Iteration over std vector unsigned vs signed index variable

Question

What is the correct way of iterating over a vector in C     Consider these two code fragments  this one works fine   for  unsigned i 0  i  lt  polygon size    i          sum    polygon i       and this one   for  int i 0  i  lt  polygon size    i          sum    polygon i       which generates warning  comparison between signed and unsigned integer expressions   I m new in the world of C    so the unsigned variable looks a bit frightening to me and I know unsigned variables can be dangerous if not used correctly  so - is this correct

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for  vector lt int gt   iterator it   polygon begin    it    polygon end    it        sum     it

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Obscure but important detail  if you say  for auto it   as follows  you get a copy of the object  not the actual element   struct Xs int i  x  x i   0  vector  lt Xs gt  v  v push back x   for auto it   v      it i   1             doesn t change the element v 0    To modify the elements of the vector  you need to define the iterator as a reference   for auto  amp it   v

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Use size t   for  size t i 0  i  lt  polygon size    i     Quoting Wikipedia   The stdlib h and stddef h header files define a datatype called size t which is used to represent the size of an object  Library functions that take sizes expect them to be of type size t  and the sizeof operator evaluates to size t  The actual type of size t is platform-dependent  a common mistake is to assume size t is the same as unsigned int  which can lead to programming errors  particularly as 64-bit architectures become more prevalent

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A call to vector lt T gt   size   returns a value of type std  vector lt T gt   size type  not int  unsigned int or otherwise   Also generally iteration over a container in C   is done using iterators  like this   std  vector lt T gt   iterator i   polygon begin    std  vector lt T gt   iterator end   polygon end     for   i    end  i         sum     i      Where T is the type of data you store in the vector   Or using the different iteration algorithms  std  transform  std  copy  std  fill  std  for each et cetera

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Regarding Johannes Schaub s answer   for std  vector lt T  gt   iterator it   v begin    it    v end      it             That may work with some compilers but not with gcc  The problem here is the question if std  vector  iterator is a type  a variable  member  or a function  method   We get the following error with gcc   In member function    void MyClass lt T gt   myMethod       error  expected     before    it    error     it    was not declared in this scope In member function    void MyClass lt T gt   sort    with T   MyClass      instantiated from    void MyClass lt T gt   run    with T   MyClass     instantiated from here dependent-name    std  vector lt T  std  allocator lt T  gt   gt   iterator    is parsed as a non-type  but instantiation yields a type note  say    typename std  vector lt T  std  allocator lt T  gt   gt   iterator    if a type is meant   The solution is using the keyword  typename  as told   typename std  vector lt T  gt   iterator it   v begin    for    it    v end      it

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A bit of history   To represent whether a number is negative or not computer use a  sign  bit   int is a signed data type meaning it can hold positive and negative values  about -2billion to 2billion    Unsigned can only store positive numbers  and since it doesn t waste a bit on metadata it can store more  0 to about 4billion    std  vector  size   returns an unsigned  for how could a vector have negative length   The warning is telling you that the right operand of your inequality statement can hold more data then the left   Essentially if you have a vector with more then 2 billion entries and you use an integer to index into you ll hit overflow problems  the int will wrap back around to negative 2 billion

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Adding this as I couldn t find it mentioned in any answer  for index-based iteration  we can use decltype vec name size    which would evaluate to std  vector lt T gt   size type Example for decltype v size    i  0    i  lt  v size    i             std  cout  lt  lt  v i

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If your compiler supports it  you could use a range based for to access the vector elements   vector lt float gt  vertices  1 0  2 0  3 0     for float vertex  vertices       std  cout  lt  lt  vertex  lt  lt           Prints  1 2 3   Note  you can t use this technique for changing the elements of the vector

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In the specific case in your example  I d use the STL algorithms to accomplish this     include  lt numeric gt    sum   std  accumulate  polygon begin    polygon end    0      For a more general  but still fairly simple case  I d go with    include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt   using namespace boost  lambda  std  for each  polygon begin    polygon end    sum     1

