warning: comparison between signed and unsigned integer expressions..how to solve it?

0
votes

i am getting a comparison between signed and unsigned integer expression in my code:

    vector<long int> row;
    long n,m;
    long int pro=1;
    cin>>n;
    for(long i=0;i<n;i++)
    {
        long int temp;
        for(long j=0;j<n;j++)
        {
            cin >> temp;
            row.push_back(temp);
        }
    }

    cin >> m;
    for(long i=0;i<row.size();i++)
        pro = pro * pow(row[i],m);

    long int mod = 1000000007;
    cout<< (long int)pro%mod;

At the line: for(long i=0;i<row.size();i++)

How can I fix this warning?

3
c++integersizeunsignedsigned
In case one does not feel like counting. pro = pro * pow(row[i],m); is row 27. Please mark problematic lines with comments denoting line number, such as // line 27 HERE. Thanks. – WhozCraig
I'm betting that in the real code, for(long i=0;i<row.size();i++) is row 27. – Drew Dormann
@DrewDormann I'd back you on that bet, not that you need it. – WhozCraig
Why are you using both long and long int? They're the same type. – emlai
Possible duplicate of C: how can I fix warnings like: "comparison between signed and unsigned" – phuclv

3 Answers

3
votes

std::vector::size returns a value of size_type, which is Unsigned integral type (usually std::size_t).

Your loop count variable is of type long which is a signed type. So in the loop condition you are comparing a signed and an unsigned type.

The solution is simple: Use std::vector<long int>::size_type (or maybe even size_t) instead of long.

2
votes

vector::size returns a size_type which is an unsigned integral value.

You can fix this one of two ways:

  1. Use an unsigned iterator in your for-loop: for(auto i = 0U; i < row.size(); ++i)
  2. Cast the return of vector::size to a signed integer: for(auto i = 0; i < static_cast<int>(row.size()); ++i)
0
votes

C++ has a thing called the range-based for loop, which relieves you from the burden of dealing with index variables. It also solves your mismatched signedness problem:

for(long r : row)
    pro = pro * pow(r,m);

Just use it.