Using Java decrement operator in a recursive function

Question

Here is a recursive funtion -

private void printVar01(int t){
    if(t != 0){
        logp.info("o: "+t);
        printVar01(t-1);
    }
}

Same funtion with a slight modification -

private void printVar02(int t){
    if(t != 0){
        logp.info("o: "+t);
        printVar02(t--);
    }
}

If I pass in a interger value like 4, printVar01 works as expected, where t decrements to 0 in successive recrsive calls, eventually causing the program to exit.

With printVar02, t stays at value 4.

Why? I am assuming this has something to do with variable assignment and/or how values are passed to funtions.

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html — Anil Reddy Yarragonda, May 09 '15 at 03:41
yup i should've read that doc twice :). also this - http://stackoverflow.com/questions/484462/difference-between-i-and-i-in-a-loop — Quest Monger, May 09 '15 at 03:53

score 3 · Answer 1 · edited May 23 '17 at 12:33

t-1 does not change t while t-- does.

t-1 gives you new value without affecting the actual value of t
t-- gives you t and than decreases the value of t

I think printVar02 should work fine and in printVar01 value remains same.

For the comment of DoubleMa

actually 01 will works not 02, in 01 t is changing, but in 02 the function is calling itself before changing the value of T, he just need to use --t;

I also definitely suspect the recursive call.

If you mean printVar = printVar01 = printVar02.

If you are calling printVar recursively than t-1 will work as recursive call will make it work and in t-- it will pass same value 4 every time as it's a postdecrement use predecrement --t instead.

actually 01 will works not 02, in 01 t is changing, but in 02 the function is calling itself before changing the value of T, he just need to use --t; — DoubleMa, May 09 '15 at 03:42

DoubleMa · Accepted Answer · 2015-05-09T03:51:14.807

2

just use --t instead of t--

private static void printVar02(int t){
if(t != 0){
    logp.info("o: "+t);
    printVar02(--t);
}

edited May 09 '15 at 03:51

answered May 09 '15 at 03:36

DoubleMa

368
1
8

Yes this is right if method name is `printVar` and not `printVar02`. – akash May 09 '15 at 03:47
1

that what he meant, i got that when he said "Here is a recursive funtion" i'm sure he was just testing it that why he used 01 and 02 – DoubleMa May 09 '15 at 03:49

score 2 · Answer 3 · answered May 09 '15 at 03:43

2

Post decrement decrees value by 1 after execution of statement printVar(t--). Hence each time 4 is being passed to the function.
Use should use --t instead which does the decrement first.

answered May 09 '15 at 03:43

Raja

831
9
22

Using Java decrement operator in a recursive function

3 Answers3