Prompt user for input then create an acronym from it

My program is complete and working, but I would like second opinions on it. The program prompts the user to enter something like a phrase or some words. Then it will create an acronym of what was entered. What are some adjustments I can make to my program to make it more efficient?

#include  
#include  
#include 
using namespace std; 

string acronym(string str); 

int main()
{
    string str;  

    while (true)
    {
        cout << "\nPlease enter a string: "; 
        getline(cin, str); 

        if (str == "") 
        {
            break; 
        }

        cout << "\n\nThe acronym is \"" << acronym(str) << "\"" << "\n";
    } 

    system("PAUSE"); 

    return 0;
}

string acronym(string str) 
{
    string phrase = "";

    phrase = str[0]; 

    for (int i = 0; i < str.length(); i++)
    {
        if (str[i] == ' ')
        {
            phrase += str[i+1]; 
        }
    }

    return phrase; 
}

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The procedure name acronym does not tell you what this procedure is doing. Naming it create_acronym would make the code more obvious.

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Error checking: You assume that the user only enters a single space between words. This will cause problems if the user enters more than one space by accident, introducing spaces in your acronym

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Most times acronyms are capitalized. You might want to capitalize each letter before adding it to the phrase.

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With renaming your procedure create_acronym, you can rename the phrase variable to acronym, which is more appropriate since an acronym is not a phrase. A phrase is one or more words.

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Pausing execution: Some say using system("PAUSE") is bad. Instead, just read from cin to a variable and discard the value. That way the application will hang waiting for input until the user presses the ENTER key.

StackExchange Code Review Q#102002, answer score: 4