Maps and function objects problem, with code

**wildcard_seven** · 09-27-2011

Code:

/*
 * failure.cpp
 *
 *  Created on: Sep 27, 2011
 *      Author: mike
 *
 *  failure.cpp - A program to read in words from stdin, tally their frequency, and add frequencies to
 *  tally up the total number of words
 */

#include <map>
#include <string>
#include <iostream>
#include <algorithm>
using namespace std;
/*
 * A function object that tallies the value elements of map pairs
 */
class Frequency_works {
	private:
		int* total;
	public:
		Frequency_works(int& x) : total(&x) {}
		void operator() (const pair<const string, int>& elem) {*total += elem.second;}
};

/*
 * A function object that tallies the value elements of map pairs
 */
class Frequency_fails {
	private:
		int total;
	public:
		Frequency_fails(int x = 0) : total(x) {}
		void operator() (const pair<const string, int>& elem) {
			total += elem.second;
			//cout << total << ' '; // For testing
		}
		int getTotal() {return total;}
};

/*
 * Reads words into a map from stdin, and accumlates their frequency
 */
void get_words ( map <string,int>& words) {

	string tmp;

	// While there is input, read from stdin
	while (cin >> tmp && tmp != "DONE") {
		words[tmp]++;  // Record word and frequency
	}
}


int main () {

	int total = 0;

	Frequency_works f1(total);
	Frequency_fails f2;

	map<string, int> w;
	get_words (w);

	// Tally and print total words, using function object of class Frequency_works
	for_each (w.begin(), w.end(), f1);
	cout << "Total words (Frequency works): " << total << '\n';

	//Tally and print total words, using function object of class Frequency_fails
	for_each (w.begin(), w.end(), f2);
	cout << "Total words (Frequency fails): " << f2.getTotal() << '\n';


	return 0;

}

OUTPUT (using the words I give as I describe how to use below)

Code:

Total words (Frequency works): 5
Total words (Frequency fails): 0

To use the program, just feed it a list of words, preferably with certain words repeated, on stdin. End with the word DONE. Like so.

Code:

./executable
box
box
hammer
hammer
nail
DONE

When you are finished, you can tell me why Frequency_fails doesn't do its job as a function object.

**hk_mp5kpdw** · 09-27-2011

Probably because of how the Frequency_fails object is passed into the for_each function. The Frequency_works object increments the total variable in main but the Frequency_fails one has an internal counter and is probably passed into the function by copy so when the for_each function call ends, the value in the object back in the main function has not been altered ...that's my guess.

Try this:

Code:

//Tally and print total words, using function object of class Frequency_fails
f2 = for_each (w.begin(), w.end(), f2);
cout << "Total words (Frequency fails): " << f2.getTotal() << '\n';

The for_each function returns the function object but in your case you were ignoring the return value from the function.

[edit]I just tested that and it seemed to make that part work.[/edit]

**grumpy** · 09-27-2011

f2 is passed by value to the function. So the counting is performed in a copy of f2.

There is a reason that for_each() returns the function passed as an argument.

**Elysia** · 09-27-2011

An alternative solution (good to know!) is to use boost::ref, eg:
for_each(w.begin(), w.end(), boost::ref(f2));

Also, in case your compiler supports it, you should be using the free functions std::begin and std::end instead of those member functions.

**wildcard_seven** · 09-27-2011

Beautiful. Thank you everyone. You guys are the best. I may be back if a search doesn't readily tell me the difference between "free function" std::begin and std::end and the member functions, because I have no idea what that's about. Otherwise, thanks again.

**Elysia** · 09-28-2011

What I simply mean is that instead of
for_each(w.begin(), w.end(), f2);
...you should do...
for_each(std::begin(w), std::end(w), f2);

With appropriate included headers, of course.

**wildcard_seven** · 09-28-2011

You were just about to explain the advantage of this, when....the doorbell rang?

Seriously, I tried a search, and I'm batting zero. Not that I know what I'm looking for.

**Elysia** · 09-28-2011

Just because it allows possible optimizations in the code.

**laserlight** · 09-28-2011

I do not know about the optimisations part, but I see them mainly as tools to make it easier to write function templates that operate on arrays in addition to containers that provide member begin() and end().

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