C++ Palindrome finder optimization

Question

I have been writing a palindrome finder in C++ and I have succeeded in writing one that is.... basic to say the least.

I am looking simply to increase the runspeed of the program, right now it takes about ~1m 5s to run a test for palindromes / 2 word palindromes on a 1500 word wordlist using the functions that I have. I would like to try running it on a much larger file but fail to see where I can optimize further?

Any help would be appreciated: P.S. This is not for school, just for leisure.

#include <iostream>
#include <ostream>
#include <vector>
#include <fstream>
#include <algorithm>
using namespace std;

bool isPal(string);

int main() {

vector<string> sVec;
vector<string> sWords;
vector<string> sTwoWords1;
vector<string> sTwoWords2;
char myfile[256]="/home/Damien/Test.txt";
ifstream fin;
string str;
fin.open(myfile);
    if(!fin){ 
        cout << "fin failed";
        return 0;
    }
while(fin){

    fin >> str;
    sWords.push_back(str);
    if(!fin){
        break;
    }
    if(isPal(str)){
      sVec.push_back(str);
    }
    else{
        getline(fin, str);
    }
}
    reverse(sVec.begin(), sVec.end());
    for(int i =0; i < sVec.size(); i++){
        cout << sVec[i] << " is a Palindrome " <<endl;
    }

    // Test 2
    for(int i=0; i<sWords.size(); i++){
        for(int j=(i+1); j<sWords.size(); j++){
            str = sWords[i]+sWords[j]; 
            if(isPal(str)){
                sTwoWords1.push_back(sWords[i]);
                sTwoWords2.push_back(sWords[j]);
            }
        }
    }
fin.close();
for(int i=0; i<sTwoWords1.size(); i++){
    cout << sTwoWords1[i] << " and " << sTwoWords2[i] << " are palindromic. \n";
}
return 0;
}


bool isPal(string& testing) {
    return std::equal(testing.begin(), testing.begin() + testing.size() / 2, testing.rbegin());
}

Answer 1

You're doing a lot of unnecessary work to test if it is a palindrome. Just use std::equal:

#include <algorithm>

bool isPal(const string& testing) {
    return std::equal(testing.begin(), testing.begin() + testing.size() / 2, testing.rbegin());
}

This will iterate from the beginning of the string to the middle and from the end of the string to the middle and compare the characters as it goes. I can't remember who showed me this, but I didn't think of it.

Edit: I learned it from Cubbi in another question about palindromes.

Answer 2

So i did some testing. In your approach Test2 takes long time.

Data: 2000 random 20 chars strings.

Your solution: 2500 ms. Seth Carnegie's: 500 ms.

Though i believe you have to multiply those by 2, because s+v can be palindrome while v+s isnt.

Idea: suppose we have a word abcd. Other words then can be palyndromes with this one are only cba and dcba. Lets check if we have those present.

...    
#include <set>
using namespace std;

bool isPal(const string&);

int main() {
    ...
    set<string> rWords;
    ...
    while(fin){

        fin >> str;

        sWords.push_back(str);

        if(!fin){
            break;
        }
        reverse(str.begin(), str.end());//add reversed str to rWords
        rWords.insert(str);
        reverse(str.begin(), str.end());

        ...
    }

    ...

    // Test 2
    for(int i = 0; i<sWords.size(); ++i)
        for(int l = 0; l<sWords[i].length(); ++l)
            if(isPal(sWords[i].substr(sWords[i].length()-l)))
            {
                string s = sWords[i].substr(0,sWords[i].length()-l);
                set<string>::iterator it = rWords.find(sWords[i].substr(0,sWords[i].length()-l));
                if(it != rWords.end())
                {
                    string s = *it;
                    reverse(s.begin(), s.end());
                    if(s == sWords[i])//isPoly to itself
                        continue;
                    sTwoWords1.push_back(sWords[i]);
                    sTwoWords2.push_back(s);
                }
            }
    ...
    return 0;
}

bool isPal(const string& testing) {
    return std::equal(testing.begin(), testing.begin() + testing.size() / 2, testing.rbegin());
}

time: 15ms