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A call to vector lt T gt   size   returns a value of type std  vector lt T gt   size type  not int  unsigned int or otherwise   Also generally iteration over a container in C   is done using iterators  like this   std  vector lt T gt   iterator i   polygon begin    std  vector lt T gt   iterator end   polygon end     for   i    end  i         sum     i      Where T is the type of data you store in the vector   Or using the different iteration algorithms  std  transform  std  copy  std  fill  std  for each et cetera

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I usually use BOOST FOREACH    include  lt boost foreach hpp gt   BOOST FOREACH  vector type  value type amp  value  v            do something with  value      It works on STL containers  arrays  C-style strings  etc

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for  vector lt int gt   iterator it   polygon begin    it    polygon end    it        sum     it

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For iterating backwards see this answer    Iterating forwards is almost identical  Just change the iterators   swap decrement by increment  You should prefer iterators  Some people tell you to use std  size t as the index variable type  However  that is not portable  Always use the size type typedef of the container  While you could get away with only a conversion in the forward iterating case  it could actually go wrong all the way in the backward iterating case when using std  size t  in case std  size t is wider than what is the typedef of size type      Using std  vector  Using iterators  for std  vector lt T gt   iterator it   v begin    it    v end      it           std  cout  lt  lt   it             Important is  always use the prefix increment form for iterators whose definitions you don t know  That will ensure your code runs as generic as possible    Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  vector lt int gt   size type i   0  i    v size    i             std  cout  lt  lt  v i                Using arrays  Using iterators  for element type  it   a  it     a    sizeof a   sizeof  a    it             std  cout  lt  lt   it             Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  size t i   0  i     sizeof a   sizeof  a   i             std  cout  lt  lt  a i              Read in the backward iterating answer what problem the sizeof approach can yield to  though

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Consider whether you need to iterate at all  The  lt algorithm gt  standard header provides us with facilities for this   using std  begin      allows argument-dependent lookup even using std  end        if the container type is unknown here auto sum   std  accumulate begin polygon   end polygon   0     Other functions in the algorithm library perform common tasks - make sure you know what s available if you want to save yourself effort

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for  vector lt int gt   iterator it   polygon begin    it    polygon end    it        sum     it

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In the specific case in your example  I d use the STL algorithms to accomplish this     include  lt numeric gt    sum   std  accumulate  polygon begin    polygon end    0      For a more general  but still fairly simple case  I d go with    include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt   using namespace boost  lambda  std  for each  polygon begin    polygon end    sum     1

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A call to vector lt T gt   size   returns a value of type std  vector lt T gt   size type  not int  unsigned int or otherwise   Also generally iteration over a container in C   is done using iterators  like this   std  vector lt T gt   iterator i   polygon begin    std  vector lt T gt   iterator end   polygon end     for   i    end  i         sum     i      Where T is the type of data you store in the vector   Or using the different iteration algorithms  std  transform  std  copy  std  fill  std  for each et cetera

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If your compiler supports it  you could use a range based for to access the vector elements   vector lt float gt  vertices  1 0  2 0  3 0     for float vertex  vertices       std  cout  lt  lt  vertex  lt  lt           Prints  1 2 3   Note  you can t use this technique for changing the elements of the vector

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Four years passed  Google gave me this answer  With the standard C  11  aka C  0x  there is actually a new pleasant way of doing this  at the price of breaking backward compatibility   the new auto keyword  It saves you the pain of having to explicitly specify the type of the iterator to use  repeating the vector type again   when it is obvious  to the compiler   which type to use  With v being your vector  you can do something like this   for   auto i   v begin    i    v end    i           std  cout  lt  lt   i  lt  lt  std  endl      C  11 goes even further and gives you a special syntax for iterating over collections like vectors  It removes the necessity of writing things that are always the same   for   auto  amp i   v         std  cout  lt  lt  i  lt  lt  std  endl      To see it in a working program  build a file auto cpp    include  lt vector gt   include  lt iostream gt   int main void        std  vector lt int gt  v   std  vector lt int gt         v push back 17       v push back 12       v push back 23       v push back 42       for   auto  amp i   v             std  cout  lt  lt  i  lt  lt  std  endl            return 0      As of writing this  when you compile this with g    you normally need to set it to work with the new standard by giving an extra flag   g   -std c  0x -o auto auto cpp   Now you can run the example       auto 17 12 23 42   Please note that the instructions on compiling and running are specific to gnu c   compiler on Linux  the program should be platform  and compiler  independent

