programming-for-dummies

Fixing a Program with a Debugger 101
For example, suppose a program asks the user to type in his age. If you run
your program and type in a valid age, such as 15, you can see how your program
handles the number 15. But what happens if the user types in an
invalid number, such as 0 or -17?
A good programmer always makes sure the program doesn’t accept invalid
data, but suppose part of your program changes the age data by mistake.
Depending on the age, sometimes the program changes the age to 0, sometimes
it changes it to a negative number (-12), sometimes it changes it to an
outrageously large number (486).
Book I
Chapter 4
Programming Tools
To find out how and why your program seems to randomly change the age,
you can step through your program and watch the number stored as the age.
By changing your variable while the program is running, you can type in different
age values to see how your program responds.
By doing this, you can suddenly see that any time the age is less than 5, your
program turns the age into 0; any time the age is greater than 5 but less than
45, the program turns the age into a negative number; and any time the age
is greater than 45, the program turns the age into a huge number, like 486.
Without the ability to change the value of the age variable while the program is
running, debugging a program is much slower and more tedious. By changing
the value of the age variable while the program is running, you can test different
values without having to trace through the program multiple times using
different values. Just run the program once and change the value of the age
variable as many times as you want, as shown in Figure 4-8.
Figure 4-8:
Watching
and
changing
variables
can show
you how a
program
reacts to
different
data.