count++ & infinite loop

Jeff Winterton · 2012-08-16T12:51:30

Working through java, a beginners guide 5th ed one of the examples: [source lang="java"]public class forDemo { // Demonstrates the for loop public static void main(String[] args) { // TODO Auto-generated method stub int count; for (count = 0; count < 5; count = count + 1); System.out.println( "This is count: " + count); System.out.println( "Done!"); } }[/source] The author goes on to say that the example is not good programming and that you will not see programmers use that example, you would probably se this instead: count++. But when I run it with the latter: [source lang="java"] public class forDemo { // Demonstrates the for loop public static void main(String[] args) { // TODO Auto-generated method stub int count; for (count = 0; count < 5; count = count++); System.out.println( "This is count: " + count); System.out.println( "Done!"); } } [/source] it runs "this is count: 0 infinitely thanks for any help.

For Beginners

Started by Jeffige August 16, 2012 03:49 AM

13 comments, last by dimitri.adamou 11 years, 8 months ago

dimitri.adamou

329

August 16, 2012 11:37 AM

The result of the expression 'count++' is the value of count before incrementing it (0). So a repeated 'count = count++' is basically the same as saying 'count = 0'.
'count = ++count' would give the expected result, although there is no point in assigning count's value to itself.

no its not. True, you assigning it 0 but then you increment it by 1.

obizues

176

August 16, 2012 12:22 PM

The problem is as stated above, the semicolon. You need to take that out. It's also good practice to add brackets after the for statement even if there is only one statement. What your code is doing is looping your entire for, and then printing out the result.

For example:



For (i=0;i<5;i++)

{

System.out.print("test: " + i);

}

Trienco

2,555

August 16, 2012 12:23 PM

no its not. True, you assigning it 0 but then you increment it by 1.

The typical implementation of a post increment would look like this:

tmp = count;
count = count + 1;
return tmp;

So no, you're not assigning first and then increment, you store the original value, then increment and the actual assignment is a completely different operator that happens last. In fact, one might argue that any compiler optimizing this code in a way that behaves the way you expect could be considered broken. You can't interrupt one operator (++) halfway through to squeeze in a different operator (=) and then go back to finish the first one.

f@dzhttp://festini.device-zero.de

SiCrane

11,840

August 16, 2012 12:25 PM

no its not. True, you assigning it 0 but then you increment it by 1.

In Java, both sides of a non-short circuit binary operator are fully evaluated, including side effects, before the operator is applied. In this case it means that count is incremented before assignment occurs, not after.

dimitri.adamou

329

August 16, 2012 12:51 PM

[quote name='dimitri.adamou' timestamp='1345117054' post='4970116']
no its not. True, you assigning it 0 but then you increment it by 1.

count++ & infinite loop

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count++ & infinite loop

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