Difference between pre-increment and post-increment in a loop?

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Is there a difference in ++i and i++ in a for loop? Is it simply a syntax thing?

a++ is known as postfix.

add 1 to a, returns the old value.

++a is known as prefix.

add 1 to a, returns the new value.

C#:

string[] items = {"a","b","c","d"};
int i = 0;
foreach (string item in items)
{
    Console.WriteLine(++i);
}
Console.WriteLine("");

i = 0;
foreach (string item in items)
{
    Console.WriteLine(i++);
}

Output:

1
2
3
4

0
1
2
3

foreach and while loops depend on which increment type you use. With for loops like below it makes no difference as you're not using the return value of i:

for (int i = 0; i < 5; i++) { Console.Write(i);}
Console.WriteLine("");
for (int i = 0; i < 5; ++i) { Console.Write(i); }

0 1 2 3 4 0 1 2 3 4

If the value as evaluated is used then the type of increment becomes significant:

int n = 0;
for (int i = 0; n < 5; n = i++) { }

Difference between pre-increment and post-increment in a loop , After seeing the difference between post-increment and pre-increment, one might notice that, since the cached value of i is never used in� Difference Between Pre-Increment and Post-Increment in C# There is no difference whether you use prefix or postfix form; the variable value will increase by 1. Then you must be wondering why there are two ways to do the same thing.

Pre-increment ++i increments the value of i and evaluates to the new incremented value.

int i = 3;
int preIncrementResult = ++i;
Assert( preIncrementResult == 4 );
Assert( i == 4 );

Post-increment i++ increments the value of i and evaluates to the original non-incremented value.

int i = 3;
int postIncrementResult = i++;
Assert( postIncrementtResult == 3 );
Assert( i == 4 );

In C++, the pre-increment is usually preferred where you can use either.

This is because if you use post-increment, it can require the compiler to have to generate code that creates an extra temporary variable. This is because both the previous and new values of the variable being incremented need to be held somewhere because they may be needed elsewhere in the expression being evaluated.

So, in C++ at least, there can be a performance difference which guides your choice of which to use.

This is mainly only a problem when the variable being incremented is a user defined type with an overridden ++ operator. For primitive types (int, etc) there's no performance difference. But, it's worth sticking to the pre-increment operator as a guideline unless the post-increment operator is definitely what's required.

There's some more discussion here: https://web.archive.org/web/20170405054235/http://en.allexperts.com/q/C-1040/Increment-operators.htm

In C++ if you're using STL, then you may be using for loops with iterators. These mainly have overridden ++ operators, so sticking to pre-increment is a good idea. Compilers get smarter all the time though, and newer ones may be able to perform optimizations that mean there's no performance difference - especially if the type being incremented is defined inline in header file (as STL implementations often are) so that the compiler can see how the method is implemented and can then know what optimizations are safe to perform. Even so, it's probably still worth sticking to pre-increment because loops get executed lots of times and this means a small performance penalty could soon get amplified.


In other languages such as C# where the ++ operator can't be overloaded there is no performance difference. Used in a loop to advance the loop variable, the pre and post increment operators are equivalent.

Correction: overloading ++ in C# is allowed. It seems though, that compared to C++, in c# you cannot overload the pre and post versions independently. So, I would assume that if the result of calling ++ in C# is not assigned to a variable or used as part of a complex expression, then the compiler would reduce the pre and post versions of ++ down to code that performs equivalently.

Stop Using i++ in Your Loops. Why ++i is often better than i++…, You will see the difference between pre and post increment/decrement. ssjgz February 25, 2020, 7:07am #3. In a for loop with an int loop� But surprisingly (to me), many candidates tell me the loop with the post-increment will print the numbers from 0 to 4, and the pre-increment loop will print 0 to 5, or 1 to 5. They usually explain the difference between pre- and post-incrementing correctly, but they misunderstand the mechanics of the for() loop.

In C# there is no difference when used in a for loop.

for (int i = 0; i < 10; i++) { Console.WriteLine(i); }

outputs the same thing as

for (int i = 0; i < 10; ++i) { Console.WriteLine(i); }

As others have pointed out, when used in general i++ and ++i have a subtle yet significant difference:

int i = 0;
Console.WriteLine(i++);   // Prints 0
int j = 0;
Console.WriteLine(++j);   // Prints 1

i++ reads the value of i then increments it.

