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Oct 02: Solving Problems with Loops

Learning Objectives

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

  • Use a loop to find and count words in a dictionary.
  • Write a loop that computes the min, max, and sum.
  • Solve problems that compare previous/next values.

Reminders

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
for key in my_dict: For each key in a dict
for key, value in my_dict.items(): For each key and value

Exercise

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

Exercise

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 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.

Exercise

Count how many characters in a string are digits.

Ex: count_digits("Today is 10/02/2023!") returns 8.

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.

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.

Extra Practice

CodingBat.com is a free site of live problems to build skill in Java and/or Python. CodingBat was created by Nick Parlante, who is Computer Science lecturer at Stanford.

Note: Do not use the built-in min(), max(), or sum() for any of these problems.

List-2 Problems

  • Easier: count_evens, has22
  • Medium: big_diff, sum13
  • Harder: centered_average, sum67

String-2 Problems

  • Easier: double_char, count_hi
  • Medium: count_code, cat_dog
  • Harder: xyz_there, end_other