Can someone explain to me how to use std::map?

[SomeAmazingGuy]

I am having a hard time completely understanding the concept behind std::map.

From what i understand so far, std::map is a container (similar to an array/vector) and is capable of storing any type of variables.. but i can't really picture it in my mind... does it work similarly to how we store values in subscripts of an array?

[mutexe]

http://www.cprogramming.com/tutorial/stl/stlmap.html

[shadowmouse]

Map is an associative container, making it fundamentally different to arrays and vectors. What a map does is store a list of keys in order, meaning the key type must have a < operator. What it does is associate a mapped value to each key, which means the keys have to be unique, although the mapped values can be similar. A simple std::map<std::string,int> can be thought of like a run-time enum. It associates an int with a string so that each object in the array now has a name (which is a string) and is accessed via map["string-here"], which returns an int. As it is an STL container (Standard Template Library), you are quite correct at thinking they can contain any type, as a mapped value, but only orderable types as key values.

[SomeAmazingGuy]

Oh i think i understand now, so when we make a map like this..

std::map<std::string , char> grades;

then we have created an "array" but we haven't specified the size of the array. So in order for us to initialize a "subscript" in this "array" then we have to do this...

grades ["SomeAmazingGuy"] = 'F';

so now i have just initialized the first element of this map.. and to access this subscript, i have to use the string literal "SomeAmazingGuy" inside the brackets

I have one more question... is there a "faster" way to initialize ,lets say, 500 subscripts in a map?

[shadowmouse]

The point of a map is that each mapped value has its own unique key value, so usually, you don't want to create things based on some pattern which you could put in a loop. In the example you used where you are linking a string and a letter (the string doesn't have to be a literal but it can be), unless there is a fixed pattern between key and mapped value and the key changes in a constant way, then no, you can't do that. Why do you want to create 500 "subscripts" (if I understand you correctly, I believe you mean elements).

[SomeAmazingGuy]

Yes i meant to say elements. And the reason i want to make 500 elements is just to experiment with maps, i don't have a special reason on why i want to do that.

So i guess the only way to initialize the elements in a map is to manually do each one individually then ....

[shadowmouse]

Technically you can use a loop to initialise a map based on predictable values, but this isn't usually what maps are used for. For example you could make a map which imitates an array std::map<int,std::string> You could then make a program that adds 500 randomly generating strings to the map, but when you are doing something like this, there is no point not just using an array.

[andywestken]

You could use a loop to add unpredictable values?

First 10 of 1000 values:

Ababali Ucivu         A
Abehic Nasujev        E
Abosod Iwow           D
Abucuje Udunu         E
Abufi Qod             E
Abumaxuki Origet      B
Abunuvamole Juqoza    E
Aceboqi Umelo         D
Acerexesi Letumaf     E
Aciqen Saguzejoq      A

I wrote the function GenerateRandomName() to help with testing search routines; the generated "names" have a similar lengths and mix of vowels and consonants to real names, which is what mattered. (The way I chose the length of the names comes form a web site where it said X% of names are Y chars long, with the help of Excel to accumulate the values and scale them to round numbers.)

Andy

#include <iostream>
#include <iomanip>
#include <string>
#include <map>
#include <cstdlib>
#include <ctime>
#include <cassert>

namespace Utils {

inline int Random(int min, int max) {
    assert(min <= max);
    double temp = static_cast<double>(rand()) / (static_cast<double>(RAND_MAX) + 1.0);
    return static_cast<int>(min + temp * (max - min + 1));
}

std::string GenerateRandomName();

} // end namespace Utils

int main() {
    std::srand(static_cast<unsigned>(std::time(0)));
    // for testing could use fixed seed

    std::cout << std::left;

    std::map<std::string, char> studentGrades;

    const int count = 1000;

    for(int i = 0; i < count; ++i) {
        std::string firstName = Utils::GenerateRandomName();
        std::string lastName  = Utils::GenerateRandomName();

        std::string name = firstName;
        name += " ";
        name += lastName;

        // assumes grade = A B C D E F
        int gradeOffset = Utils::Random(0, 5);
        char grade = static_cast<char>('A' + gradeOffset);
        //for debugging
        //std::cout << "Add : " << name << " " << grade << "\n";
        studentGrades.insert(make_pair(name, grade));
    }

    const int maxDisplayCount = 10;
    int displayCount = 0;

    std::cout << "First "
        << std::min(maxDisplayCount, static_cast<int>(studentGrades.size()))
        << " of " << studentGrades.size() << " values:\n\n";

    std::map<std::string, char>::iterator iter = studentGrades.begin();

    while((iter != studentGrades.end()) && (maxDisplayCount > displayCount)) {
        std::cout << std::setw(20) << (*iter).first << "  " << (*iter).second << "\n";
        ++iter;
        ++displayCount;
    }

    return 0;
}

namespace RandNameData {

static const char* vowels[]= { "a", "e", "i", "o", "u" };
static const size_t vowelCount = sizeof(vowels)/sizeof(vowels[0]);

static const char* consonants[] = { "b", "c", "d", "f", "g", "h", "j",
    "k", "l", "m", "n", "p", "q", "r", "s", "t", "v", "w", "x", "z" };
static const size_t consonantCount = sizeof(consonants)/sizeof(consonants[0]);

} // end namespace RandNameData

std::string Utils::GenerateRandomName() {
    using namespace RandNameData;

    std::string name;

    bool vowelWanted = (1 == Random(0, 1));

    size_t targetLen = 0;

    int temp = Random(1, 663);
    if(650 < temp) targetLen = 11;
    else if(615 < temp) targetLen = 10;
    else if(525 < temp) targetLen = 9;
    else if(525 < temp) targetLen = 8;
    else if(400 < temp) targetLen = 7;
    else if(225 < temp) targetLen = 6;
    else if( 80 < temp) targetLen = 5;
    else if( 12 < temp) targetLen = 4;
    else if(  1 < temp) targetLen = 3;
    else targetLen = 2;

    while(targetLen > name.length()) {
        if(vowelWanted) {
            int index = Random(0, vowelCount - 1);
            name += vowels[index];
            vowelWanted = false;
        } else {
            int index = Random(0, consonantCount - 1);
            name += consonants[index];
            vowelWanted = true;
        }
    }

    name[0] = static_cast<char>(toupper(name[0]));

    return name;
}