What is the difference between ++i and i++?

Question

In C, what is the difference between using ++i and i++, and which should be used in the incrementation block of a for loop?

Answer 1

• ++i will increment the value of i, and then return the incremented value.

   i = 1;  j = ++i;  (i is 2, j is 2) 

• i++ will increment the value of i, but return the original value that i held before being incremented.

   i = 1;  j = i++;  (i is 2, j is 1) 

For a for loop, either works. ++i seems more common, perhaps because that is what is used in K&R.

In any case, follow the guideline "prefer ++i over i++" and you won't go wrong.

There's a couple of comments regarding the efficiency of ++i and i++. In any non-student-project compiler, there will be no performance difference. You can verify this by looking at the generated code, which will be identical.

The efficiency question is interesting... here's my attempt at an answer: Is there a performance difference between i++ and ++i in C?

As @OnFreund notes, it's different for a C++ object, since operator++() is a function and the compiler can't know to optimize away the creation of a temporary object to hold the intermediate value.

Answer 2

i++ is known as Post Increment whereas ++i is called Pre Increment.

i++

i++ is post increment because it increments i's value by 1 after the operation is over.

Lets see the following example:

int i = 1, j; j = i++; 

Here value of j = 1 but i = 2. Here value of i will be assigned to j first then i will be incremented.

++i

++i is pre increment because it increments i's value by 1 before the operation. It means j = i; will execute after i++.

Lets see the following example:

int i = 1, j; j = ++i; 

Here value of j = 2 but i = 2. Here value of i will be assigned to j after the i incremention of i. Similarly ++i will be executed before j=i;.

For your question which should be used in the incrementation block of a for loop? the answer is, you can use any one.. doesn't matter. It will execute your for loop same no. of times.

for(i=0; i<5; i++)    printf("%d ",i); 

And

for(i=0; i<5; ++i)    printf("%d ",i); 

Both the loops will produce same output. ie 0 1 2 3 4.

It only matters where you are using it.

for(i = 0; i<5;)     printf("%d ",++i); 

In this case output will be 1 2 3 4 5.