t in the value zero. Declared as
above, <code>a[1]</code> will be a size-zero one-dimensional array. For
comparison, declaring</p>
<pre class="stan"><code>array[0, 3] real b;</code></pre>
<p>also produces an array with an overall size of zero, but in this case,
there is no way to index legally into <code>b</code>, because <code>b[0]</code> is
undefined. The array will behave at run time as if it’s a <span class="math inline">\(0 \times 0\)</span> array. For example, the result of <code>to_matrix(b)</code>