What Is the Difference Between ++I and I++?

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.

Let’s see the following example:

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

Here value of j = 1, but i = 2. Here the value of i will be assigned to j first, and 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++.

Let’s see the following example:

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

Here the value of j = 2 but i = 2. Here the 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... It doesn't matter. It will execute your for loop same number 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 the same output. I.e., 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.

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.

Please don't worry about the "efficiency" (speed, really) of which one is faster. We have compilers these days that take care of these things. Use whichever one makes sense to use, based on which more clearly shows your intent.

++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.

The only difference is the order of operations between the increment of the variable and the value the operator returns.

This code and its output explains the the difference:

#include<stdio.h>

int main(int argc, char* argv[])
{
  unsigned int i=0, a;
  printf("i initial value: %d; ", i);
  a = i++;
  printf("value returned by i++: %d, i after: %d\n", a, i);
  i=0;
  printf("i initial value: %d; ", i);
  a = ++i;
  printf(" value returned by ++i: %d, i after: %d\n",a, i);
}

The output is:

i initial value: 0; value returned by i++: 0, i after: 1
i initial value: 0;  value returned by ++i: 1, i after: 1

So basically ++i returns the value after it is incremented, while i++ return the value before it is incremented. At the end, in both cases the i will have its value incremented.

Another example:

#include<stdio.h>

int main ()
  int i=0;
  int a = i++*2;
  printf("i=0, i++*2=%d\n", a);
  i=0;
  a = ++i * 2;
  printf("i=0, ++i*2=%d\n", a);
  i=0;
  a = (++i) * 2;
  printf("i=0, (++i)*2=%d\n", a);
  i=0;
  a = (++i) * 2;
  printf("i=0, (++i)*2=%d\n", a);
  return 0;
}

Output:

i=0, i++*2=0
i=0, ++i*2=2
i=0, (++i)*2=2
i=0, (++i)*2=2

Many times there is no difference

Differences are clear when the returned value is assigned to another variable or when the increment is performed in concatenation with other operations where operations precedence is applied (i++*2 is different from ++i*2, but (i++)*2 and (++i)*2 returns the same value) in many cases they are interchangeable. A classical example is the for loop syntax:

for(int i=0; i<10; i++)

has the same effect of

for(int i=0; i<10; ++i)