Determining whether one vector is a prefix of the other

I am learning from C++ Primer 5th edition. This is exercise 5.17:


Given two vectors of ints, write a program to determine whether one vector is a prefix of the other. For vectors of unequal length, compare the number of elements of the smaller vector. For example, given the vectors containing 0, 1, 1, and 2 and 0, 1, 1, 2, 3, 5, 8, respectively your program should return true.

#include 
#include 
#include 

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;
using std::end;
using std::begin;

int main()
{
    vector i{0,1,1,2}, n{0,1,1,2,3,5,8}, smlr;
    char sml, bgr;
    long j = (i.end()-i.begin()) ,k = (n.end()-n.begin());
    if ( j > k)
    {
        sml = 'n', bgr = 'i', smlr = n;
    }

    else if (j ::size_type a = 0; a < i.size() ; ++a ) {
            if (i[a] != n[a]) {
                cout << "both vectors are unequal and are not prefixes of the other";
                return -1;
            }
        }
    }

    for ( int l = 0; l < smlr.size() && i[l] == n[l]; ++l )
    {
            if(l == (smlr.size()-1))
            {
            cout << sml << " is a prefix of " << bgr;
            return 0;
            }
    }
    cout << sml << " is not a prefix of " << bgr;
    return 0;
}

• Is my code efficient?

• Am I using too many variables?

• Is it better to write more code and use less variables or use less variables and less code?

I am a beginner, so please be harsh.

Ok first, we want to determine if some condition is true or not based on some input. That calls for a boolean function! In particular, our inputs are two vectors. :

bool isPrefixOf(const std::vector& smaller, const std::vector& larger) 
{
    ...
}

I named the first variable smaller, but it might not actually be smaller! Let's make sure that it is:

if (smaller.size() > larger.size())
    {
        std::swap(smaller, larger); // this very efficiently swaps our vectors
                                    // in constant time
    }

Now once we got that, the algorithm in your j == k is exactly correct, but pretty hard to read. The convention is to use variables like i, j, k solely as indices in loops, not as important variables. So I'll rewrite it like this:

for (int i = 0; i < smaller.size(); ++i) 
    {
        if (smaller[i] != larger[i]) 
        {
            return false; // clearly not a prefix
        }
    }

    return true; // we match up, so we must be done!
}

So cool, we got our function that checks if a smaller vector is a prefix of a larger vector, so now our main just has to determine which way we call it:

int main() 
{
    vector vec1{0, 1, 1, 2};
    vector vec2{0, 1, 1, 2, 3, 5, 8};

    cout << "vec1 is a prefix of vec2?: " << isPrefixOf(vec1, vec2) << endl;
}

Which should print true. So in general, your variable names don't line up with convention, and you coded the same logic twice - don't repeat yourself! Take advantage of functions to encode conceptually distinct functionality.

StackExchange Code Review Q#33404, answer score: 3