C What is the difference between i and i

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

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a i   means a contains current i value a   i means a contains incremented i value

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i   and   i This little code may help to visualize the difference from a different angle than the already posted answers  int i   10  j   10     printf   quot i is  i  n quot   i   printf   quot i   is  i  n quot   i     printf   quot i is  i  n n quot   i      printf   quot j is  i  n quot   j   printf   quot   j is  i  n quot     j   printf   quot j is  i  n quot   j    The outcome is    Remember that the values are i   10  and j   10  i is 10  i   is 10       Assigns  print out   then increments i is 11   j is 10    j is 11      Increments  then assigns  print out  j is 11   Pay attention to the before and after situations  for loop As for which one of them should be used in an incrementation block of a for loop  I think that the best we can do to make a decision is use a good example  int i  j   for  i   0  i  lt   3  i        printf   quot   gt  iteration   i quot   i    printf   quot  n quot     for  j   0  j  lt   3    j      printf   quot   gt  iteration   i quot   j    The outcome is   gt  iteration  0  gt  iteration  1  gt  iteration  2  gt  iteration  3  gt  iteration  0  gt  iteration  1  gt  iteration  2  gt  iteration  3   I don t know about you  but I don t see any difference in its usage  at least in a for loop

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The Main Difference is          i   Post After Increment  and      i Pre   Before Increment          post if i  1  the loop increments like 1 2 3 4 n   pre  if i  1  the loop increments like 2 3 4 5 n

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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 evaluated  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

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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

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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

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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

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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 lt stdio h gt   int main int argc  char  argv        unsigned int i 0  a    printf  quot i initial value   d   quot   i     a   i      printf  quot value returned by i     d  i after   d n quot   a  i     i 0    printf  quot i initial value   d   quot   i     a     i    printf  quot  value returned by   i   d  i after   d n quot  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 lt stdio h gt   int main      int i 0    int a   i   2    printf  quot i 0  i   2  d n quot   a     i 0    a     i   2    printf  quot i 0    i 2  d n quot   a     i 0    a      i    2    printf  quot i 0     i  2  d n quot   a     i 0    a      i    2    printf  quot i 0     i  2  d n quot   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 lt 10  i     has the same effect of for int i 0  i lt 10    i   Rule to remember To not make any confusion between the two operators I adopted this rule  Associate the position of the operator    with respect to the variable i to the order of the    operation with respect to the assignment Said in other words      before i means incrementation must be carried out before assignment     after i means incrementation must be carried out after assignment

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Pre-crement means increment on the same line  Post-increment means increment after the line executes                int j 0  System out println j     0 System out println j       0  post-increment  It means after this line executes j increments   int k 0  System out println k     0 System out println   k     1  pre increment  It means it increments first and then the line executes   When it comes with OR  AND operators  it becomes more interesting   int m 0  if  m    0    m      0   amp  amp   m      1       false    in OR condition if first line is already true then compiler doesn t check the rest  It is technique of compiler optimization    System out println  post-increment   m      int n 0  if  n    0    n      0   amp  amp     n    1       true System out println  pre-increment   n     1     In Array  System out println  In Array    int   a     55  11  15  20  25     int ii  jj  kk   1  mm  ii     a 1      ii   12  a 1    a 1    1 System out println a 1      12  jj   a 1       12 System out println a 1      a 1    13  mm   a 1    13 System out printf     n d  d  d n   ii  jj  mm       12  12  13  for  int val  a         System out print      val     55  13  15  20  25     In C   post pre-increment of pointer variable   include  lt iostream gt  using namespace std   int main          int x 10      int  p    amp x       std  cout lt  lt  address     lt  lt p lt  lt   n     prints address of x     std  cout lt  lt  address     lt  lt p lt  lt   n     prints  address of x    sizeof int      std  cout lt  lt  address     lt  lt  amp x lt  lt   n     prints address of x      std  cout lt  lt  address     lt  lt    amp x lt  lt   n     error  reference can t re-assign because it is fixed  immutable

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i  Prefix operation   Increments and then assigns the value  eg   int i   5  int b     i        In this case  6 is assigned to b first and then increments to 7 and so on   i    Postfix operation   Assigns and then increments the value  eg   int i   5  int b   i          In this case  5 is assigned to b first and then increments to 6 and so on   Incase of for loop  i   is mostly used because  normally we use the starting value of i before incrementing in for loop  But depending on your program logic it may vary

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The difference can be understood by this simple C   code below   int i  j  k  l  i   1    initialize int i with 1 j   i 1    add 1 with i and set that as the value of j  i is still 1 k   i      k gets the current value of i  after that i is incremented  So here i is 2  but k is 1 l     i     i is incremented first and then returned  So the value of i is 3 and so does l  cout  lt  lt  i  lt  lt       lt  lt  j  lt  lt       lt  lt  k  lt  lt      lt  lt  l  lt  lt  endl  return 0

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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 amp 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  OnFreund 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

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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

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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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You can think of internal conversion of that as a multiple statements     case 1   i      you can think it as   i  i   i 1     case 2     i    you can think it as   i   i i  i

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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

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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

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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

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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 lt 5  i       printf   d   i     And  for i 0  i lt 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 lt 5       printf   d     i     In this case output will be 1 2 3 4 5

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The effective result of using either in a loop 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    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  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

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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

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

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