C: 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++ and ++i

This little code may help to visualize the difference from a different angle than the already posted answers:

int i = 10, j = 10;
  
printf ("i is %i \n", i);
printf ("i++ is %i \n", i++);
printf ("i is %i \n\n", i);
  
printf ("j is %i \n", j);
printf ("++j is %i \n", ++j);
printf ("j is %i \n", j);

The outcome is:

//Remember that the values are i = 10, and j = 10

i is 10 
i++ is 10     //Assigns (print out), then increments
i is 11 

j is 10 
++j is 11    //Increments, then assigns (print out)
j is 11 

Pay attention to the before and after situations.

for loop

As for which one of them should be used in an incrementation block of a for loop, I think that the best we can do to make a decision is use a good example:

int i, j;

for (i = 0; i <= 3; i++)
    printf (" > iteration #%i", i);

printf ("\n");

for (j = 0; j <= 3; ++j)
    printf (" > iteration #%i", j);

The outcome is:

> iteration #0 > iteration #1 > iteration #2 > iteration #3
> iteration #0 > iteration #1 > iteration #2 > iteration #3 

I don't know about you, but I don't see any difference in its usage, at least in a for loop.

Answer 2

++i increments the value, then returns it.

i++ returns the value, and then increments it.

It's a subtle difference.

For a for loop, use ++i, as it's slightly faster. i++ will create an extra copy that just gets thrown away.

Answer 3

You can think of internal conversion of that as a multiple statements;

• case 1

i++;

you can think it as,

i;
i = i+1;

• case 2

++i;

you can think it as,

i = i+i;
i;

Answer 4

i++: In this scenario first the value is assigned and then increment happens.

++i: In this scenario first the increment is done and then value is assigned

Below is the image visualization and also here is a nice practical video which demonstrates the same.

Answer 5

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.

Answer 6

Here is the example to understand the difference

int i=10;
printf("%d %d",i++,++i);

output: 10 12/11 11 (depending on the order of evaluation of arguments to the printf function, which varies across compilers and architectures)

Explanation: i++->i is printed, and then increments. (Prints 10, but i will become 11) ++i->i value increments and prints the value. (Prints 12, and the value of i also 12)