October 3: Loops

Lesson Outline¶

Mini Lecture: More on for loops

• For Loops

Inclass Activity

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() require a loop
• 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]))

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

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

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.

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(), 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

_{*Thanks to Dr. Mayfield for these exercises}

Your To-Do List¶

Due Wednesday October 4th at 11pm*

• Submit HW 6 through Canvas and check out the Help page. Remember the attribution statement

Prepare for Quiz 3A

• Thursday, beginning of class, 25 minutes, in EnGeo 2204.
• Two parts: concepts and programming (will use lab machine account, canvas and gradescope)
• Study chapters 5 and 6: both reading and activities
• Review class activities

• Review homework programming assignments By today

• Start Chapter 7 "orange" textbook activities

• Start Homework 7: HW 7

Complete by Monday October 9th

• Complete the Chapter 7 "orange" textbook activities. You must do this through CANVAS to receive credit.

Due Wednesday October 11th at 11pm

• Submit HW 7 through Canvas and check out the Help page