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I usually use BOOST FOREACH    include  lt boost foreach hpp gt   BOOST FOREACH  vector type  value type amp  value  v            do something with  value      It works on STL containers  arrays  C-style strings  etc

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The first is type correct  and correct in some strict sense    If you think about is  size can never be less than zero    That warning strikes me as one of the good candidates for being ignored  though

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In the specific case in your example  I d use the STL algorithms to accomplish this     include  lt numeric gt    sum   std  accumulate  polygon begin    polygon end    0      For a more general  but still fairly simple case  I d go with    include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt   using namespace boost  lambda  std  for each  polygon begin    polygon end    sum     1

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The two code segments work the same  However  unsigned int  route is correct  Using unsigned int types will work better with the vector in the instance you used it  Calling the size   member function on a vector returns an unsigned integer value  so you want to be comparing the variable  i  to a value of its own type   Also  if you are still a little uneasy about how  unsigned int  looks in your code  try  uint   This is basically a shortened version of  unsigned int  and it works exactly the same  You also don t need to include other headers to use it

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for  vector lt int gt   iterator it   polygon begin    it    polygon end    it        sum     it

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To be complete  C  11 syntax enables just one another version for iterators  ref    for auto it std  begin polygon   it  std  end polygon     it         do something with  it     Which is also comfortable for reverse iteration  for auto it std  end polygon -1  it  std  begin polygon -1  --it         do something with  it

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I usually use BOOST FOREACH    include  lt boost foreach hpp gt   BOOST FOREACH  vector type  value type amp  value  v            do something with  value      It works on STL containers  arrays  C-style strings  etc

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Adding this as I couldn t find it mentioned in any answer  for index-based iteration  we can use decltype vec name size    which would evaluate to std  vector lt T gt   size type Example for decltype v size    i  0    i  lt  v size    i             std  cout  lt  lt  v i

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For iterating backwards see this answer    Iterating forwards is almost identical  Just change the iterators   swap decrement by increment  You should prefer iterators  Some people tell you to use std  size t as the index variable type  However  that is not portable  Always use the size type typedef of the container  While you could get away with only a conversion in the forward iterating case  it could actually go wrong all the way in the backward iterating case when using std  size t  in case std  size t is wider than what is the typedef of size type      Using std  vector  Using iterators  for std  vector lt T gt   iterator it   v begin    it    v end      it           std  cout  lt  lt   it             Important is  always use the prefix increment form for iterators whose definitions you don t know  That will ensure your code runs as generic as possible    Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  vector lt int gt   size type i   0  i    v size    i             std  cout  lt  lt  v i                Using arrays  Using iterators  for element type  it   a  it     a    sizeof a   sizeof  a    it             std  cout  lt  lt   it             Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  size t i   0  i     sizeof a   sizeof  a   i             std  cout  lt  lt  a i              Read in the backward iterating answer what problem the sizeof approach can yield to  though

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The first is type correct  and correct in some strict sense    If you think about is  size can never be less than zero    That warning strikes me as one of the good candidates for being ignored  though

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A bit of history   To represent whether a number is negative or not computer use a  sign  bit   int is a signed data type meaning it can hold positive and negative values  about -2billion to 2billion    Unsigned can only store positive numbers  and since it doesn t waste a bit on metadata it can store more  0 to about 4billion    std  vector  size   returns an unsigned  for how could a vector have negative length   The warning is telling you that the right operand of your inequality statement can hold more data then the left   Essentially if you have a vector with more then 2 billion entries and you use an integer to index into you ll hit overflow problems  the int will wrap back around to negative 2 billion

