C Loop through Map

Question

I want to iterate through each element in the map lt string  int gt  without knowing any of its string-int values or keys   What I have so far   void output map lt string  int gt  table           map lt string  int gt   iterator it         for  it   table begin    it    table end    it                           How do I access each element

User · Answer

Try the following  for   const auto  amp p   table        std  cout  lt  lt  p first  lt  lt    t   lt  lt  p second  lt  lt  std  endl       The same can be written using an ordinary for loop  for   auto it   table begin    it    table end      it         std  cout  lt  lt  it- gt first  lt  lt    t   lt  lt  it- gt second  lt  lt  std  endl       Take into account that value type for std  map is defined the following way  typedef pair lt const Key  T gt  value type   Thus in my example p is a const reference to the value type where Key is std  string and T is int  Also it would be better if the function would be declared as  void output  const map lt string  int gt   amp table

User · Answer

The value type of a map is a pair containing the key and value as it s first and second member  respectively   map lt string  int gt   iterator it  for  it   symbolTable begin    it    symbolTable end    it          std  cout  lt  lt  it- gt first  lt  lt       lt  lt  it- gt second  lt  lt    n       Or with C  11  using range-based for   for  auto const amp  p   symbolTable        std  cout  lt  lt  p first  lt  lt       lt  lt  p second  lt  lt    n

User · Answer

As  Vlad from Moscow says  Take into account that value type for std  map is defined the following way   typedef pair lt const Key  T gt  value type   This then means that if you wish to replace the keyword auto with a more explicit type specifier  then you could this   for   const pair lt const string  int gt   amp p   table        std  cout  lt  lt  p first  lt  lt    t   lt  lt  p second  lt  lt  std  endl       Just for understanding what auto will translate to in this case

User · Answer

You can achieve this like following   map lt string  int gt   iterator it   for  it   symbolTable begin    it    symbolTable end    it          std  cout  lt  lt  it- gt first       string  key                 lt  lt                     lt  lt  it- gt second      string s value                 lt  lt  std  endl      With C  11   and onwards    for  auto const amp  x   symbolTable        std  cout  lt  lt  x first     string  key                 lt  lt                      lt  lt  x second    string s value                 lt  lt  std  endl      With C  17   and onwards    for  auto const amp   key  val    symbolTable        std  cout  lt  lt  key           string  key                 lt  lt                       lt  lt  val           string s value                lt  lt  std  endl

User · Answer

As P0W has provided complete syntax for each C   version  I would like to add couple of more points by looking at your code  Always take const  amp  as argument as to avoid extra copies of the same object  use unordered map as its always faster to use  See this discussion  here is a sample code   include  lt iostream gt   include  lt unordered map gt  using namespace std   void output const auto amp  table       for  auto const  amp   k  v    table               std  cout  lt  lt   quot Key   quot   lt  lt  k  lt  lt   quot  Value   quot   lt  lt  v  lt  lt  std  endl          int main         std  unordered map lt string  int gt  mydata               quot one quot   1             quot two quot   2             quot three quot   3             output mydata       return 0

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