Learning Processing: A Beginner's Guide to Programming Images ...

144 Learning Processing
9.2
What is an array?
From Chapter 4, you may recall that a variable is a named pointer to a location in memory where data is
stored. In other words, variables allow programs to keep track of information over a period of time. An
array is exactly the same, only instead of pointing to one singular piece of information, an array points to
multiple pieces. See Figure 9.1 .
Variable
fi g. 9.1
7
Array
4 8 15 16 23 42
You can think of an array as a list of variables. A list, it should be noted, is useful for two important
reasons. Number one, the list keeps track of the elements in the list themselves. Number two, the list
keeps track of the order of those elements (which element is the fi rst in the list, the second, the third,
etc.). Th is is a crucial point since in many programs, the order of information is just as important as the
information itself.
In an array, each element of the list has a unique index, an integer value that designates its position in the
list (element #1, element #2, etc.). In all cases, the name of the array refers to the list as a whole, while
each element is accessed via its position.
Notice how in Figure 9.2 , the indices range from 0 to 9. Th e array has a total of 10 elements, but the fi rst
element number is 0 and the last element is 9. We might be tempted to stomp our feet and complain:
“ Hey, why aren’t the elements numbered from 1 to 10? Wouldn’t that be easier? ”
Array index values
0
fi g. 9.2
1 2 3 4 5 6 7 8 9
While at fi rst, it might intuitively seem like we should start counting at one (and some programming
languages do), we start at zero because technically the fi rst element of the array is located at the start of
the array, a distance of zero from the beginning. Numbering the elements starting at 0 also makes many
array operations (the process of executing a line of code for every element of the list) a great deal more
convenient. As we continue through several examples, you will begin to believe in the power of counting
from zero.