How do I create a frequency table of strings?

[infinite1212]

I need to write a piece of code counts the string inputted and shows how many times each string occurred, essentially a frequency table of strings. So, user enters multiple strings (i.e. aaa, bbb, ccc, aaa) and the code is supposed to print out how many times each string occurs.

I know that I am supposed to use maps, but I have no clue how to proceed. I've looked at examples of frequency tables using maps, but I am lost. Any help would be greatly appreciated. Thank you!

[keskiverto]

You did gove example that has four strings. From those you would create a table that has two column; the string and its frequency:

aaa 2
bbb 1
ccc 1

A std::map has a list of pairs. Each pair has a key and a value. Keys are unique, just like the strings in the above table.

See
http://www.cplusplus.com/reference/map/map/operator[]/
http://en.cppreference.com/w/cpp/container/map/operator_at

[infinite1212]

I understand that keys are unique. But, am I able to generate keys automatically for each new entry that the user enters? I still don't know how I am supposed to use map to make a frequency table..

[keskiverto]

What does the (linked) descriptions of map::operator[] say about the case when you call it with value that is not an existing key?

[infinite1212]

Okay, so I went scouring through my notes and I found code that translated to a frequency table of strings. But, I'm having a bit of trouble.

int main() {
map<string, int> F;  //Stores frequency table
string s;                  //holds input;

while (cin>> s) F[s]++;

for (map<string, int>::iterator i=F.begin(); i != F.end(); i++){
      cout<< (*i).first << ":\t" << (*i).second<< "\n";
}

return 0;

}

I tried the code, it works, but I don't understand it.

I understand that .first prints the key, and .second gives the value. But, I don't understand the purpose of the while loop.

As for the for loop: What is the purpose of the iterator? What's the point of printing out *i?

[lastchance]

The while loop puts strings s into the map. If s isn't yet in the map it creates F[s] and sets its value to 1; if it is already there then it increases the value by 1. So it creates a frequency table. The while loop will run until cin can supply no more strings; then the condition of the stream will become 'false'.

The for loop outputs the frequency table by listing the pairs in the map. An iterator allows you to go through a container one by one when you don't have a natural array index. An iterator is a bit like a pointer, so you would get the element it points to by dereferencing as *i. Each element is a pair, with the first part accessed with '.first' and the second with '.second'. The iterator notation is incredibly verbose. You would get the same from
for ( auto e : F ) cout << e.first << ":\t" << e.second << '\n'
which says: for each element in F print out the key (here, a string), then a colon and tab, then the value (here, the count).

Different case - upper or lower - will give different strings, so depending on what you want to do you may want to filter the strings before putting them in the map.