CoCalc Public FilesSkills 2020 / Skill-02-IntroPython-Part2-Student.ipynbOpen with one click!
Authors: Alyssa Gadsby, Prof Michael Huster, Asia Parker
Compute Environment: Ubuntu 20.04 (Default)

Introduction to Python, Part 2

This skill homework shows a few extensions to the previous homework on variables, lists, and functions, then covers some completely new territory.

More on Operators

Python, of course, has the traditional mathematical operators +, -, *, and / for addition, subtraction, multiplication and division. The one important note is that division with integers (since we are using python 3) will always give a float.

In the cell below try some integer division like

print(33/33)
print(33/2)
print(33/2)

Additional Mathematical Operators

There are three additional math operators you should know.

The remainder operator, %, does a division and returns the remainder. It works with both integer and float division. Try the following remainders:

print(255 % 16)
print(4.2%1.5)

Next the power operator is **. (Many languages use the ^, but in python this is the bitwise XOR operator, so be careful!) It raises the first number to the power of the second. If both operators are integers, it will return an integer if possible. Try the following:

print(2**3)
print(3**2)
print(2**(-1))
print(1.2**1.2)

Note: I usually do not put spaces around the power operator because it has higher precedence than the other mathematical operators.

Finally, the integer division or floor division operator is //. This acts like normal integer division if the operators are integers. The odd feature is that the answer is rounded down, even if the answer is negative. If either or both of the operands are floats, the answer will be a float round down. Try the following floor divisions:

print(3//2)
print(-3//2)
print(100.0//3.0)

Look at the results to make sure you understand how these operators work.

In [1]:
print(33/33) print(33/2) print(33/2)
1.0 16.5 16.5
In [2]:
print(255 % 16) print(4.2%1.5)
15 1.2000000000000002
In [3]:
print(2**3) print(3**2) print(2**(-1)) print(1.2**1.2)
8 9 0.5 1.2445647472039776
In [4]:
print(3//2) print(-3//2) print(100.0//3.0)
1 -2 33.0

Compound Assignments Like +=

Like C and most modern languages, python supports compound assignments. These are statements that modify the current value of a variable by performing an operation on it. They are equivalent to assigning the result of an operation to the first operand:

The following pairs of statements have the same result:

y += x or y = y + x

even

price *= units + 1 is the same as price = price * (units+1)

In the code cell below, create some variables and try compound assignments on them. Here are my suggestions, but feel free to try your own.

x = 2
y  = 3
y += x
x *= 1.6
y += x
print(x)
print(y)

Are You Remembering Tab Completion?

Keep trying tab completion. Once you get used to it, you will regret using a software development system that doesn't have it.

In [6]:
r=7 print(r) r += 7 print(r) r *= 7 print(r)
7 14 98

Boolean Variables and Operators (True & False)

You often need to make true or false decisions in computer code. This is often done with Boolean or true-false expressions. In python the two values are True and False. Boolean variable can also be defined. In addition zero is False and a non-zero number is True

Some Boolean operators are not, and, and or. Note that in python, these are spelled out.

In the cell below create the variable tr with the value True, and the varaible fa with the value False. Then print out not tr, tr and fr, and tr or fa. Make sure you understand the results before moving on.

My examples are

tr = True
fa = False
print(not tr)
print(not fa)
print(tr and fa)
print(tr or fa)
In [7]:
tr = True fa = False print(not tr) print(not fa) print(tr and fa) print(tr or fa)
False True False True

Comparison Operators ==, >, etc.

Python has the usual comaprison operators == (for equal to), != (not equal to), and the math-type comparisons >=, >, <, <=. All of these operators evaulate to either True or False. In the cell below try each of these by typing something like

print(3 < 2)

Make one example of each comparison operator.

In [10]:
print(27>8) print(17==18)
True False

Flow control with if, elif, and else

The following is an example of an if statement:

answer = 3
if answer > 2:
    print("Greater than 2")
    print("Go fish")
print("Done")

Note the : after the if statement. This is required. Also note that the lines indented are only executed if the expression is true and the line dedented to the same position as the if statement is always executed. This is a reminder that indentation is very important in python.

Go ahead and run the lines above in the code cell below.

In [11]:
answer = 3 if answer > 2: print("Greater than 2") print("Go fish") print("Done")
Greater than 2 Go fish Done

The else Statement

Many times you want to choose between two strategies. The else statement works like this:

answer = 5
if answer > 2:
    print("Greater than the mystery number")
    print("Go fish")
else:
    print("Less than or equal to the mystery number")
    print("Try another guess")
print("Done")

Again, the block that is executed when the if experssion is false must be indented. Try this in the cell below. Rerun the cell with different values for answer.

In [13]:
answer = 2 if answer > 2: print("Greater than the mystery number") print("Go fish") else: print("Less than or equal to the mystery number") print("Try another guess") print("Done")
Less than or equal to the mystery number Try another guess Done

Multiple Choices. The elif Statement

Finally if there are multiple branches you can use the elif statement which stands for else if condition. You can stack together multiple elif statements in after an if statement. Here is an example:

answer = 2
if answer > 2:
    print("Guess is too big")
    print("Go fish")
elif answer == 2:
    print("Correct! Game Over!")
else:
    print("Guess is too small")
    print("Try another guess")
print("Done")

Enter this code and play the game using different guesses for answer.

In [19]:
answer = -1 if answer > 2: print("Guess is too big") print("Go fish") elif answer == 2: print("Correct! Game Over!") else: print("Guess is too small") print("Try another guess") print("Done")
Guess is too small Try another guess Done

Make This a Function

Many times I will develop code in a cell (usually more complicated than our guessing game), then when it is working make it into a function. In the cell below, write a function with one argument being the guess, then test your code by guessing answers that are too small, too big and just right. Define the answer as a global variable in the cell. You also want your function to return True when the guess is correct and False when the answer is wrong.

I have given you an outline for the function. You have to fill it in and test it.

BTW, the # symbol starts a comment. The # and anything after it in a line is ignored.

Note: The code in the cell below WILL NOT WORK. You have to fix it to make it run.

In [29]:
answer =20 # set answer an integer number # next start the function definition def game(guess): # print the value guessed print("Your guess is ", guess) if answer>20: print("Your guess is too big") print("Go fish") elif answer==20: print ("your guess is correct") else: print("Try another guess") print("Done")
your guess is correct

while Loops

Many times your code has to loop until some ending condition is met. Now that you have your function game, here is how you can get input from the user, play the game, and quit when the answer is right. I also generate a random integer from 1 to 10.

The code in the cell is not quite right. You should change it to correctly keep track of the number of guesses.

Note: this code uses the function game you fixed above.

In [30]:
from random import randint nGuesses = 1 answer = randint(1,10) print("Guess a number from 1 to 10") yourGuess = int(input("Enter your guess: ")) while game(yourGuess) == True: yourGuess = int(input("Enter your guess: ")) print("It took you", nGuesses, "guesses to get it right!")
Guess a number from 1 to 10
Enter your guess:
Your guess is 20 It took you 1 guesses to get it right!

Other Topics

There are many other topics, but the material covered should get you started.

Conclusion

This ends the second skills homework for python. This tour through operators and function should get you started. Feel free to contact me with any questions by e-mail.

The next topics are libraries, arrays from the numpy library, and reading and writing data files.

In [1]:
from IPython.core.display import HTML def css_styling(): styles = open("custom.css", "r").read() return HTML(styles) css_styling()