming 1 and I've asked all my sources, and can't really get an answer.

Posted 16-Oct-14 5:55am

Member 11158840

Updated 16-Oct-14 6:06am

George Jonsson

v2

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[no name] 16-Oct-14 12:04pm

Because that is the way the postfix operator works. http://msdn.microsoft.com/en-us/library/aa691363(v=vs.71).aspx

Richard MacCutchan 16-Oct-14 12:29pm

How can you be teaching programming if you do not understand something as basic as this? I suggest you go to http://download.oracle.com/javase/tutorial/index.html and study the language in greater detail.

Member 11158840 16-Oct-14 20:20pm

You know, you might want to think before you post something so rude. I happen to be teaching the class because nobody else was brave enough to try to learn JAVA and teach it in my school. So I'm learning it and teaching it at the same time so the 29 kids who signed up for it can actually take it. No, I'm not an expert, but obviously I'm trying my best since I came to this forum to make sure I knew the correct explanation for this situation which wasn't fully explained in the three textbooks I've been studying, or the online tutorials I've watched.

Richard MacCutchan 17-Oct-14 4:23am

That's not rude it's common sense. If you do not even understand the basics then you should not be teaching it. The chances are quite high, given your question, that you are going to be teaching your class quite a few wrong things. And given the number of samples of really bad code we see in these forums I suspect there are other equally poorly trained teachers around. Think about what might happen if one of your students is tasked with writing an aircraft navigation system and does not understand the difference between i++ and ++i.

Member 11158840 16-Oct-14 14:05pm

I'm teaching it for the first time. I'm a math person, not a programmer, this is new to me. I understood the prefix/postfix for print, but I wanted to confirm it also worked the same way for the if statement. Believe me I'm studying it as much as possible! (Also I asked the teacher who taught it for years and he also wasn't sure... )

Richard MacCutchan 17-Oct-14 4:24am

Also I asked the teacher who taught it for years and he also wasn't sure... )
Unbelievable!

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George Jonsson · Accepted Answer · 2014-10-16T06:12:00

Solution 1

Look into the difference between.

Java

if ((j++) == k)     // postfix operator

and

JavaScript

if ((++j) == k)  // prefix operator

For the postfix operator the comparison is made BEFORE j is incremented.
(128.22 == 129.22)

For the prefix operator the comparison is made AFTER j is incremented.
(129.22 == 129.22)

[Update]
As Sergey mentioned, it is pretty unusual to use the ++ operator on floating point types. Floating points are a bit complicated and can be tricky to explain.
As we touched on this subject I add two links for further reading:
Look at Solution 5 for this CP question. The problem with floating numbers[^]

As a math/programming teacher, you might find this article interesting. What Every Computer Scientist Should Know About Floating-Point Arithmetic[^]

Good luck with your teaching.

Posted 16-Oct-14 6:12am

George Jonsson

Updated 16-Oct-14 23:52pm

v3

Comments

Member 11158840 16-Oct-14 12:15pm

Thank you!!! I knew it had something to do with that, but I couldn't figure out how. Now I can explain it to my students!

Sergey Alexandrovich Kryukov 16-Oct-14 17:18pm

Sure, as simple as that, my 5.
I would add:
1) using ++ operators with double is pretty weird;
2) combination of operators with conditional statement and assignment looks laconic and cool, but isn't clarity and readability more important? I would advise: keep it simple (I don't want to say KISS, because I don't want to say OP is "S" :-)
—SA

George Jonsson 17-Oct-14 5:53am

I added some links that explains floating points. I hope you find them helpful.

Sergey Alexandrovich Kryukov 16-Oct-14 17:18pm

Sure, as simple as that, my 5.
I would add:
1) using ++ operators with double is pretty weird;
2) combination of operators with conditional statement and assignment looks laconic and cool, but isn't clarity and readability more important. I would advise: keep it simple (I don't want to say KISS, because I don't want to say OP is "S" :-)
—SA

Member 11158840 16-Oct-14 20:24pm

I agree with you 100%.

Sergey Alexandrovich Kryukov 16-Oct-14 21:03pm

Very good. But knowing what's going on is useful; I appreciate your desire to get to the essence of things.
Good luck.
—SA

Member 11158840 19-Oct-14 20:45pm

Thanks, it is difficult as all my training is in math. I would have a much easier time with AP Calculus I imagine, but I'm enjoying learning the coding. I'm open to any sources of information, texts, tutorials, etc. The textbook the school ordered is the AP book, but I'm teaching programming one, mainly to beginners, so a colleague gave me the Blue Pelican Java and Java for Everyone which I'm using to supplement the AP text which assumes more advanced knowledge. The only problem is that the Blue Pelican book has great lessons, but the explanations are not as detailed as I would like. Hence, my question here. I thought it was the prefix/postfix, but my colleagues who know more than me weren't sure, so I posted it here. But, like you said, it is weird for an actual programmer to use ++ with a double... so not sure why it is a question asked of students. I think it was just a way to get across the prefix/postfix difference again.

George Jonsson 16-Oct-14 22:15pm

Good point with the double, Sergey, and KISS is always a valuable advice.

Sergey Alexandrovich Kryukov 16-Oct-14 22:54pm

A. Einstein:
"Everything should be made as simple as possible, but no simpler."
—SA

George Jonsson 17-Oct-14 5:54am

A virtual 5 for that proverb. :)

Sergey Alexandrovich Kryukov 17-Oct-14 11:04am

Thank you, George, thank you, Albert. :-)
—SA