assignment/initialization of container - map

Discussion in 'C++' started by xuatla, Jul 18, 2006.

  1. xuatla

    xuatla Guest

    Hi,

    I want to define a map:

    std::map<string, int> myMap;

    e.g., the score of students. Then I can assign the value as follows:
    myMap["stud1"] = 90;
    myMap["stud2"] = 60;
    ....

    My question now is: can I assign the name of many students in one line?
    e.g., for array we have the following way:
    int myArray[] = { 1, 3, 4, 5 };

    Do we have similar way for map?
    std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    // wrong code

    Another question: how can I print the score of a given student's name?

    void getScore(std::map<string, int> myMap, const std::string& stuName)
    {
    return myMap.find(stuName)->second();
    }

    Is this correct? Any better solution?

    Thanks in advance!

    -X
     
    xuatla, Jul 18, 2006
    #1
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  2. xuatla wrote:

    > I want to define a map:
    >
    > std::map<string, int> myMap;
    >
    > e.g., the score of students. Then I can assign the value as follows:
    > myMap["stud1"] = 90;
    > myMap["stud2"] = 60;
    > ...
    >
    > My question now is: can I assign the name of many students in one line?
    > e.g., for array we have the following way:
    > int myArray[] = { 1, 3, 4, 5 };
    >
    > Do we have similar way for map?


    No.

    > std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    > // wrong code
    >
    > Another question: how can I print the score of a given student's name?
    >
    > void getScore(std::map<string, int> myMap, const std::string& stuName)
    > {
    > return myMap.find(stuName)->second();
    > }
    >
    > Is this correct? Any better solution?


    First, unless you want to DRASTICALLY inefficient, better to pass the
    map by reference instead of by value. Second, you can't return a value
    if you declare the function void. Finally, although theoretically
    slightly less efficient, more clear in my view is to use operator[] and
    dump the separate function altogether:

    myMap[stuName]

    or if you insist on the separate function, make it:

    int getScore(std::map<string, int>& myMap, const std::string& stuName)
    {
    return myMap[stuName];
    }

    Best regards,

    Tom
     
    Thomas Tutone, Jul 18, 2006
    #2
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  3. xuatla

    Ron Natalie Guest

    xuatla wrote:
    > Hi,
    >
    > I want to define a map:
    >
    > std::map<string, int> myMap;
    >
    > e.g., the score of students. Then I can assign the value as follows:
    > myMap["stud1"] = 90;
    > myMap["stud2"] = 60;
    > ...
    >
    > My question now is: can I assign the name of many students in one line?
    > e.g., for array we have the following way:
    > int myArray[] = { 1, 3, 4, 5 };
    >
    > Do we have similar way for map?
    > std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    > // wrong co

    No, map's not an aggregate.

    The best you can do is something like:

    struct apair {
    const char* s;
    int i;
    } maptab[] = { { "stud1", 90 }, ....

    for(apair* ap = maptab; ap != sizeof maptab/sizeof (apair); ++ap)
    myMap[ap->s] = ap->i;
     
    Ron Natalie, Jul 18, 2006
    #3
  4. Ron Natalie wrote:
    > xuatla wrote:
    >> Hi,
    >>
    >> I want to define a map:
    >>
    >> std::map<string, int> myMap;
    >>
    >> e.g., the score of students. Then I can assign the value as follows:
    >> myMap["stud1"] = 90;
    >> myMap["stud2"] = 60;
    >> ...
    >>
    >> My question now is: can I assign the name of many students in one
    >> line? e.g., for array we have the following way:
    >> int myArray[] = { 1, 3, 4, 5 };
    >>
    >> Do we have similar way for map?
    >> std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    >> // wrong co

    > No, map's not an aggregate.
    >
    > The best you can do is something like:
    >
    > struct apair {
    > const char* s;
    > int i;
    > } maptab[] = { { "stud1", 90 }, ....
    >
    > for(apair* ap = maptab; ap != sizeof maptab/sizeof (apair); ++ap)
    > myMap[ap->s] = ap->i;


    Shouldn't this work

    std::map<string,int> myMap(maptab, maptab +
    sizeof(maptab)/sizeof(*maptab));

    ? You might find that using [] instead of .insert() is less efficient.

