de>boot_idx</code> to index the predictors as <code>x[boot_idx]</code> and outcomes as <code>y[boot_idx]</code>. This generates a new size-<span class="math inline">\(N\)</span> vector whose entries are defined by <code>x[boot_idx][n] = x[boot_idx[n]]</code> and similarly for <code>y</code>. For example, if <span class="math inline">\(N = 4\)</span> and <code>boot_idxs = {2, 1, 1, 3}</code>, then <code>x[boot_idxs] = [x[2], x[1], x[1], x[3]]'</code> and <code>y[boot_idxs] = [y[2], y[1], y[1], y[3]]'</code>.