A better way to implement a pattern repeating in several different places in the code

I have the following pattern repeating in several different places in my code:

if (m_condition)
{
    auto& item = m_vector[m_index];
    Do something with 'item'
}
else
{
    for (auto& item : m_vector)
    {
        Do something with 'item'
    }
}

Where m_condition, m_vector and m_index are class members.

The Do something with 'item' part is different in each occurrence of the pattern.

I am looking for a nicer/cleaner/shorter way to implement this (performance is not an issue).

I came up with this, but it feels kinda "nasty":

auto iBgnIndex = m_condition ? m_index : 0;
auto iEndIndex = m_condition ? m_index : m_vector.size()-1;
for (auto i = iBgnIndex; i <= iEndIndex; i++)
{
    Do something with 'm_vector[i]'
}

What better options do I have?

Thanks