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CS 140: Introduction
to Computer Programming
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Input and Output
There are quite a few ways that a program can
receive information from the "outside world"
while executing. The first method that
comes to mind is probably to have the user type
something on the keyboard, which is what we will
consider below. But a program could also
detect mouse clicks and read data from a file,
among other possibilities.
There are two input commands: raw_input and
input. We use the first one to prompt the
user to enter a string (such as a name, word, or
zip code) while the second is used to ask the
user to enter a number (either an integer or a
float). Fortunately, the syntax for these
two commands is very similar. To obtain a
string, type
last_name = raw_input("Please enter your last
name:")
When encountering this command, the program will
print the statement Please
enter your last name: on
the screen, then record whatever is typed at the
cursor (including lower and upper case letters,
spaces, and digits) until the user hits the
[RETURN] or [ENTER] keys. This string of
keystrokes is then called last_name
for later reference. It is possible to use
the command without assigning a variable name,
as in
raw_input("Press the [RETURN] key to
continue.")
In this case it doesn't matter what is typed,
because the program won't remember anyway, since
the input is not given a name.
The same process can be used to get a number
from the user, by typing
age = input("Enter your current age:")
Python will treat age as an integer if the
number typed does not have a decimal point, but
will designate it as a float if it does contain
a decimal point. In other words, Python is
bright enough to figure out what data type to
assign age
based on the user input. By the way, you
will get an error message (and the program will
halt) if the user does not type a number
at an input prompt! For instance, this
would occur above if the user typed eight
or just hit the [RETURN] key.
Now let's take a quick look at printing text and
numbers to the screen. The print command
is used to display any data type. One can
even combine several different types on a single
line using commas to separate the various items
to be printed. For instance,
total = 8
color = "purple"
print "I bought", total,
color, "shirts for
the semester."
would result in
I bought 8 purple
shirts for the semester.
Several print statements in a row will display
items on separate rows, unless the print
statement is followed by a comma. Thus the
code
print "Once
upon",
print "a midnight clear,"
print "A jolly fellow",
print "did appear."
would give the following output:
Once upon a midnight clear,
A jolly fellow did appear.
Note that commas inside quotes are simply
printed, while the ones outside quotes are used
to separate items and prevent carriage returns.
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