How can I loop through a C map of maps

Question

How can I loop through a std  map in C    My map is defined as   std  map lt  std  string  std  map lt std  string  std  string gt   gt    For example  the above container holds data like this   m  name1    value1      data1   m  name1    value2      data2   m  name2    value1      data1   m  name2    value2      data2   m  name3    value1      data1   m  name3    value2      data2     How can I loop through this map and access the various values

User · Answer

As einpoklum mentioned in their answer  since C  17 you can also use structured binding declarations  I want to extend on that by providing a full solution for iterating over a map of maps in a comfortable way   int main         std  map lt std  string  std  map lt std  string  std  string gt  gt  m             name1      value1    data1      value2    data2                name2      value1    data1      value2    data2                name3      value1    data1      value2    data2                 for  const auto amp   k1  v1    m          for  const auto amp   k2  v2    v1              std  cout  lt  lt   m    lt  lt  k1  lt  lt        lt  lt  k2  lt  lt        lt  lt  v2  lt  lt  std  endl       return 0      Note 1  For filling the map  I used an initializer list  which is a C  11 feature   This can sometimes be handy to keep fixed initializations compact   Note 2  If you want to modify the map m within the loops  you have to remove the const keywords   Code on Coliru

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Do something like this   typedef std  map lt std  string  std  string gt  InnerMap  typedef std  map lt std  string  InnerMap gt  OuterMap   Outermap mm        set the initial values  for  OuterMap  iterator i   mm begin    i    mm end      i        InnerMap  amp im   i- gt second      for  InnerMap  iterator ii   im begin    ii    im end      ii            std  cout  lt  lt   map                       lt  lt  i- gt first                     lt  lt                           lt  lt  ii- gt first                     lt  lt                            lt  lt  ii- gt second                     lt  lt    n

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You can use an iterator   typedef std  map lt std  string  std  map lt std  string  std  string gt  gt   iterator it type  for it type iterator   m begin    iterator    m end    iterator             iterator- gt first   key        iterator- gt second   value        Repeat if you also want to iterate through the second map

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for std  map lt std  string  std  map lt std  string  std  string gt   gt   iterator outer iter map begin    outer iter  map end      outer iter        for std  map lt std  string  std  string gt   iterator inner iter outer iter- gt second begin    inner iter  outer iter- gt second end      inner iter            std  cout  lt  lt  inner iter- gt second  lt  lt  std  endl            or nicer in C  0x   for auto outer iter map begin    outer iter  map end      outer iter        for auto inner iter outer iter- gt second begin    inner iter  outer iter- gt second end      inner iter            std  cout  lt  lt  inner iter- gt second  lt  lt  std  endl

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Old question but the remaining answers are outdated as of C  11 - you can use a ranged based for loop and simply do   std  map lt std  string  std  map lt std  string  std  string gt  gt  mymap   for auto const  amp ent1   mymap         ent1 first is the first key   for auto const  amp ent2   ent1 second           ent2 first is the second key        ent2 second is the data         this should be much cleaner than the earlier versions  and avoids unnecessary copies    Some favour replacing the comments with explicit definitions of reference variables  which get optimised away if unused    for auto const  amp ent1   mymap      auto const  amp outer key   ent1 first    auto const  amp inner map   ent1 second    for auto const  amp ent2   inner map        auto const  amp inner key     ent2 first      auto const  amp inner value   ent2 second

User · Answer

First solution is Use range based for loop  like   Note  When range expression   s type is std  map then a range declaration   s type is std  pair   for   range declaration   range expression             loop statement   Code 1   typedef std  map lt std  string  std  map lt std  string  std  string gt  gt  StringToStringMap   StringToStringMap my map   for const auto  amp pair1   my map           Type of pair1 is std  pair lt std  string  std  map lt std  string  std  string gt  gt        pair1 first point to std  string  first key        pair1 second point to std  map lt std  string  std  string gt   inner map     for const auto  amp pair2   pair1 second                  pair2 first is the second inner  key           pair2 second is the value          The Second Solution   Code 2  typedef std  map lt std  string  std  string gt  StringMap  typedef std  map lt std  string  StringMap gt  StringToStringMap   StringToStringMap my map   for StringToStringMap  iterator it1   my map begin    it1    my map end    it1             it1- gt first point to first key        it2- gt second point to inner map     for StringMap  iterator it2   it1- gt second begin    it2    it1- gt second end    it2                      it2- gt second point to value            it2- gt first point to second inner  key

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C  11   std  map lt  std  string  std  map lt std  string  std  string gt   gt  m  m  name1    value1      data1   m  name1    value2      data2   m  name2    value1      data1   m  name2    value2      data2   m  name3    value1      data1   m  name3    value2      data2    for  auto i   m      for  auto j   i second          cout  lt  lt  i first c str    lt  lt       lt  lt  j first c str    lt  lt       lt  lt  j second c str    lt  lt  endl    output   name1 value1 data1 name1 value2 data2 name2 value1 data1 name2 value2 data2 name3 value1 data1 name3 value2 data2

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use std  map lt  std  string  std  map lt std  string  std  string gt   gt   const iterator when map is const

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With C  17  or later   you can use the  structured bindings  feature  which lets you define multiple variables  with different names  using a single tuple pair  Example   for  const auto amp   name  description    planet descriptions        std  cout  lt  lt   Planet    lt  lt  name  lt  lt     n   lt  lt  description  lt  lt    n n       The original proposal  by luminaries Bjarne Stroustrup  Herb Sutter and Gabriel Dos Reis  is fun to read  and the suggested syntax is more intuitive IMHO   there s also the proposed wording for the standard which is boring to read but is closer to what will actually go in

[c++] How can I loop through a C++ map of maps?

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