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

0 votes
asked Aug 24, 2008 by the-anti-9

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

15 Answers

0 votes
answered Aug 20, 2008 by andy-lester

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.

0 votes
answered Aug 24, 2008 by ryan-fox

++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.

0 votes
answered Aug 24, 2008 by mark-harrison
  • ++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 On Freund 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.

0 votes
answered Aug 24, 2008 by oysterd

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.

0 votes
answered Aug 4, 2011 by scott-urban

I assume you understand the difference in semantics now (though honestly I wonder why people ask 'what does operator X mean' questions on stack overflow 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) ...
0 votes
answered Aug 6, 2011 by dusktreader

The effective result of using either is identical. In other words, the loop will do the same exact thing in both instances.

In terms of efficiency, there could be a penalty involved with choosing i++ over ++i. In terms of the language spec, using the post-increment operator should create an extra copy of the value on which the operator is acting. This could be a source of extra operations.

However, you should consider two main problems with the preceding logic.

  1. Modern compilers are great. All good compilers are smart enough to realize that it is seeing an integer increment in a for-loop, and it will optimize both methods to the same efficient code. If using post-increment over pre-increment actually causes your program to have a slower running time, then you are using a terrible compiler.

  2. In terms of operational time-complexity, the two methods (even if a copy is actually being performed) are equivalent. The number of instructions being performed inside of the loop should dominate the number of operations in the increment operation significantly. Therefore, in any loop of significant size, the penalty of the increment method will be massively overshadowed by the execution of the loop body. In other words, you are much better off worrying about optimizing the code in the loop rather than the increment.

In my opinion, the whole issue simply boils down to a style preference. If you think pre-increment is more readable, then use it. Personally, I prefer the post-incrment, but that is probably because it was what I was taught before I knew anything about optimization.

This is a quintessential example of premature optimization, and issues like this have the potential to distract us from serious issues in design. It is still a good question to ask, however, because there is no uniformity in usage or consensus in "best practice."

0 votes
answered Aug 28, 2012 by parag

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.. no 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.

0 votes
answered Aug 14, 2013 by srinath

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)

0 votes
answered Aug 17, 2013 by scitech

++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
0 votes
answered Aug 22, 2013 by munna

a=i++ means i value will be stored in a without incrementing a=++i means i value will be stored with incrementing...

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