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

Question

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?

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, Aug 08 '15 at 04:21
@DrewDormann I'd back you on that bet, not that you need it. — WhozCraig, Aug 08 '15 at 04:23
Why are you using _both_ `long` and `long int`? They're the same type. — Emil Laine, Aug 08 '15 at 09:41
Possible duplicate of [C: how can I fix warnings like: "comparison between signed and unsigned"](http://stackoverflow.com/questions/859943/c-how-can-i-fix-warnings-like-comparison-between-signed-and-unsigned) — phuclv, Mar 24 '17 at 10:52
did you ever copy the warning and google it? results will come immediately and there are tons of duplicates on SO — phuclv, Mar 24 '17 at 10:54

score 4 · Accepted Answer · answered Aug 08 '15 at 04:27

4

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.

answered Aug 08 '15 at 04:27

Some programmer dude

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Or `decltype(row)::size_type` so you don't have to change it if the element type of `row` happened to change. – Emil Laine Aug 09 '15 at 09:23

score 2 · Answer 2 · answered Aug 08 '15 at 04:27

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

You can fix this one of two ways:

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

score 0 · Answer 3 · answered Aug 08 '15 at 09:35

0

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.

answered Aug 08 '15 at 09:35

Emil Laine

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

3 Answers3