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I usually use BOOST FOREACH    include  lt boost foreach hpp gt   BOOST FOREACH  vector type  value type amp  value  v            do something with  value      It works on STL containers  arrays  C-style strings  etc

User · Answer

To be complete  C  11 syntax enables just one another version for iterators  ref    for auto it std  begin polygon   it  std  end polygon     it         do something with  it     Which is also comfortable for reverse iteration  for auto it std  end polygon -1  it  std  begin polygon -1  --it         do something with  it

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The first is type correct  and correct in some strict sense    If you think about is  size can never be less than zero    That warning strikes me as one of the good candidates for being ignored  though

User · Answer

The first is type correct  and correct in some strict sense    If you think about is  size can never be less than zero    That warning strikes me as one of the good candidates for being ignored  though

User · Answer

A bit of history   To represent whether a number is negative or not computer use a  sign  bit   int is a signed data type meaning it can hold positive and negative values  about -2billion to 2billion    Unsigned can only store positive numbers  and since it doesn t waste a bit on metadata it can store more  0 to about 4billion    std  vector  size   returns an unsigned  for how could a vector have negative length   The warning is telling you that the right operand of your inequality statement can hold more data then the left   Essentially if you have a vector with more then 2 billion entries and you use an integer to index into you ll hit overflow problems  the int will wrap back around to negative 2 billion

User · Answer

The two code segments work the same  However  unsigned int  route is correct  Using unsigned int types will work better with the vector in the instance you used it  Calling the size   member function on a vector returns an unsigned integer value  so you want to be comparing the variable  i  to a value of its own type   Also  if you are still a little uneasy about how  unsigned int  looks in your code  try  uint   This is basically a shortened version of  unsigned int  and it works exactly the same  You also don t need to include other headers to use it

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Regarding Johannes Schaub s answer   for std  vector lt T  gt   iterator it   v begin    it    v end      it             That may work with some compilers but not with gcc  The problem here is the question if std  vector  iterator is a type  a variable  member  or a function  method   We get the following error with gcc   In member function    void MyClass lt T gt   myMethod       error  expected     before    it    error     it    was not declared in this scope In member function    void MyClass lt T gt   sort    with T   MyClass      instantiated from    void MyClass lt T gt   run    with T   MyClass     instantiated from here dependent-name    std  vector lt T  std  allocator lt T  gt   gt   iterator    is parsed as a non-type  but instantiation yields a type note  say    typename std  vector lt T  std  allocator lt T  gt   gt   iterator    if a type is meant   The solution is using the keyword  typename  as told   typename std  vector lt T  gt   iterator it   v begin    for    it    v end      it

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Use size t   for  size t i 0  i  lt  polygon size    i     Quoting Wikipedia   The stdlib h and stddef h header files define a datatype called size t which is used to represent the size of an object  Library functions that take sizes expect them to be of type size t  and the sizeof operator evaluates to size t  The actual type of size t is platform-dependent  a common mistake is to assume size t is the same as unsigned int  which can lead to programming errors  particularly as 64-bit architectures become more prevalent

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A call to vector lt T gt   size   returns a value of type std  vector lt T gt   size type  not int  unsigned int or otherwise   Also generally iteration over a container in C   is done using iterators  like this   std  vector lt T gt   iterator i   polygon begin    std  vector lt T gt   iterator end   polygon end     for   i    end  i         sum     i      Where T is the type of data you store in the vector   Or using the different iteration algorithms  std  transform  std  copy  std  fill  std  for each et cetera

