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


Answers

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.

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?




I assume you understand the difference in semantics now (though honestly I wonder why people ask 'what does operator X mean' questions on rather than reading, you know, a book or web tutorial or something.

But anyway, as far as which one to use, ignore questions of performance, which are unlikely important even in C++. This is the principle you should use when deciding which to use:

Say what you mean in code.

If you don't need the value-before-increment in your statement, don't use that form of the operator. It's a minor issue, but unless you are working with a style guide that bans one version in favor of the other altogether (aka a bone-headed style guide), you should use the form that most exactly expresses what you are trying to do.

QED, use the pre-increment version:

for (int i = 0; i != X; ++i) ...



++i is pre-increment the other is post-increment

i++: gets the element and then increments it.
++i: increments i and then returns the element

Example:

int i = 0;
printf("i: %d\n", i);
printf("i++: %d\n", i++);
printf("++i: %d\n", ++i);

Output:

i: 0
i++: 0
++i: 2



a=i++ means a contains current i value a=++i means a contains incremented i value




++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.




Shortly : ++i and i++ works same if you are not writing them in a function. If you use something like function(i++) or function(++i) you can see the difference.

function(++i) says first increment i by 1, after that put this i into the function with new value.

function(i++) says put first i into the function after that increment i by 1.

int i=4;
printf("%d\n",pow(++i,2));//it prints 25 and i is 5 now
i=4;
printf("%d",pow(i++,2));//it prints 16 i is 5 now



They both increment the number. ++i is equivalent to i = i + 1.

i++ and ++i are very similar but not exactly the same. Both increment the number, but ++i increments the number before the current expression is evaluted, whereas i++ increments the number after the expression is evaluated.

Example :

int i = 1;
int x = i++; //x is 1, i is 2
int y = ++i; //y is 3, i is 3



The reason ++i can be slightly faster than i++ is that i++ can require a local copy of the value of i before it gets incremented, while ++i never does. In some cases, some compilers will optimize it away if possible... but it's not always possible, and not all compilers do this.

I try not to rely too much on compilers optimizations, so I'd follow Ryan Fox's advice: when I can use both, I use ++i.




The following C code fragment illustrates the difference between the pre and post increment and decrement operators:

int i; int j;

// Increment operators

i = 1;

j = ++i; // i is now 2, j is also 2

j = i++; // i is now 3, j is 2




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