std::valarray::operator[] — cppreference.com

const T& operator[]( std::size_t pos ) const; T& operator[]( std::size_t pos );

valarray<T> operator[]( std::slice slicearr ) const;

std::slice_array<T> operator[]( std::slice slicearr );

valarray<T> operator[]( const std::gslice& gslicearr ) const;

gslice_array<T> operator[]( const std::gslice& gslicearr );

valarray<T> operator[]( const valarray<bool>& boolarr ) const;

valarray<T> operator[]( const valarray<size_t>& indarr ) const;

indirect_array<T> operator[]( const valarray<size_t>& indarr );