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For iterating backwards see this answer    Iterating forwards is almost identical  Just change the iterators   swap decrement by increment  You should prefer iterators  Some people tell you to use std  size t as the index variable type  However  that is not portable  Always use the size type typedef of the container  While you could get away with only a conversion in the forward iterating case  it could actually go wrong all the way in the backward iterating case when using std  size t  in case std  size t is wider than what is the typedef of size type      Using std  vector  Using iterators  for std  vector lt T gt   iterator it   v begin    it    v end      it           std  cout  lt  lt   it             Important is  always use the prefix increment form for iterators whose definitions you don t know  That will ensure your code runs as generic as possible    Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  vector lt int gt   size type i   0  i    v size    i             std  cout  lt  lt  v i                Using arrays  Using iterators  for element type  it   a  it     a    sizeof a   sizeof  a    it             std  cout  lt  lt   it             Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  size t i   0  i     sizeof a   sizeof  a   i             std  cout  lt  lt  a i              Read in the backward iterating answer what problem the sizeof approach can yield to  though

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In C  11 I would use general algorithms like for each to avoid searching for the right type of iterator and lambda expression to avoid extra named functions objects  The short  quot pretty quot  example for your particular case  assuming polygon is a vector of integers   for each polygon begin    polygon end      amp sum  int i   sum    i       tested on  http   ideone com i6Ethd Dont  forget to include  algorithm and  of course  vector    Microsoft has actually also a nice example on this  source  http   msdn microsoft com en-us library dd293608 aspx  include  lt algorithm gt   include  lt iostream gt   include  lt vector gt  using namespace std   int main            Create a vector object that contains 10 elements     vector lt int gt  v     for  int i   1  i  lt  10    i          v push back i               Count the number of even numbers in the vector by        using the for each function and a lambda     int evenCount   0     for each v begin    v end      amp evenCount   int n          cout  lt  lt  n        if  n   2    0             cout  lt  lt   quot  is even  quot   lt  lt  endl             evenCount          else            cout  lt  lt   quot  is odd  quot   lt  lt  endl                        Print the count of even numbers to the console     cout  lt  lt   quot There are  quot   lt  lt  evenCount           lt  lt   quot  even numbers in the vector  quot   lt  lt  endl

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Use size t   for  size t i 0  i  lt  polygon size    i     Quoting Wikipedia   The stdlib h and stddef h header files define a datatype called size t which is used to represent the size of an object  Library functions that take sizes expect them to be of type size t  and the sizeof operator evaluates to size t  The actual type of size t is platform-dependent  a common mistake is to assume size t is the same as unsigned int  which can lead to programming errors  particularly as 64-bit architectures become more prevalent

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In C  11 I would use general algorithms like for each to avoid searching for the right type of iterator and lambda expression to avoid extra named functions objects  The short  quot pretty quot  example for your particular case  assuming polygon is a vector of integers   for each polygon begin    polygon end      amp sum  int i   sum    i       tested on  http   ideone com i6Ethd Dont  forget to include  algorithm and  of course  vector    Microsoft has actually also a nice example on this  source  http   msdn microsoft com en-us library dd293608 aspx  include  lt algorithm gt   include  lt iostream gt   include  lt vector gt  using namespace std   int main            Create a vector object that contains 10 elements     vector lt int gt  v     for  int i   1  i  lt  10    i          v push back i               Count the number of even numbers in the vector by        using the for each function and a lambda     int evenCount   0     for each v begin    v end      amp evenCount   int n          cout  lt  lt  n        if  n   2    0             cout  lt  lt   quot  is even  quot   lt  lt  endl             evenCount          else            cout  lt  lt   quot  is odd  quot   lt  lt  endl                        Print the count of even numbers to the console     cout  lt  lt   quot There are  quot   lt  lt  evenCount           lt  lt   quot  even numbers in the vector  quot   lt  lt  endl

