print("Halt!") s = 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
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 = float(input("Type in a Number: ")) str = 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 <class 'float'> num * 2 = 24.68 str = Hello str is a <class 'str'> str * 2 = HelloHello
num was gotten with float(input(...)) while
str was gotten with
input returns a string and the function float converts it to a floating point number. There is also a function
int that converts a string or a floating point number into an integer.
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. Integers and floats
can be worked on by mathematical functions, strings cannot. Notice
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 repeated: for example
str * 2 = HelloHello.
The operations with strings do slightly different things than operations with numbers. Here are some interative mode examples to show that some more.
>>> "This"+" "+"is"+" joined." 'This is joined.' >>> "Ha, "*5 'Ha, Ha, Ha, Ha, Ha, ' >>> "Ha, "*5+"ha!" 'Ha, Ha, Ha, Ha, Ha, ha!' >>>
Here is the list of some string operations:
#This programs calculates rate and distance problems print("Input a rate and a distance") rate = float(input("Rate:")) distance = float(input("Distance:")) print("Time:", distance/rate)
> python3 rate_times.py Input a rate and a distance Rate:5 Distance:10 Time: 2.0 > python3 rate_times.py Input a rate and a distance Rate:3.52 Distance:45.6 Time: 12.9545454545
#This program calculates the perimeter and area of a rectangle print("Calculate information about a rectangle") length = float(input("Length:")) width = float(input("Width:")) print("Area", length*width) print("Perimeter", 2*length+2*width)
> python3 area.py Calculate information about a rectangle Length:4 Width:3 Area 12.0 Perimeter 14.0 > python3 area.py Calculate information about a rectangle Length:2.53 Width:5.2 Area 13.156 Perimeter 15.46
#Converts Fahrenheit to Celsius temp = float(input("Farenheit temperature:")) print((temp-32.0)*5.0/9.0)
> python3 temperature.py Farenheit temperature:32 0.0 > python3 temperature.py Farenheit temperature:-40 -40.0 > python3 temperature.py Farenheit temperature:212 100.0 > python3 temperature.py Farenheit temperature:98.6 37.0
Write a program that gets 2 string variables and 2 integer variables from the user, concatenates (joins them together with no spaces) and displays the strings, then multiplies the two numbers on a new line.