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Oct 02: Prac3, for Statements

Learning Objectives

After today's class, you should be able to:

  • Explain the syntax and meaning of a for statement.
  • Describe examples of how a for loop can be used.
  • Predict how range() works given 1, 2, or 3 arguments
  • Use for loops to obtain information from containers

Reminders

HW6 Debrief

  • Testing with doctest Real Python Tutorial
  • random.randint() versus random.random()
  • if-elif-else inside of a while loop
  • Importing functions from another module

Practice Quiz 3

  • Two portions to the quiz: written and coding
  • Log in as student
  • Start exam recording
  • Log in to Canvas only
  • Raise your hand when complete with Canvas written portion
  • Open Thonny for coding portion only
  • Submit code through Canvas (will go to Gradescope)
  • Log out when finished

POGIL Activity

  • For loops
    • If you are absent today, complete this activity at home
    • Bring your completed activity to class or office hours

Example Code

Model 1 

print("hello")
for x in [2, 7, 1]:
    print("the number is", x)
print("goodbye")

Model 2 

range(5)
list(range(5))
x = range(3)
print(x)
print(list(x))
list(range(5, 10))
list(range(-3, 4))
list(range(4, 10, 2))
for i in range(5):
    print(i)
Question 8 
for c in "Hi!":
    print(c)
Model 3 
names = ["emma", "liam", "aisha", "mateo", "sofia", "ravi"]

for i in range(len(names)):
    name = names[i]
    # replace element at index i
    names[i] = name.capitalize()
Question 24 
prices = [19.99, 6.34, 1.00, 12.79, 2.50]

Bonus Work

1. Given a string, print each uppercase letter along with its index. Ex: print_upper("James Madison Computer Science") should print: 

J at 0
M at 6
C at 14
S at 23
2. Given a sentence, count how many times each word occurs. Use a dictionary to keep track of the count of each word. Ex: count_words("mat the cat sat on the mat") should return: 
{'mat': 2, 'the': 2, 'cat': 1, 'sat': 1, 'on': 1}

Min, Max, Sum

  • the built-in functions min(), max(), sum(). The implementation of these require a loop. The code for these may look something like this:
    def my_min(sequence):
        result = sequence[0]
        for value in sequence:
            if value < result:  # found new minimum
              result = value
     return result

    def my_max(sequence):
        result = sequence[0]
        for value in sequence:
            if value > result:  # found new maximum
                result = value
        return result

    def my_sum(sequence):
        result = 0
        for value in sequence:
            result += value     # add running total
        return result


    if __name__ == "__main__":
        print("min:", my_min([13, -5, 100, 0, 77]))
        print("max:", my_max([13, -5, 100, 0, 77]))
        print("sum:", my_sum([13, -5, 100, 0, 77]))
  • What happens with this line of code: sum(grades)/max (grades)? How many loops are executed?

3. Find the index of the maximum value in a list. Ex. index_max([13, -5, 100, 0, 77]) returns 2.

4. Count how many characters in a string are digits. Ex. count_digits("Today is 10/03/2023!") returns 8

Indexing

  • Sometimes a loop needs to look at index [i + 1].
  • In that case, the range should end at len(s) - 1.
def is_sorted(seq):
    for i in range(len(seq) - 1):
         if seq[i] > seq[i + 1]:
            return False
    return True

if __name__ == "__main__":
    print("yes sorted:", is_sorted([1, 5, 10, 13, 16]))
    print("not sorted:", is_sorted([1, 5, 13, 16, 10]))

5. Write a function, two_in_row, that returns True if two consecutive values are equal. Ex. two_in_row(["Pizza", "Soda", "Soda", "Candy", "Salad"]) returns True.

6. Write a function, three_in_row, that returns True if three consecutive values are equal. Ex. three_in_row(["Apple", "Banana", "Cherry", "Cherry", "Cherry"]) returns True.