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Oct 02: Solving Problems with Loops

Meet in King 248

Learning Objectives

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

  • Use a loop to accumulate or filter values into a new list.
  • Write a loop that computes the min, max, and sum of a list.
  • Modify a list with append(), index assignment, removal, and sort.

Lesson Outline

Presentation [10 min]

  • Editing lists (appending, assignment, removal, sorting)

Loop Problems [40 min]

  • I'll explain each section below, and you'll solve the problems

Your To-Do List

for Statements

Code Meaning
for value in my_list: For each value in a list
for index in range(len(my_list)): For each index in a list
for index, value in enumerate(my_list): For each index and value

Exercise

Given a string, print each uppercase letter along with its index. Hint: use the str.isupper() method.

Ex: print_upper("James Madison Computer Science") should print: 

J at 0
M at 6
C at 14
S at 23

Exercise

Given a positive integer n, build a list of all numbers from 1 to n that are divisible by 3 or 5. Use range() to loop over the numbers, and append() each one that matches.

Ex: multiples(20) should return: 

[3, 5, 6, 9, 10, 12, 15, 18, 20]

Min, Max, Sum

  • The built-in min(), max(), and sum() functions require a loop.
  • Therefore, calling sum(grades) / max(grades) runs two loops.
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]))

Exercise

Find the index of the maximum value in a list.

Ex: index_max([13, -5, 100, 0, 77]) returns 2, because the maximum value 100 is at index 2.

Exercise

Count how many characters in a string are digits. Hint: use the str.isdigit() method.

Ex: count_digits("Today is 10/02/2023!") returns 8, because 8 of the characters are digits.

Looking Ahead

  • 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]))

Exercise

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 because of two "Soda" in a row.

Exercise

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 because of three "Cherry" in a row.