Iterating over a vector in reverse direction

Question

I need to iterate over a vector from the end to the beginnig. The \"correct\" way is

for(std::vector::reverse_iterator rit = v.rbegin(); rit !=          


        

Answer 1

Hi i think better way use iterator as you use in first sample and if you need get iterator index you can use std::distance to calculate it, if i understand your question

Answer 2

Try out a do while :

std::vector<Type> v;
// Some code 
if(v.size() > 0)
{
    unsigned int i = v.size() - 1;
    do
    {
        // Your stuff
    }
    while(i-- > 0);
}

Answer 3

for (it = v.end()-1; it != v.begin()-1; --it)
{
}

The "goes to" operator definitely messes with my head.

Answer 4

I would prefer the reverse iterator variant, because it's still easy to interpret and allows to avoid index-related errors.

Sometimes you can simply use the BOOST_REVERSE_FOREACH, which would make your code look the following way:

reverse_foreach (int value, vector) {
   do_something_with_the_value;
}

---

Actually speaking, you can always use foreach statements for these kinds of loops, but then they become a bit unobvious:

size_t i = 0;

foreach (int value, vector) {
   do_something;
   ++i;
}

Answer 5

As you've noted, the problem with a condition of i >= 0 when it's unsigned is that the condition is always true. Instead of subtracting 1 when you initialize i and then again after each iteration, subtract 1 after checking the loop condition:

for (unsigned i = v.size(); i-- > 0; )

I like this style for several reasons:

• Although i will wrap around to UINT_MAX at the end of the loop, it doesn't rely on that behavior — it would work the same if the types were signed. Relying on unsigned wraparound feels like a bit of a hack to me.
• It calls size() exactly once.
• It doesn't use >=. Whenever I see that operator in a for loop, I have to re-read it to make sure there isn't an off-by-one error.
• If you change the spacing in the conditional, you can make it use the "goes to" operator.

Answer 6

loop condition i != std::numeric_limits<unsigned>::max() ... or use UINT_MAX if you think its to verbose. or another way:
for(unsigned j=0, end=v.size(), i=end-1; j<end; --i, ++j)
or
for(unsigned end=v.size(), i=end-1; (end-i)<end; --i)