Exercises for Lesson 9

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Exercise 1: Writing functions that return values

Part a: Working with strings

Implement the following functions.

def getMiddleChar(s):
    """
    Returns the middle character from a string.  Rounds down if even length.

    s: a string
    returns: a single character at the middle position of s (a string)
    """
    # TODO
def getOutside(s):
    """
    Returns the concatentation of the first and last characters in a string.

    s: a string (assume its length is at least 2)
    returns: the first and last characters, concatenated together (a string)
    """
    # TODO
def getInnerString(s):
    """
    Returns a string that is missing its first and last characters.

    s: a string (assume its length is at least 2)
    returns: the inner part of s, without the first and last characters (a string)
    """
    # TODO

Here is an example of how these could be used:

s = "pineapple"
res1a = getMiddleChar(s)
res1b = getMiddleChar("pine")
print(res1a, res1b) # what does this give?

res2 = getOutside(s)
res3 = getInnerString(s)
print(res2, res3) # what about this one?

Part b: Element-wise addition

Write a function that takes in two lists of numbers, and returns a list in which each value is the sum of the two corresponding input values. You can assume both input lists have the same length.

Here is an example output from this program:

firstList = [1,2,3,4]
secondList = [8,2,6,1]
sumList = perElementAddition(firstList, secondList)
print(sumList) # prints: [9, 4, 9, 5]

Here is your function to define:

def perElementAddition(list1, list2):
    """
    Returns the per-element sum of the two lists.

    list1: a list of numbers
    list2: a list of numbers of the same length as list1
    returns: a list of the per-element totals (a list of numbers)
    """
    # TODO

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Exercise 2: What’s in a name? (variable scope)

Now, let’s discuss the following function, and some code that uses it.

def cube(x):
    answer = x * x * x
    return answer

Why does the following code print 4 27 and not 27 27?

answer = 4
result = cube(3)
print(answer, result) # prints: 4 27

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Exercise 3: True and False

For each of the following expressions, predict what will be printed.

a)  3 < 4

b)  3 >= 4

c)  3 != 4

d)  3 == 4

e)  "hello" == "hello"

f)  "hello" < "hello"

g)  "Hello" < "Hello"

h)  "hello" < "Hello"

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Exercise 4: Two-way decisions

For each of the following code snippets, predict what will be printed.

x = 10
y = 12
z = 4

# (a)
if x > 10:
    print("yes")
else:
    print("no")

# (b)
if x == y:
    print("yep")
else:
    print("nope")

# (c)
if z * 3 < y:
    print("si")
else:
    print("no")

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Exercise 5: Functions that return multiple values

Read through the following code:

def sumDiff(x, y):
    total = x + y
    diff = x - y
    return total, diff

def main():
    valString = input("Please enter two numbers, separated by a comma: ")

    vals = valString.split(',')
    numStr1, numStr2 = vals # "unpack" the values of the list

    num1 = float(numStr1)
    num2 = float(numStr2)

    resultVals = sumDiff(num1, num2)
    total, diff = resultVals # unpack again, this time from the resulting tuple

    print("The sum is", total, "and the difference is", diff)

main()

Let’s say the user enters 7,3. What do you expect the output to be?

What would happen if the return line at the end of sumDiff weren’t there?

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Exercise 6: Modifying function parameters

The following function converts a temperature in degrees Celsius to the temperate in Fahrenheit.

def convertToFahrenheit(celsiusTemp):
    return 9/5 * celsiusTemp + 32

Write a function that takes in a list of temperatures in Celsius and modifies that list, converting each temperature to its corresponding value in Fahrenheit. It does not need to return anything.

def convertListToFahrenheit(tempList):
    # TODO

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Exercise 7: Input validation

Up until now, we’ve assumed that users provide the type of input we want. Add error checking to the following function. In your modified version, the program should just return from main early if the user types anything 2 or smaller for n.

from graphics import *

def main():
    # Get the number of points from the user
    n = int(input("How many points would you like? "))

    # Make the window
    win = GraphWin("A shape with " + str(n) + " points", 800, 600)

    # Get the points
    points = []
    for i in range(n):
        p = win.getMouse()
        points.append(p)
        p.draw(win)

    # Draw the polygon
    polygon = Polygon(points)
    polygon.setFill("forest green")
    polygon.draw(win)

main()

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