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
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You have multiple syntax errors in your code? How does it run? int i notdefined, no semicolon after num++?Juned Ahsan– Juned Ahsan2013-08-22 05:22:50 +00:00Commented Aug 22, 2013 at 5:22
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num=num++ where is ; ?Pandiyan Cool– Pandiyan Cool2013-08-22 05:24:10 +00:00Commented Aug 22, 2013 at 5:24
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possible duplicate of Post increment operator not incrementing in for loopRohit Jain– Rohit Jain2013-08-22 05:42:10 +00:00Commented Aug 22, 2013 at 5:42
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7 Answers
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.
1 Comment
Thomas W
Good answer. 'Self-increment' operators don't have to be assigned to their variable, trying to do so is a mistake! +1.
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;
1 Comment
SpringLearner
Op is asking for reason and you are providing the solutin
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
...
answered Aug 22, 2013 at 5:22
Subhrajyoti Majumder
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2 Comments
Thomas W
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
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.
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.
Comments
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.
answered Aug 22, 2013 at 5:39
Ruchira Gayan Ranaweera
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Comments
1 Comment
Thomas W
Has everyone lost their semicolons?
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.
Comments
