[Java] Diference Between i-- and --i

[PedroBarbosa]

Hey

Title says it all...

 

exemple:

 

int i = 0;

--i; or i--;

 

Thanks

[TheKDub]

What exactly do you mean by i-- and --i?

Do you mean like this:

int i = 0;

int 0 = i;

[PedroBarbosa]

What exactly do you mean by i-- and --i?

Do you mean like this:

int i = 0;

int 0 = i;

 

My question is if this is the same thing:

 

 

for( int i = 10 ; i >= 0 ; --i )

System.out.println( i );

 

for( int i = 10 ; i >= 0 ; i-- )

System.out.println( i );

[Sauron]

--i subtracts 1 right now. i-- uses the current value of i, then subtracts 1.

 

So in pseudocode:

int i = 5;

print(i--);

print(i);

 

this prints 5 and then 4.

 

print(--i);

print(i);

 

this prints 4 twice.

[madknight3]

The following will modify the value before it assigns to the variable

int i = 5;int value = --i;// i = 4 and value = 4 int i = 5;int value = ++i;// i = 6 and value = 6

The following will assign the variable and then modify the value

int i = 5;int value = i--;// i = 4 and value = 5int i = 5;int value = i++;// i = 6 and value = 5

If you aren't assigning to a variable or passing to a method, then there's not really much of a difference. They will both still increment or decrement the value.

int i = 5;--i; // i = 4i--; // i = 3

Technically --i and ++i are slightly more efficient than i-- and i++ when considering the compiled code (without compiler optimization). However I'd be very surprised if compilers aren't optimizing it for you thus making them equivalent. Regardless, this micro-optimization wouldn't really be noticeable but I still prefer to use --i and ++i when I can anyway.

[minibois]

 

My question is if this is the same thing:

 

 

for( int i = 10 ; i >= 0 ; --i )

System.out.println( i );

 

for( int i = 10 ; i >= 0 ; i-- )

System.out.println( i );

 

I am not sure what --i is, but i-- basically means:

i = i - 1;

So everytime you code comes across i--, it lower the var i with 1.

 

EDIT: better explanations above

[spenser_l]

These are types of incrementers. i-- is called a post-decrementer, and --i is a pre-decrementer. 

 

Basically, if you were to use one of these within an operation, the time at which i is subtracted from would be different.

 

for example:

int a = 1;int c = --a;

so here both c and a would equal 0 because of the use of the PRE-decrementation. It would look at the value of a, which is 1, and then subtract from it, then assign that value to c.

 

If it was like this:

int a = 1;int c = a--;

c would equal 1 and a would equal 0 because of the POST-decrementation. c would look at the value of a, which is 1, assign it, and then a would be subtracted by 1.

 

These same principles apply to pre and post incrementers like i++ and ++i.

[Nineshadow]

i-- means the expression has the value i, then i gets decremented.

--i decrements i before setting the value of the expression.

 

Example

x = 0

y = x++ => y is 0, x is 1

// y is set to x, then x gets incremented.

z = ++x => z is 1, z is 1

// x gets incremented , then z is equal to the incremented x.

[fizzlesticks]

I'm not sure if it ever matters in Java but in C++ with operator overloading, unless you specifically want a post increment it's generally better to use pre increment. Since a pre increment just returns the same object it can be much faster than a post increment which much create a copy of the object, increment the old object then return the copy.