programming-for-dummies

164
Using Math to Manipulate Numbers
Organizing equations with operator precedence
To do multiple calculations, you can type one mathematical calculation after
another, such as
X = 34 + 7
Y = X * 89
Although this works, it can get clumsy, especially if you need to write more
than a handful of equations. As a simple solution, you can cram multiple
equations into a single, big equation, such as
Y = 34 + 7 * 89
The problem is, how does the computer calculate this equation? Does it first
add 34 + 7 and then use this result (41) to multiple with 89? Or does it first
multiply 7 by 89 and then add this result (623) to 34?
Depending on the order it calculates its mathematical operators, the result is
either 3649 or 657, two obviously different answers.
To calculate any equation with multiple mathematical operators, computers
follow rules that define which mathematical operators get calculated first
(known as operator precedence). Table 3-3 lists common operator precedence
for most programming languages where the top operators have the
highest precedence, and the lowest operators at the bottom of the table
have the lowest precedence.
Table 3-3
Operator
Symbol
Exponentiation
^
Multiplication *
Division /
Integer division \
Modulus arithmetic % or mod
Addition +
Subtraction –
Operator Precedence
If an equation contains operators that have equal precedence, the computer
calculates the result from left to right, such as
X = 8 - 3 + 7
First, the computer calculates 8 – 3, which is 5 Then it calculates 5 + 7,
which is 12.