programming-for-dummies

146
Declaring Variables
Declaring the data type of a variable
You can never stop users from typing in the wrong type of data, such as
typing in the words twenty thousand instead of the number 20000. However,
you can protect yourself from trying to do something wrong, like multiplying
a number by text. In the Python programming language, the following program
actually runs:
salary = input(“What is your annual salary?”)
taxrate = “Thirty percent”
print “This is how much you owe in taxes = “, salary *
taxrate
In this program, the third line tries to multiple the salary variable by the
taxrate variable. If the program asks the user, “What is your annual
salary?”, and the user types 20000, the third line of the program tries to
multiply 20000 (stored in the salary variable) by Thirty percent.
Obviously, you can’t multiply a number by a word, so this program would
appear to work but prints only:
This is how much you owe in taxes =
The problem occurs because the program multiplies the salary variable by
the text Thirty percent. Even though the program appears to run, it
doesn’t run correctly.
If you’re writing a small program, you could examine your program, line-byline,
to see where the problem might be, but in a large program that might
consist of thousands of lines, trying to track down this simple problem could
be time-consuming and frustrating.
To avoid this problem, many programming languages force you to define
(declare) your variable names plus the type of data they can hold. By doing
this, you can have the compiler check your program to make sure you aren’t
mixing data types accidentally, such as trying to multiply a number by text.
If your program does try to mix data types by multiplying a number by text,
the compiler refuses to run your program, displays an error message, and
highlights the problem so you can fix it right away, as shown in Figure 2-2.