1. A method of performing a conditional vector output operation in a processor, the method comprising: receiving electrical signals representative of an input vector; receiving electrical signals representative of a condition vector, the number of values in the input vector being equal to the number of values in the condition vector, values in the input vector and in the condition vector being in one-to-one correspondence with one another, and each value in the condition vector being a result of evaluating a predetermined conditional expression using data corresponding to a value in the input vector; and generating electrical signals representative of an output vector containing values in the input vector for which corresponding values in the condition vector are equal to a predetermined value, a length of the output vector being equal to a number of values in the condition vector that are equal to the predetermined value.