Incrementing in C - When to use x or x

Question

I m currently learning C   and I ve learned about the incrementation a while ago  I know that you can use    x  to make the incrementation before and  x    to do it after   Still  I really don t know when to use either of the two    I ve never really used    x  and things always worked fine so far - so  when should I use it   Example  In a for loop  when is it preferable to use    x    Also  could someone explain exactly how the different incrementations  or decrementations  work  I would really appreciate it

User · Answer

Postfix form of    -- operator follows the rule use-then-change   Prefix form    x --x  follows the rule change-then-use   Example 1  When multiple values are cascaded with  lt  lt  using cout then calculations if any  take place from right-to-left but printing takes place from left-to-right e g    if val if initially 10   cout lt  lt    val lt  lt  quot   quot  lt  lt  val   lt  lt  quot   quot  lt  lt  val   will result into 12    10    10   Example 2  In Turbo C    if multiple occurrences of    or  in any form  are found in an expression  then firstly all prefix forms are computed then expression is evaluated and finally postfix forms are computed e g   int a 10 b  b a       a     a   a  cout lt  lt b lt  lt a lt  lt endl   It s output in Turbo C   will be 48 13  Whereas it s output in modern day compiler will be  because they follow the rules strictly  45 13   Note  Multiple use of increment decrement operators on same variable in one expression is not recommended  The handling results of such expressions vary from compiler to compiler

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Understanding the language syntax is important when considering clarity of code   Consider copying a character string  for example with post-increment   char a 256     Hello world    char b 256   int i   0  do     b i    a i     while  a i        We want the loop to execute through encountering the zero character  which tests false  at the end of the string   That requires testing the value pre-increment and also incrementing the index   But not necessarily in that order - a way to code this with the pre-increment would be   int i   -1  do       i    b i    a i     while  a i      It is a matter of taste which is clearer and if the machine has a handfull of registers both should have identical execution time  even if a i  is a function that is expensive or has side-effects   A significant difference might be the exit value of the index

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Just wanted to re-emphasize that   x is expected to be faster than x     especially if x is an object of some arbitrary type   so unless required for logical reasons    x should be used

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From cppreference when incrementing iterators      You should prefer pre-increment   operator    iter  to post-increment   operator  iter    if you are not going   to use the old value  Post-increment   is generally implemented as follows       Iter operator   int           Iter tmp  this      store the old value in a temporary object         this             call pre-increment      return tmp          return the old value          Obviously  it s less efficient than   pre-increment    Pre-increment does not generate the temporary object  This can make a significant difference if your object is expensive to create

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I just want to notice that the geneated code is offen the same if you use pre post incrementation where the semantic  of pre post  doesn t matter   example   pre cpp    include  lt iostream gt   int main       int i   13    i      for    i  lt  42  i                std  cout  lt  lt  i  lt  lt  std  endl            post cpp    include  lt iostream gt   int main        int i   13      i    for    i  lt  42    i              std  cout  lt  lt  i  lt  lt  std  endl                 gt  g   -S pre cpp   gt  g   -S post cpp   gt  diff pre s post s    1c1  lt     file    pre cpp  ---  gt     file    post cpp

User · Answer

I agree with  BeowulfOF  though for clarity I would always advocate splitting the statements so that the logic is absolutely clear  i e    i    x    i    or  x    i  i      So my answer is if you write clear code then this should rarely matter  and if it matters then your code is probably not clear enough

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You explained the difference correctly  It just depends on if you want x to increment before every run through a loop  or after that  It depends on your program logic  what is appropriate   An important difference when dealing with STL-Iterators  which also implement these operators  is  that it   creates a copy of the object the iterator points to  then increments  and then returns the copy    it on the other hand does the increment first and then returns a reference to the object the iterator now points to  This is mostly just relevant when every bit of performance counts or when you implement your own STL-iterator   Edit  fixed the mixup of prefix and suffix notation

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The most important thing to keep in mind  imo  is that x   needs to return the value before the increment actually took place -- therefore  it has to make a temporary copy of the object  pre increment   This is less effecient than   x  which is incremented in-place and returned   Another thing worth mentioning  though  is that most compilers will be able to optimize such unnecessary things away when possible  for instance both options will lead to same code here   for  int i 0  i lt 10   i  for  int i 0  i lt 10 i

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Scott Meyers tells you to prefer prefix except on those occasions where logic would dictate that postfix is appropriate      More Effective C    item  6 - that s sufficient authority for me   For those who don t own the book  here are the pertinent quotes   From page 32      From your days as a C programmer  you may recall that the prefix form of the increment operator is sometimes called  increment and fetch   while the postfix form is often known as  fetch and increment    The two phrases are important to remember  because they all but act as formal specifications      And on page 34      If you re the kind who worries about efficiency  you probably broke into a sweat when you first saw the postfix increment function   That function has to create a temporary object for its return value and the implementation above also creates an explicit temporary object that has to be constructed and destructed   The prefix increment function has no such temporaries

User · Answer

It s not a question of preference  but of logic   x   increments the value of variable x after processing the current statement     x increments the value of variable x before processing the current statement   So just decide on the logic you write   x      i will increment i and add i 1 to x  x    i   will add i to x  then increment i

[c++] Incrementing in C++ - When to use x++ or ++x?

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Example 2:

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