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Iteration over std::vector: unsigned vs signed index variable

Asked 2009-01-03T16:52:24.593
493

What is the correct way of iterating over a vector in C++?

Consider these two code fragments, this one works fine:

for (unsigned i=0; i < polygon.size(); i++) {
    sum += polygon[i];
}

and this one:

for (int i=0; i < polygon.size(); i++) {
    sum += polygon[i];
}

which generates warning: comparison between signed and unsigned integer expressions.

The unsigned variable looks a bit frightening to me and I know unsigned variables can be dangerous if not used correctly, so - is this correct?

c++ stl unsigned signed
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43

In the specific case in your example, I'd use the STL algorithms to accomplish this. 

#include <numeric> 

sum = std::accumulate( polygon.begin(), polygon.end(), 0 );

For a more general, but still fairly simple case, I'd go with:

#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>

using namespace boost::lambda;
std::for_each( polygon.begin(), polygon.end(), sum += _1 );
answered 2009-01-03T18:03:29.803

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