Question

I am trying to port the following JavaScript code to C++:

var vector = new array();     
for (var i = 0; i < points.length; i++)
        {
            var newX = points[i].X * cos - points[i].Y * sin;
            var newY = points[i].Y * cos + points[i].X * sin;
            vector[vector.length] = newX;
            vector[vector.length] = newY;
            sum += newX * newX + newY * newY;
        }

I can’t seem to understand what is happening in these lines:

vector[vector.length] = newX;
vector[vector.length] = newY;

What purpose does it serve to overwrite the value at the same location in the array?

Answer 1

JavaScript arrays dynamically expand to hold new elements, so to append new items you need only assign to the next available index.

Array indexes are zero-based, so given an array called “vector”, vector.length is one past the last element. Nothing is being over-written; The line vector[vector.length] = x appends x to the end of the array.

In JavaScript, the following methods of appending elements are identical, though using push more clearly indicates your intent:

vector = [1,2,3];
vector[vector.length] = 4; // [1,2,3,4]
vector.push(5); // [1,2,3,4,5]

The equivalent C++ code (assuming you’re using std::vector) would

my_vector.push_back(newX);