Can't get the right answer with c++ STL map

Question

I have wrote a small map test program which is below:

void main()
{
    vector<double> price;
    vector<string> time;

    price.push_back(5.70);
    price.push_back(5.77);
    price.push_back(5.81);
    price.push_back(5.72);
    price.push_back(5.81);

    time.push_back("10:40");
    time.push_back("10:43");
    time.push_back("10:44");
    time.push_back("10:46");
    time.push_back("10:48");

    map<double,string> myMap;

    for (int i=0 ; i<price.size() ; i++)
    {   myMap[price[i]] = time[i]; }

    for (int i=0 ; i<price.size() ; i++)
    {   
        if (price[i] == 5.81)
        { cout << myMap[price[i]] << endl; }
    }
}

My expect outputs should be:

10:44
10:48

But the outputs I get is :

10:48
10:48

I don't know why it is wrong.

std::map is one-to-one. Since you have insert in 5.81 twice, '10:44' is replaced with '10:48'. — cbel, Apr 04 '14 at 07:58
I suppose that using floats as keys could yield surprising results because (1) **The result of a computation usually is not exact.** If you try to find an entry in a map with a computed float result you may not find it. (2) There are even **more subtle problems** with floats, e.g. your literal 5.81 may not equal g in `float f = 10.0; g = 58.1/f`. Cf. http://stackoverflow.com/questions/22715793/floating-point-number-comparison-via-integer-casting-in-c-language, and http://stackoverflow.com/questions/4238122/hash-function-for-floats — Peter - Reinstate Monica, Apr 04 '14 at 08:26

score 2 · Accepted Answer · edited May 23 '17 at 10:31

2

Note the Unique keys section of the map specification.

Unique keys
No two elements in the container can have equivalent keys.

This means that for each unique key (in your case, 5.81), there can only be one value: the last one set (10:48). For non-unique maps, use multimap

For discussion on the difference between multimap and map: see this

edited May 23 '17 at 10:31

Community

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answered Apr 04 '14 at 07:57

Avery

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score 1 · Answer 2 · answered Apr 04 '14 at 07:56

myMap can only store one value per key: the key 5.81 is duplicated.

You are overwriting the value "10:44" with your subsequent insertion of 5.81 -> "10.48".

If you want duplicate values for a single key then use std::multimap.

(Also, beware of using floating point types as keys. You'll hit problems with floating point comparison.)

score 1 · Answer 3 · answered Apr 04 '14 at 07:57

1

On a last iteration in first cycle you are updating myMap[5.81] previously added 2 iteration above (not add new element). Consider using std::multimap

answered Apr 04 '14 at 07:57

αλεχολυτ

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score 0 · Answer 4 · edited Jun 20 '20 at 09:12

0

From the documentation

Maps are associative containers that store elements formed by a combination of a key value and a mapped value, following a specific order.

In a map, the key values are generally used to sort and uniquely identify the elements, while the mapped values store the content associated to this key.

So you can only have one value per key, which is why it printed two time the same value.

Also, you can replace this portion of your code:

for (int i=0 ; i<price.size() ; i++)
{   
    if (price[i] == 5.81)
    { cout << myMap[price[i]] << endl; }
}
// by
cout << myMap[5.81] << endl;

It's more or less the same, if you don't consider the problem with double keys.

edited Jun 20 '20 at 09:12

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answered Apr 04 '14 at 07:59

Jaffa

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After the replace, it only gives one output which is 10:48. – user3368506 Apr 04 '14 at 08:06
Yes, but there's only one value anyway ! – Jaffa Apr 04 '14 at 08:13
My wish is to print out all the time where the price is 5.81. So I need there be 2 times. But I know my way is wrong. – user3368506 Apr 04 '14 at 08:17
Using a map, there's only one time where the price is 5.81. You can also use http://www.cplusplus.com/reference/map/multimap/ – Jaffa Apr 04 '14 at 08:19

Can't get the right answer with c++ STL map

4 Answers4