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The differences between a++ and ++a

Dec 6, 2013 at 2:10pm
icebreath123 (3)
Hello everyone,I would like to ask a question here.What are the differences between a++ and ++a?

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do{
      a++;
      cout<<a;

}while (a < 5);


//this is the other code

do{
      ++a;
      cout<<a;

}while (a < 5);


Can someone explain briefly for these 2 examples?
Dec 6, 2013 at 2:16pm
S G H (2638)
In this context it doesn't do anything different.
The difference is here:

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int a = 0;
int b = a++; // b = 0, a = 1


Where

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int a = 0;
int b = ++a; // b = 1, a = 1
Dec 6, 2013 at 2:21pm
icebreath123 (3)
So,how do I know when a++ and ++a will do different things?
Dec 6, 2013 at 2:22pm
mutexe (2372)
Dec 6, 2013 at 2:39pm
icebreath123 (3)
After I reading the contents of the website,I still do not know when a++ and ++a will do different things .
Dec 6, 2013 at 2:59pm
MikeyBoy (5631)
a++ increments a after the expression has been evaluated. So in:

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a = 1;
b = a++;


The expression a++ evaluates to 1, so b is set to 1, and then a is incremented to 2.

++a increments a before the expression is evaluated, so in:

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a = 1;
b = ++a;


a is incremented first, and the expression then evaluates to 2, so b is set to 2.
Last edited on Dec 6, 2013 at 2:59pm
Dec 6, 2013 at 3:11pm
kooth (746)
As EssGeEich said, the context of your example doesn't really do anything different. These operators are convenient for looping when you want to either used a variable's current value or its new value.

Here is an example that might help:

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int
    LegalAge = 21,
    age = 0;

while( age++ < 21 )
    {
    cout << "Your age is " << age << "." << endl;

    cout << "You can't have a beer until you are " <<
        LegalAge; << "!" << endl << endl;
    }


In the example above, you will have the following output:

Your age is 1.
You can't have a beer until you are 21!

Your age is 2.
You can't have a beer until you are 21!

Your age is 3.
You can't have a beer until you are 21!

Your age is 4.
You can't have a beer until you are 21!

Your age is 5.
You can't have a beer until you are 21!

Your age is 6.
You can't have a beer until you are 21!

Your age is 7.
You can't have a beer until you are 21!

Your age is 8.
You can't have a beer until you are 21!

Your age is 8.
You can't have a beer until you are 21!

Your age is 10.
You can't have a beer until you are 21!

Your age is 11.
You can't have a beer until you are 21!

Your age is 12.
You can't have a beer until you are 21!

Your age is 13.
You can't have a beer until you are 21!

Your age is 14.
You can't have a beer until you are 21!

Your age is 15.
You can't have a beer until you are 21!

Your age is 16.
You can't have a beer until you are 21!

Your age is 17.
You can't have a beer until you are 21!

Your age is 18.
You can't have a beer until you are 21!

Your age is 19.
You can't have a beer until you are 21!

Your age is 20.
You can't have a beer until you are 21!

If you changed the example to:

 
 
while( ++age < 21 )


you would have the following output:

Your age is 1.
You can't have a beer until you are 21!

Your age is 2.
You can't have a beer until you are 21!

Your age is 3.
You can't have a beer until you are 21!

Your age is 4.
You can't have a beer until you are 21!

Your age is 5.
You can't have a beer until you are 21!

Your age is 6.
You can't have a beer until you are 21!

Your age is 7.
You can't have a beer until you are 21!

Your age is 8.
You can't have a beer until you are 21!

Your age is 8.
You can't have a beer until you are 21!

Your age is 10.
You can't have a beer until you are 21!

Your age is 11.
You can't have a beer until you are 21!

Your age is 12.
You can't have a beer until you are 21!

Your age is 13.
You can't have a beer until you are 21!

Your age is 14.
You can't have a beer until you are 21!

Your age is 15.
You can't have a beer until you are 21!

Your age is 16.
You can't have a beer until you are 21!

Your age is 17.
You can't have a beer until you are 21!

Your age is 18.
You can't have a beer until you are 21!

Your age is 19.
You can't have a beer until you are 21!

Your age is 20.
You can't have a beer until you are 21!

Notice that the first line starts with "Your age is 1" in this case.

I hope this helped to explain the difference.

EDIT: Thanks to cire who caught my cut and paste error!

Last edited on Dec 13, 2013 at 2:08pm
Dec 6, 2013 at 5:14pm
thefasninja (28)
Difference between both a++ and ++a are quite easy.
in a++ we are adding one in a and other side we are add a to some other variable.
Dec 7, 2013 at 2:53pm
MikeyBoy (5631)
in a++ we are adding one in a and other side we are add a to some other variable.


Um... what? Unless I've totally misunderstood what you're trying to say, that's nonsense. ++a increments a by 1, just as a++ does. The difference is in what the expression evaluates to.
Last edited on Dec 7, 2013 at 3:52pm
Dec 7, 2013 at 3:45pm
cire (8284)
kooth wrote:
In the example above, you will have the following output:

Your output is wrong in the first case.

http://ideone.com/hvg1m3
Dec 11, 2013 at 7:32pm
kooth (746)
cire you're right: It was a cut and paste error. Thanks for catching what my old eyes can't.
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