CODEWITHSUNDEEP
c++dictionary
vvolis
vvolis

Reputation: 23

C++ Map returning class object based on variable key

I reacently learned about map structures and I am trying to use one, but can't seem to solve one problem.

I have tried the code below:

map<string, valuePair> translator;

The class valuePair is just a combination of 2 objects (string and a number).

Im assigning values to the map

translator[currentWord] = valuePair(stateNo, "state");

Where currentWord is a variable string, stateNo is an int.

Now later I want to get back the valuePair number value from the map, but can;t seem to be able to do it. Heres a screenshot of my watch window trying to access the variable x. https://i.sstatic.net/ccask.png

These are all the ways I managed to find online to return the value, yet none of them seem to work. As you can see the key "a" is in the map. What am I doing wrong?

[EDIT] Thanks guys, I used the tips you gave in comments and found out, that actually it works the way I expected - translator["a"].x prints the values I need. I have nothing to mark as "Correct answer" though, and I'm not sure what to do with this thread now :/

Upvotes: 1

Views: 2504

Answers (3)

Spacemoose
Spacemoose

Reputation: 4016

As Jonathan Henson posted in the comments, you would be better off posting your code than your debugger output. Ideally you want a minimal example that reproduces the error you are having.

Based on the debugger output, I'm wondering if you have a scope issue -- you are trying to access the map data outside of the scope where you have it defined. Without seeing the source code though, there is no way to know.

Here is a working example that does precisely what you are trying to do. The only modification is I have used a struct for valuePair, and c++ 11 initializer lists. This won't affect the map access code, but you might need to turn on c++ 11 support to compile it.

As a first step, look check it out in your debugger and see if you get the same difficulty. If so, your problem is the debugger or debugger setup, not your code.

If the debugger works for the sample code (posted below), gradually transform my code into your code (making minimal changes, building, and see if it still works). This is a very useful approach to learning the fine points of a language.

#include <iostream>
#include <map>
#include <string>

using namespace std;

struct valuePair 
{
 int num;
 string str;
};



int main()
{
 map<string, valuePair> translator;
 translator["a"] = {0,"bla"};
 translator["b"] = {1, "alb"};
 translator["c"] = {2, "lab"};
 valuePair res = translator["c"];
 cout << "c:" << res.num << "," << res.str << "\n";
 res = translator.at("b");
 cout << "b:" << res.num << "," << res.str << "\n";
 res = translator.find("a")->second;
 cout << "a:" << res.num << "," << res.str << "\n";
 return 0;
}

Upvotes: 2

Danvil
Danvil

Reputation: 22981

If you have an std::map<K,V> m; you can add elements and change values by using m[k] = v;. However the operator[] is an operation which always creates a key/value pair, if the key you are looking for is not contained in the map. Thus it is not allowed when you have a const reference or pointer, e.g. const std::map<K,V>&.

With a std::map you always have to consider the case that the key you are looking for is actually not contained in the map!

To look for the value stored under a given key you have to use std::map::find (link).

Example:

std::map<std::string,valuePair>::const_iterator it = translator.find(currentWord);
if(it != translator.end()) {
    // the map contains an element with this key
    const valuePair& value = it->second; // this is the value
}
else {
    // the map *does not* contain an element with this key
}

As mentioned in the comments std::map::at (link) may be an alternative for C++11. But then you have to take care of the possible exception which is thrown when you use a key which does not exist in the map.

Upvotes: 2

user743414
user743414

Reputation: 936

this works for me

#include <map>
#include <sstream>
#include <string>


int main()
{
  std::map<std::wstring, double> items;

  items[L"0"] = 0.123;
  items[L"1"] = 1.234;
  items[L"2"] = 2.234;
  items[L"3"] = 3.345;
  items[L"4"] = 4.567;


  for (int i = 0; i < 5; i++)
  {
    std::wstringstream oss;
    oss << i;

    std::wstring key = oss.str();
    double value = items[key];
  }


  return 0;
}

Upvotes: -2

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