    V
    --
    Please remove capital 'A's when replying by e-mail
    I do not respond to top-posted replies, please don't ask
     
    Victor Bazarov, Jul 18, 2006
    #4
  5. xuatla

    Marcus Kwok Guest

    Thomas Tutone <> wrote:
    > xuatla wrote:
    >> Another question: how can I print the score of a given student's name?
    >>
    >> void getScore(std::map<string, int> myMap, const std::string& stuName)
    >> {
    >> return myMap.find(stuName)->second();
    >> }
    >>
    >> Is this correct? Any better solution?

    >
    > First, unless you want to DRASTICALLY inefficient, better to pass the
    > map by reference instead of by value. Second, you can't return a value
    > if you declare the function void. Finally, although theoretically
    > slightly less efficient, more clear in my view is to use operator[] and
    > dump the separate function altogether:
    >
    > myMap[stuName]
    >
    > or if you insist on the separate function, make it:
    >
    > int getScore(std::map<string, int>& myMap, const std::string& stuName)
    > {
    > return myMap[stuName];
    > }


    However, if stuName is not already in the map, the myMap[stuName]
    version will automatically create an entry for it, and
    default-initialize(?, maybe it's zero-initialize) it. Using
    myMap.find(), this case can be handled separately if desired.

    // untested and uncompiled
    int getScore(const std::map<std::string, int>& myMap,
    const std::string& stuName)
    {
    std::map<std::string, int>::const_iterator i = myMap.find(stuName);

    if (i != myMap.end()) {
    return i->second;
    }
    else {
    return -1;
    }
    }

    I do agree that the myMap[stuName] syntax is easier to read though,
    however it cannot be used on a const map (or a reference to a const map)
    because of the automatic-entry-creation behavior.

    --
    Marcus Kwok
    Replace 'invalid' with 'net' to reply
     
    Marcus Kwok, Jul 18, 2006
    #5
  6. Victor Bazarov wrote:
    > Ron Natalie wrote:
    > > xuatla wrote:


    > >> I want to define a map:
    > >>
    > >> std::map<string, int> myMap;
    > >>
    > >> e.g., the score of students. Then I can assign the value as follows:
    > >> myMap["stud1"] = 90;
    > >> myMap["stud2"] = 60;
    > >> ...
    > >>
    > >> My question now is: can I assign the name of many students in one
    > >> line? e.g., for array we have the following way:
    > >> int myArray[] = { 1, 3, 4, 5 };
    > >>
    > >> Do we have similar way for map?
    > >> std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    > >> // wrong co

    > > No, map's not an aggregate.
    > >
    > > The best you can do is something like:
    > >
    > > struct apair {
    > > const char* s;
    > > int i;
    > > } maptab[] = { { "stud1", 90 }, ....
    > >
    > > for(apair* ap = maptab; ap != sizeof maptab/sizeof (apair); ++ap)
    > > myMap[ap->s] = ap->i;

    >
    > Shouldn't this work
    >
    > std::map<string,int> myMap(maptab, maptab +
    > sizeof(maptab)/sizeof(*maptab));


    I could be wrong about this, but I don't think that would work. The
    map<string, int> constructor would be expecting iterators pointing to
    pair<string, int>, and would get instead an iterator to struct { const
    char*, int }. Unless there's some sort of implicit conversion going on
    that I don't understand, your example shouldn't compile. Even if it
    were a map<const char*, int>, I think it still wouldn't work, because a
    struct { const char*, int } is different from a pair<const char*, int>.

    Or maybe I'm wrong.

