What is the difference between i and i

Question

I ve seen them both being used in numerous pieces of C  code  and I d like to know when to use i   or   i  i being a number variable like int  float  double  etc   Anyone who knows this

User · Accepted Answer

Oddly it looks like the other two answers don t spell it out  and it s definitely worth saying     i   means  tell me the value of i  then increment     i means  increment i  then tell me the value     They are Pre-increment  post-increment operators   In both cases the variable is incremented  but if you were to take the value of both expressions in exactly the same cases  the result will differ

User · Answer

int i   0  Console WriteLine i        Prints 0  Then value of  i  becomes 1  Console WriteLine --i      Value of  i  becomes 0  Then prints 0    Does this answer your question

User · Answer

If you have   int i   10  int x     i    then x will be 11   But if you have   int i   10  int x   i      then x will be 10   Note as Eric points out  the increment occurs at the same time in both cases  but it s what value is given as the result that differs  thanks Eric     Generally  I like to use   i unless there s a good reason not to  For example  when writing a loop  I like to use   for  int i   0  i  lt  10    i        Or  if I just need to increment a variable  I like to use     x    Normally  one way or the other doesn t have much significance and comes down to coding style  but if you are using the operators inside other assignments  like in my original examples   it s important to be aware of potential side effects

User · Answer

Just for the record  in C    if you can use either  i e   you don t care about the ordering of operations  you just want to increment or decrement and use it later  the prefix operator is more efficient since it doesn t have to create a temporary copy of the object  Unfortunately  most people use posfix  var    instead of prefix    var   just because that is what we learned initially   I was asked about this in an interview   Not sure if this is true in C   but I assume it would be

User · Answer

The way the operator works is that it gets incremented at the same time  but if it is before a variable  the expression will evaluate with the incremented decremented variable   int x   0      x is 0 int y     x    x is 1 and y is 1   If it is after the variable the current statement will get executed with the original variable  as if it had not yet been incremented decremented   int x   0      x is 0 int y   x       y   x  is evaluated with x 0  but x is still incremented  So  x is 1  but y is 0   I agree with dcp in using pre-increment decrement    x  unless necessary  Really the only time I use the post-increment decrement is in while loops or loops of that sort  These loops are the same   while  x  lt  5     evaluates conditional statement         some code       x          increments x     or  while  x    lt  5    evaluates conditional statement with x value before increment  and x is incremented         some code     You can also do this while indexing arrays and such   int i   0  int   MyArray   new int 2   MyArray i      1234    sets array at index 0 to  1234  and i is incremented MyArray i    5678      sets array at index 1 to  5678  int temp   MyArray --i     temp is 1234  becasue of pre-decrement     Etc  etc

User · Answer

The typical answer to this question  unfortunately posted here already  is that one does the increment  quot before quot  remaining operations and the other does the increment  quot after quot  remaining operations  Though that intuitively gets the idea across  that statement is on the face of it completely wrong   The sequence of events in time is extremely well-defined in C   and it is emphatically not the case that the prefix    var  and postfix  var    versions of    do things in a different order with respect to other operations  It is unsurprising that you ll see a lot of wrong answers to this question  A great many  quot teach yourself C  quot  books also get it wrong  Also  the way C  does it is different than how C does it  Many people reason as though C  and C are the same language  they are not  The design of the increment and decrement operators in C  in my opinion avoids  the design flaws of these operators in C  There are two questions that must be answered to determine what exactly the operation of prefix and postfix    are in C   The first question is what is the result  and the second question is when does the side effect of the increment take place  It is not obvious what the answer to either question is  but it is actually quite simple once you see it  Let me spell out for you precisely what x   and   x do for a variable x  For the prefix form    x    x is evaluated to produce the variable The value of the variable is copied to a temporary location The temporary value is incremented to produce a new value  not overwriting the temporary   The new value is stored in the variable The result of the operation is the new value  i e  the incremented value of the temporary   For the postfix form  x      x is evaluated to produce the variable The value of the variable is copied to a temporary location The temporary value is incremented to produce a new value  not overwriting the temporary   The new value is stored in the variable The result of the operation is the value of the temporary  Some things to notice  First  the order of events in time is exactly the same in both cases  Again  it is absolutely not the case that the order of events in time changes between prefix and postfix  It is entirely false to say that the evaluation happens before other evaluations or after other evaluations  The evaluations happen in exactly the same order in both cases as you can see by steps 1 through 4 being identical  The only difference is the last step - whether the result is the value of the temporary  or the new  incremented value  You can easily demonstrate this with a simple C  console app  public class Application       public static int currentValue   0       public static void Main                 Console WriteLine  quot Test 1    x quot               currentValue  TestMethod             Console WriteLine  quot  nTest 2  x   quot             currentValue    TestMethod             Console WriteLine  quot  nTest 3    x quot               currentValue  TestMethod             Console ReadKey             public static class ExtensionMethods        public static void TestMethod this int passedInValue                 Console WriteLine  quot Current  0  Passed-in  1  quot               Application currentValue              passedInValue            Here are the results    Test 1    x Current 1 Passed-in 1  Test 2  x   Current 2 Passed-in 1  Test 3    x Current 3 Passed-in 3  In the first test  you can see that both currentValue and what was passed in to the TestMethod   extension show the same value  as expected  However  in the second case  people will try to tell you that the increment of currentValue happens after the call to TestMethod    but as you can see from the results  it happens before the call as indicated by the  Current 2  result  In this case  first the value of currentValue is stored in a temporary  Next  an incremented version of that value is stored back in currentValue but without touching the temporary which still stores the original value  Finally that temporary is passed to TestMethod     If the increment happened after the call to TestMethod   then it would write out the same  non-incremented value twice  but it does not   It s important to note that the value returned from both the currentValue   and   currentValue operations are based on the temporary and not the actual value stored in the variable at the time either operation exits  Recall in the order of operations above  the first two steps copy the then-current value of the variable into the temporary   That is what s used to calculate the return value  in the case of the prefix version  it s that temporary value incremented while in the case of the suffix version  it s that value directly non-incremented  The variable itself is not read again after the initial storage into the temporary  Put more simply  the postfix version returns the value that was read from the variable  i e  the value of the temporary  while the prefix version returns the value that was written back to the variable  i e  the incremented value of the temporary    Neither return the variable s value  This is important to understand because the variable itself could be volatile and have changed on another thread which means the return value of those operations could differ from the current value stored in the variable   It is surprisingly common for people to get very confused about precedence  associativity  and the order in which side effects are executed  I suspect mostly because it is so confusing in C   C  has been carefully designed to be less confusing in all these regards  For some additional analysis of these issues  including me further demonstrating the falsity of the idea that prefix and postfix operations  quot move stuff around in time quot  see  https   ericlippert com 2009 08 10 precedence-vs-order-redux  which led to this SO question  int   arr  0   int value   arr arr 0      Value   1  You might also be interested in my previous articles on the subject  https   ericlippert com 2008 05 23 precedence-vs-associativity-vs-order  and https   ericlippert com 2007 08 14 c-and-the-pit-of-despair  and an interesting case where C makes it hard to reason about correctness  https   docs microsoft com archive blogs ericlippert bad-recursion-revisited Also  we run into similar subtle issues when considering other operations that have side effects  such as chained simple assignments  https   docs microsoft com archive blogs ericlippert chaining-simple-assignments-is-not-so-simple And here s an interesting post on why the increment operators result in values in C  rather than in variables  Why can  39 t I do   i   in C-like languages

[c#] What is the difference between i++ and ++i?

Examples related to c#