4

I have a little doubt here,i have a code as

 int num=0;
 for(int i=0;i<5;i++){
   num=num++;
   System.out.print(num);
 }

why is the output always 00000

MichelleJS
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Hmahwish
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7 Answers7

10

The ++ operator increments num last, so when num is 0, you are setting it to 0 again.

It has to deal with how the ++ operator increments num, and what num is truly pointing to. To avoid it, just use num++

Interestingly enough, num=++num will correctly increment and assign the incremented value, though the whole purpose of the ++ operator, either pre or post, is that it modifies the value directly. You do not have to re-assign it back to the value.

hotforfeature
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    Good answer. 'Self-increment' operators don't have to be assigned to their variable, trying to do so is a mistake! +1. – Thomas W Aug 22 '13 at 05:29
3

use num++;

int num=0;
for(int i=0;i<5;i++){
   num++;
   System.out.print(num);
}

Output 12345

num=num++;

is equals to num=num;

num=++num;

is equals to num=num+1;

Pandiyan Cool
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2
num=num++;

is equal - 

num = num;
num ++;

First it assign then it try to increment the num which is already assigned. For better calrification - 

 0  iconst_0
 1  istore_1 [num]
 2  iconst_0
 3  istore_2 [i]
 4  goto 22
 7  iload_1 [num] // Load first
 8  iinc 1 1 [num] // incement but no reload
11  istore_1 [num] // store old load value
12  getstatic java.lang.System.out : java.io.PrintStream [16]
15  iload_1 [num]
16  invokevirtual java.io.PrintStream.print(int) : void [22]
19  iinc 2 1 [i]
22  iload_2 [i]
23  iconst_5
24  if_icmplt 7
27  return

if we consider num=++num;

then generated byte code would be -

 0  iconst_0
 1  istore_1 [num]
 2  iconst_0
 3  istore_2 [i]
 4  goto 22
 7  iinc 1 1 [num] // Increment 
10  iload_1 [num] // load the incremented value
11  istore_1 [num] // store the loaded incremented value
...
Subhrajyoti Majumder
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  • Not strictly correct. The order in which assignments are executed is arbitrary (not defined by the language). I assume that in the poster's problem, the assignment (to the original value) was being executed *after* the post-increment. Thus overwriting any increment. – Thomas W Aug 22 '13 at 05:25
  • Exactly. The language _does not define behaviour_ where assignments & increments are present, within the same statement. Neither do C/C++ (though there, the smallest unit of sequencing can be a comma-operator clause.) Corner-cases like this are left unspecified deliberately, to allow flexibility for optimization & discourage code from relying on highly-obscure/ or implementation-dependent outcomes. – Thomas W Aug 22 '13 at 10:21
1

That what postfix ++ does.

You can use:

num++;
System.out.print(num);
Community
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BobTheBuilder
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1

num=num++;
Here you are using postfix ++ operator with assignment.So that means you assigned the value first and then incremented it.
So 

 num = num++;

equivalent to 

 num = num;//line1
 num+1;//line2

Deep look at line2 .The result of num+1 is not assigned to anything.So num is always have the value assigned at line1.i.e.,0 in your case.

Prabhaker A
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1

Ok, In java = operator works as follows

L.H.S=R.H.S right hand side value will assign to left hand side variable.

In here initial value of num=0 and

num=num++ this increment not influence in place to num. if you do ++num it will effect at once in palace. so again you are assign 0 for num. So entire process this will happen continuously until loop stop.

Ruchira Gayan Ranaweera
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1

for explanation,

Post Increment(n++) : First execute the statement and then increase the value by one.

here, value of 'num++' is assigned to num and that is before increment and is 0. so num will have always value 0.

you can use simply num++ there.

Rajj
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