Strings and Print Statements

We have already used print() statements numerous times in this course, but our strings have always used double quotation marks ". There are actually several ways to signify a string in Python, and we will use several of them this year.

>>> print("Hello, World!")
Hello, World!
>>> print('Hello, World!')
Hello, World!
>>> print("""Hello, World!""")
Hello, World!
>>> print ('''Hello, World!''')
Hello, World!
>>> print("""Hello, World!
This is my first multi-line string.
The new line values (\\n) are included with the other
normal characters.""")
Hello, World!
This is my first multi-line string.
The new line values (\n) are included with the other
normal characters.                                      # This could have been useful in Project 1.
>>> print('Maggie's project')
      
SyntaxError: invalid syntax	#Python reads the apostrophe in Maggie's as a string mark.
>>> print("Maggie's project")
Maggie's project

Variables

We have also talked about how to create variables and assign values to them. This is pretty straightforward, but we have to keep in mind that we can do operations with a variable without changing the value of the variable. In order to change the value of the variable, we need to reassign the variable.

>>> score = 0
>>> print(score)
0
>>> score + 20
20
>>> print(score)
0
>>> score = score + 10	# This is variable reassignment
>>> print(score)
10
>>> score + 10
20
>>> print(score)
10
>>> score = 1000
>>> print(score)
1000				# The value of the variable can change often throughout the program.

Variables can be any length we choose, but good variables have these characteristics:

  • They are human-readable and understandable (score not s).
  • They begin with a letter or underscore (for now, yours should always start with a letter).
  • They don’t have any spaces or symbols (use underscore, if needed: number_books).
  • They don’t use a reserved word in Python (like class, try, from, etc.)

Integer and Modulo Division

Finally, we need to learn two new math operators that are extremely important in computer programming. We have already seen how to do regular math with the Python interpreter, including float division (4 / 2). There are two other kinds of division in Python.

>>> 9 // 4
2					# Integer division returns the whole number of the result
>>> 9 % 4
1					# Modulo division returns the remainder of the result

Alarm Clock Problem

Many people keep time using a 24 hour clock (11 is 11am and 23 is 11pm, 0 is midnight). If it is currently 13 and you set your alarm to go off in 50 hours, it will be 15 (3pm). Write a Python program to solve the general version of the above problem. Ask the user for the time now (in hours), and then ask for the number of hours to wait for the alarm. Your program should output what the time will be on the clock when the alarm goes off.

We will learn how to ask the user for data later in this unit. For now, let’s write a program that will calculate what time the clock will show when the alarm goes off.

Solution

#alarm_clock.py

#define variables
current_time = 13
added_hours = 5

#calculate alarm time
alarm_time = (current_time + added_hours) % 24

#print result to screen
print("Your alarm will go off in ", added_hours,"hours. Your clock will read", str(alarm_time) + ".")

Homework

  • Read Simple Python Data, 2.1-2.7 in the online textbook.
  • Do ex. 3-4
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