C++ Character Frequency Linked List - c++

Question

For my project, I need to read in a file and count the times each character appears and store it in a linked list. Below is what I have for the reading in the file portion of the program:
ifstream inFile;
ofstream outFile;
inFile.open(inputfile.txt);
char ch;
list<charFrequency> charFreqList;
list<charFrequency>::iterator i;
inFile >> ch;
while (!inFile.eof())
{
charFrequency cf(ch);
charFreqList.push_back(cf);
for (i = charFreqList.begin(); i != charFreqList.end(); ++i)
{
if (i->getCharacter() == cf.getCharacter())
{
i->increment();
charFreqList.pop_back();
}
}
inFile >> ch;
}
inFile.close();
I need the program to go through and if the character is already in the linked list, it just needs to increment the count of the character but only leave one instance of the character in the list, however, I get an error message stating "list iterator not implementable". I know it has to do with the pop_back() as it removes the last element, but I don't know about avoiding this issue.
Thanks in advance for the help!

Answer 1

First, only add a charFrequency if the character doesn't exist, otherwise you increment the count. It would also be easier if a new charFrequency started with a count of 1.
Second, change your input loop to not check for eof(). This is explained in many threads on SO.
class charFrequency
{
int count;
char ch;
public:
void increment() { ++count; }
char getCharacter() const { return ch; }
int getCount() const { return count; }
charFrequency(char c) : ch(c), count(1) {}
};
Last, for kicks, let's use some C++ and use the std::find_if() algorithm function instead of writing a loop. Introduce a function object below that finds a character.
struct FindCharacter
{
char ch;
FindCharacter(char c) : ch(c) {}
bool operator()(charFrequency& cf) const
{ return cf.getCharacter() == ch; }
};
So now putting this all together, we have this:
#include <list>
#include <algorithm>
#include <fstream>
//...
while (ifs)
{
ifs >> ch;
// Search for character
std::list<charFrequency>::iterator it = std::find_if(charFreqList.begin(), charFreqList.end(), FindCharacter(ch));
// if not found, add new charFrequency to list
if ( it == charFreqList.end())
charFreqList.push_back(charFrequency(ch));
else
it->increment(); // increment
}
//...

Answer 2

You don't need to push into the list and remove it back. You could do something like below, or break from the if condition in your code and then remove the last element (not in the loop)
list<charFrequency> charFreqList;
list<charFrequency>::iterator i;
while (!inFile.eof())
{
charFrequency cf(ch);
Predicate pre(ch); //Write your predicate functor
//check if the char is present
i = std::find_if(charFreqList.begin(), charFreqList.end(), pred);
if( i != charFreqList.end() )
i->increment();
else
charFreqList.push_back(ch); //insert only when not present
inFile >> ch;
}
However, std::list is a wrong choice of datastructure here, it shall give a bad run time complexity.