++i increments the value of i then reads it.

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The question is:

Is there a difference in ++i and i++ in a for loop?

The answer is: No.

Why does each and every other answer have to go into detailed explanations about pre and post incrementing when this is not even asked?

This for-loop:

for (int i = 0; // Initialization
     i < 5;     // Condition
     i++)       // Increment
{
   Output(i);
}

Would translate to this code without using loops:

int i = 0; // Initialization

loopStart:
if (i < 5) // Condition
{
   Output(i);

   i++ or ++i; // Increment

   goto loopStart;
}

Now does it matter if you put i++ or ++i as increment here? No it does not as the return value of the increment operation is insignificant. i will be incremented AFTER the code's execution that is inside the for loop body.

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Since you ask about the difference in a loop, i guess you mean

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

In that case, you have no difference in most languages: The loop behaves the same regardless of whether you write i++ and ++i. In C++, you can write your own versions of the ++ operators, and you can define separate meanings for them, if the i is of a user defined type (your own class, for example).

The reason why it doesn't matter above is because you don't use the value of i++. Another thing is when you do

for(int i=0, a = 0; i<10; a = i++) 
    ...;

Now, there is a difference, because as others point out, i++ means increment, but evaluate to the previous value, but ++i means increment, but evaluate to i (thus it would evaluate to the new value). In the above case, a is assigned the previous value of i, while i is incremented.

Pre-increment and Post-increment in C/C++, The difference between functions and operators is thus in the syntax and terminology, rather than in the concepts. We use operator notation to� The post-increment and post-decrement operators increase (or decrease) the value of their operand by 1, but the value of the expression is the operand's value prior to the increment (or decrement) operation. In languages where increment/decrement is not an expression (e.g., Go), only one version is needed (in the case of Go, post operators only).

Pre-increment vs Post-increment Operator in Loops – Incus Data Blog, Unless your loop runs a million times in a real time shoot em up game nobody will notice the difference. This is because post-increment (or decrement) requires a copy of i to be But if there is a difference, then ++i is expected to perform better. for a consistent and minimal definition of post-increment and pre-increment,� Pre Increment Operation a = 11 x = 11. Post-increment operator: A post-increment operator is used to increment the value of variable after executing expression completely in which post increment is used. In the Post-Increment, value is first used in a expression and then incremented. Syntax:

pre-increment vs. post-increment, What is the difference between pre increment operator and post increment operator in C? - re increment operator is used to increment variable value etc. The increment is adding one to the current value. We all have a little confusion about post-increment and pre-increment. Now let us get over this confusion with a detailed explanation. I hope this will help you to understand the difference between post-increment and pre-increment in Java. Post-Increment VS Pre-Increment in Java

What is the difference between pre increment operator and post , I habitually used post-increments in for loops because the examples C Does it really make a difference, or is it merely a style thing? Pre-increment Vs Post-increment Operator Pre-increment (++i) Operator When we use Pre-increment operator in our program, value is first incremented and then incremented value is used in an expression. For example - Let's take a very basic example.

Comments
  • Dupe: stackoverflow.com/questions/467322/…
  • I'm amazed at how many answers completely missed the point of the question.
  • Perhaps we should be amazed that no one edited the question to be more clear :)
  • This question could apply to C, Java, C++, PHP, C#, Javascript, JScript, Objective C: en.wikipedia.org/wiki/Category:C_programming_language_family
  • Good answer posted here: stackoverflow.com/a/4706225/214296
  • This is not even what the user asked for.
  • Wouldn't it have been great if C++ was named ++C indicating that you can write a well optimized code using it..
  • Shouldn't modern compilers be able to optimize this when the resulting value is obviously going to be trashed anyway?
  • @che - they do when it's a simple type, however classes that overload operator++ (such as iterators) are a different story.
  • @che: That's a good question. The reason that C++ compilers don't replace "CustomType++;" with "++CustomType;" is because there's no guarantee that both user-defined functions have the same effect. They SHOULD...but there's no guarantee.
  • @michael.bartnett: Good point, overloading ++ in C# does appear to be available. It seems though, that compared to c++, in c# you cannot overload the pre and post versions independently. So, I would assume that if the result of calling ++ in C# is not assigned to a variable or used as part of a complex expression, then the compiler would reduce the pre and post versions of ++ down to code that performs equivalently.