Best way to trim a vector from custom values

Question

is there a smarter/nicer way to trim a vector given a value I want to erase, something similar to what trim does for strings, e.g. from

0 0 0 1 2 3 56 0 2 4 0 0

to

1 2 3 56 0 2 4

I cannot find a trim-like function, is find/erase the best option?

Answer 1

Suppose we are working with a

std::vector<int> v;

We want to trim zeros, so

auto const is_not_zero = [](int i){ return i != 0; };

To find the first non-zero element and delete everything before it,

auto const first_non_zero = std::find_if(v.begin(), v.end(), is_not_zero);
v.erase(b.begin(), first_non_zero);

To find the last non-zero, do the same with reverse iterators:

auto const last_non_zero = std::find_if(v.rbegin(), v.rend(), is_not_zero);
v.erase(last_non_zero.base(), v.end());

(std::reverse_iterator::base returns the underlying ordinary iterator from a reverse one).

Answer 2

In many cases, it's best to just not do anything with the vector and simply work with an iterator pair for the relevant range of elements. You could also use a span:

#include <gsl/span>

// ...

auto non_zeros = gsl::span(nonzeros_start_iter, nonzeros_end_iter);

and you can use this span like any standard library container. In C++20 you can switch gsl::span to std::span.