cified, <code class="computeroutput">plus</code> will be used. If no predicate is specified, <code class="computeroutput">equal_to</code> will be used.</p>
<p>
The <code class="computeroutput">reduce_by_key()</code> algorithm assumes that the binary reduction function is associative. When used with non-associative functions the result may be non-deterministic and vary in precision. Notably this affects the <code class="computeroutput">plus&lt;float&gt;()</code> function as floating-point addition is not ass