For example, we might want to remove all prime numbers from
bigArrayOfNumbers
.
We’ll use an array method called
splice
, which can add or remove elements from an
array (see
). The following will not work as expected:
for
(
let
i
=
0
;
i
<
bigArrayOfNumbers
.
length
;
i
++
) {
if
(
isPrime
(
bigArrayOfNumbers
[
i
]))
bigArrayOfNumbers
.
splice
(
i
,
1
);
}
Because the index is increasing, and we’re removing elements, there’s a possibility that
we might skip over primes (if they are adjacent). We can solve this problem by using
descending indexes:
for
(
let
i
=
bigArrayOfNumbers
.
length
-
1
;
i
>=
0
;
i
--
) {
if
(
isPrime
(
bigArrayOfNumbers
[
i
]))
bigArrayOfNumbers
.
splice
(
i
,
1
);
}
Note carefully the initial and test conditions: we have to start with one less than the
length of the array, because arrays indexes are zero-based. Also, we continue the loop
as long as
i
is greater than or equal to 0; otherwise, this loop won’t cover the first ele‐
ment in the array (which would cause problems if the first element were a prime).
Conclusion
Control flow is really what makes our programs tick. Variables and constants may
contain all the interesting information, but control flow statements allow us to make
useful choices based on that data.
Flowcharts are a useful way to describe control flow visually, and very often it can be
helpful to describe your problem with a high-level flowchart before you start writing
code. At the end of the day, however, flowcharts are not very compact, and code is a
more efficient and (with practice) natural way to express control flow (many attempts
have been made to construct programming languages that were visual only, yet text-
based languages have never been threatened by this usurper).
78 | Chapter 4: Control Flow
