print "Halt!" s = raw_input("Who Goes there? ") print "You may pass,", s
When I ran it here is what my screen showed:
Halt! Who Goes there? Josh You may pass, Josh
Of course when you run the program your screen will look different because of the raw_input
statement. When you ran the program you probably noticed (you did run the program right?) how you had to type in your name and then press Enter. Then the program printed out some more text and also your name. This is an example of input. The program reaches a certain point and then waits for the user to input some data that the program can use later.
Of course, getting information from the user would be useless if we didn't have anywhere to put that information and this is where variables come in. In the previous program s is a variable. Variables are like a box that can store some piece of data. Here is a program to show examples of variables:
a = 123.4 b23 = 'Spam' first_name = "Bill" b = 432 c = a + b print "a + b is", c print "first_name is", first_name print "Sorted Parts, After Midnight or",b23
And here is the output:
a + b is 555.4 first_name is Bill Sorted Parts, After Midnight or Spam
Variables store data. The variables in the above program are a, b23, first_name
, b, and c. The two basic types are strings and numbers. Strings are a sequence of letters, numbers and other characters. In this example b23 and first_name
are variables that are storing strings. Spam, Bill, a + b is, and first_name is
are the strings in this program. The characters are surrounded by " or '. The other type of variables are numbers.
Okay, so we have these boxes called variables and also data that can go into the variable. The computer will see a line like first_name = "Bill"
and it reads it as Put the string Bill into the box (or variable) first_name
. Later on it sees the statement c = a + b and it reads it as Put a + b or 123.4 + 432 or 555.4 into c.
Here is another example of variable usage:
a = 1 print a a = a + 1 print a a = a * 2 print a
And of course here is the output:
1 2 4
Even if it is the same variable on both sides the computer still reads it as: First find out the data to store and than find out where the data goes.
One more program before I end this chapter:
num = input("Type in a Number: ") str = raw_input("Type in a String: ") print "num =", num print "num is a ",type(num) print "num * 2 =",num*2 print "str =", str print "str is a ",type(str) print "str * 2 =",str*2
The output I got was:
Type in a Number: 12.34 Type in a String: Hello num = 12.34 num is a <type 'float'> num * 2 = 24.68 str = Hello str is a <type 'string'> str * 2 = HelloHello
Notice that num
was gotten with input while str
was gotten with raw_input
. raw_input
returns a string while input returns a number. When you want the user to type in a number use input but if you want the user to type in a string use raw_input
.
The second half of the program uses type which tells what a variable is. Numbers are of type int or float (which are short for 'integer' and 'floating point' respectively). Strings are of type string. Notice also how when python multiples a number by a integer the expected thing happens. However when a string is multiplied by a integer the string has that many copies of it added i.e. str * 2 = HelloHello
.