    Best regards,

    Tom
     
    Thomas Tutone, Jul 18, 2006
    #6
  7. xuatla

    Ron Natalie Guest

    Victor Bazarov wrote:
    \
    > Shouldn't this work
    >
    > std::map<string,int> myMap(maptab, maptab +
    > sizeof(maptab)/sizeof(*maptab));
    >
    > ? You might find that using [] instead of .insert() is less efficient.
    >


    I would have if I had used pairs. However, you can't use aggregate
    initializers on pairs so I had to choose between making the insert
    nice or the static initializer.
     
    Ron Natalie, Jul 18, 2006
    #7
  8. xuatla

    mlimber Guest

    Thomas Tutone wrote:
    > xuatla wrote:
    >
    > > I want to define a map:
    > >
    > > std::map<string, int> myMap;
    > >
    > > e.g., the score of students. Then I can assign the value as follows:
    > > myMap["stud1"] = 90;
    > > myMap["stud2"] = 60;
    > > ...
    > >
    > > My question now is: can I assign the name of many students in one line?
    > > e.g., for array we have the following way:
    > > int myArray[] = { 1, 3, 4, 5 };
    > >
    > > Do we have similar way for map?

    >
    > No.


    There are several similar but not identical ways for std::map. See the
    other responses in this thread for initializing from an array, and also
    consider a helper class that uses method chaining (see the FAQ for more
    on that):

    template<class K, class V>
    class MapInitializer
    {
    typedef std::map<K,V> Map;
    Map m_;
    public:
    operator Map() const { return m_; }

    MapInitializer& Add( const K& k, const V& v )
    {
    m_[k] = v;
    return *this;
    }
    };

    const std::map<int,std::string> msgMap
    = MapInitializer<int,std::string>()
    .Add( 1, "Msg 1" )
    .Add( 2, "Msg 2" )
    .Add( 42, "Msg 3" );

    Cheers! --M
     
    mlimber, Jul 18, 2006
    #8
  9. xuatla

    xuatla Guest

    Thomas Tutone wrote:
    > xuatla wrote:
    >
    > > I want to define a map:
    > >
    > > std::map<string, int> myMap;
    > >
    > > e.g., the score of students. Then I can assign the value as follows:
    > > myMap["stud1"] = 90;
    > > myMap["stud2"] = 60;
    > > ...
    > >
    > > My question now is: can I assign the name of many students in one line?
    > > e.g., for array we have the following way:
    > > int myArray[] = { 1, 3, 4, 5 };
    > >
    > > Do we have similar way for map?

    >
    > No.
    >
    > > std::map<string, int> myMap = { ("stud1", 90), ("stud2", 60) };
    > > // wrong code
    > >
    > > Another question: how can I print the score of a given student's name?
    > >
    > > void getScore(std::map<string, int> myMap, const std::string& stuName)
    > > {
    > > return myMap.find(stuName)->second();
    > > }
    > >
    > > Is this correct? Any better solution?

    >
    > First, unless you want to DRASTICALLY inefficient, better to pass the
    > map by reference instead of by value. Second, you can't return a value
    > if you declare the function void. Finally, although theoretically
    > slightly less efficient, more clear in my view is to use operator[] and
    > dump the separate function altogether:
    >
    > myMap[stuName]
    >
    > or if you insist on the separate function, make it:
    >
    > int getScore(std::map<string, int>& myMap, const std::string& stuName)
    > {
    > return myMap[stuName];
    > }


    Thanks for your reply. "void" is my typo. I think I was dumb when I
    stated my question in above way. I got the answers from the replies
    here now. Thanks to all.

    - X

    btw : an off-topic question: I use thunderbird to read newsgroups.
    Today I found that I couldn't read the latest threads in it (the most
    recent ones shown was posted at yesterday). Is there anyone else
    encountered same questions? Which software do you think is best for
    newsgroups reading/posting? Now I am using Google groups to read the
    threads here. (I just came back from Mars and this is my first time
    using Google groups for newsgroups...)


    >
    > Best regards,
    >
    > Tom
     
    xuatla, Jul 18, 2006
    #9
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