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Four years passed  Google gave me this answer  With the standard C  11  aka C  0x  there is actually a new pleasant way of doing this  at the price of breaking backward compatibility   the new auto keyword  It saves you the pain of having to explicitly specify the type of the iterator to use  repeating the vector type again   when it is obvious  to the compiler   which type to use  With v being your vector  you can do something like this   for   auto i   v begin    i    v end    i           std  cout  lt  lt   i  lt  lt  std  endl      C  11 goes even further and gives you a special syntax for iterating over collections like vectors  It removes the necessity of writing things that are always the same   for   auto  amp i   v         std  cout  lt  lt  i  lt  lt  std  endl      To see it in a working program  build a file auto cpp    include  lt vector gt   include  lt iostream gt   int main void        std  vector lt int gt  v   std  vector lt int gt         v push back 17       v push back 12       v push back 23       v push back 42       for   auto  amp i   v             std  cout  lt  lt  i  lt  lt  std  endl            return 0      As of writing this  when you compile this with g    you normally need to set it to work with the new standard by giving an extra flag   g   -std c  0x -o auto auto cpp   Now you can run the example       auto 17 12 23 42   Please note that the instructions on compiling and running are specific to gnu c   compiler on Linux  the program should be platform  and compiler  independent

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Use size t   for  size t i 0  i  lt  polygon size    i     Quoting Wikipedia   The stdlib h and stddef h header files define a datatype called size t which is used to represent the size of an object  Library functions that take sizes expect them to be of type size t  and the sizeof operator evaluates to size t  The actual type of size t is platform-dependent  a common mistake is to assume size t is the same as unsigned int  which can lead to programming errors  particularly as 64-bit architectures become more prevalent

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A bit of history   To represent whether a number is negative or not computer use a  sign  bit   int is a signed data type meaning it can hold positive and negative values  about -2billion to 2billion    Unsigned can only store positive numbers  and since it doesn t waste a bit on metadata it can store more  0 to about 4billion    std  vector  size   returns an unsigned  for how could a vector have negative length   The warning is telling you that the right operand of your inequality statement can hold more data then the left   Essentially if you have a vector with more then 2 billion entries and you use an integer to index into you ll hit overflow problems  the int will wrap back around to negative 2 billion

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In the specific case in your example  I d use the STL algorithms to accomplish this     include  lt numeric gt    sum   std  accumulate  polygon begin    polygon end    0      For a more general  but still fairly simple case  I d go with    include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt   using namespace boost  lambda  std  for each  polygon begin    polygon end    sum     1

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Consider whether you need to iterate at all  The  lt algorithm gt  standard header provides us with facilities for this   using std  begin      allows argument-dependent lookup even using std  end        if the container type is unknown here auto sum   std  accumulate begin polygon   end polygon   0     Other functions in the algorithm library perform common tasks - make sure you know what s available if you want to save yourself effort

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Obscure but important detail  if you say  for auto it   as follows  you get a copy of the object  not the actual element   struct Xs int i  x  x i   0  vector  lt Xs gt  v  v push back x   for auto it   v      it i   1             doesn t change the element v 0    To modify the elements of the vector  you need to define the iterator as a reference   for auto  amp it   v

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For iterating backwards see this answer    Iterating forwards is almost identical  Just change the iterators   swap decrement by increment  You should prefer iterators  Some people tell you to use std  size t as the index variable type  However  that is not portable  Always use the size type typedef of the container  While you could get away with only a conversion in the forward iterating case  it could actually go wrong all the way in the backward iterating case when using std  size t  in case std  size t is wider than what is the typedef of size type      Using std  vector  Using iterators  for std  vector lt T gt   iterator it   v begin    it    v end      it           std  cout  lt  lt   it             Important is  always use the prefix increment form for iterators whose definitions you don t know  That will ensure your code runs as generic as possible    Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  vector lt int gt   size type i   0  i    v size    i             std  cout  lt  lt  v i                Using arrays  Using iterators  for element type  it   a  it     a    sizeof a   sizeof  a    it             std  cout  lt  lt   it             Using Range C  11  for auto const amp  value  a            std  cout  lt  lt  value           Using indices  for std  size t i   0  i     sizeof a   sizeof  a   i             std  cout  lt  lt  a i              Read in the backward iterating answer what problem the sizeof approach can yield to  though

[c++] Iteration over std::vector: unsigned vs signed index variable

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