Should I Use a Signed or Unsigned Index Variable When Iterating Over a std::vector?

Iteration over std::vector: Signed vs Unsigned Index Variable

When iterating over a vector in C , you can use either a signed or unsigned index variable. However, there are some subtle differences to be aware of.

Using an unsigned index variable is generally preferred because it eliminates the possibility of a negative index, which would result in undefined behavior. For example, this code works fine:

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

However, this code would generate a warning:

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

The warning occurs because the comparison i < polygon.size() is between a signed and an unsigned integer expression. This can lead to unexpected behavior in some cases.

Therefore, it is best to always use an unsigned index variable when iterating over a vector.

You may also prefer to use iterators instead of indices. Iterators provide a more abstract way of accessing the elements of a vector, and they can help to prevent you from making mistakes. For example, you could use the following code to iterate over a vector:

for (std::vector<int>::iterator it = polygon.begin(); it != polygon.end(); ++it) {
    sum += *it;
}

In general, it is considered good practice to use iterators rather than indices when iterating over a vector.

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