Lab 7: Loops

Creating the necessary folder

Create a subfolder called lab7 within your cs111 folder, and put all of the files for this lab in that folder.

Write functions that use a loop

In Part II of Problem Set 5, you will write a number of functions that use a loop to process a sequence. This task of the lab helps to prepare you for those problems.

In Spyder, open a new editor window and save it to your lab7 folder using the name lab7.py.

1. Earlier in the course, we covered a recursive function called num_vowels that took an arbitrary string of lower-case letters as input and returned the number of vowels in the string. For example:

   >>> num_vowels('hello')
   result: 2
   
   >>> num_vowels('aloha')
   result: 3
   

   Write a version of this function that uses a loop instead of recursion. Here’s a template:

   def num_vowels(s):
       """ returns the number of vowels in a string s consisting 
           of lower-case letters
       """
       count = 0
   
       for c in s:
   
       # complete the rest of the function!
   

   Note that this function should perform a cumulative computation that gradually builds up the final count of the number of vowels.

2. We also covered a recursive function called rem_all that took an arbitrary list of values vals and a single element elem and that used recursion to create a new list in which all occurrences of elem were removed. For example:

   >>> rem_all(10, [3, 5, 10, 7, 10])
   result: [3, 5, 7]
   
   >>> rem_all(4, [2, 4, 8, 4, 4])
   result: [2, 8]
   

   Write a version of this function that uses a loop instead of recursion. Here’s a template:

   def rem_all(elem, values):
       """ returns a list in which all occurrences of elem 
           in values have been removed
       """
       result = []
   
       for ___ in ____:
   
       # complete the rest of the function!
   

   Note that this function also needs to perform a cumulative computation! We have called the accumulator variable result, and we’ve initialized it to be an empty list. Inside the loop, you should use list concatenation to gradually build up a list containing all of the values that belong in the final result.

   Important: Although an element-based loop would work here, you should use an index-based loop to get practice with that type of loop. If you need help starting that type of loop, feel free to ask a staff member.

3. In lecture, we learned how to use the input() function to get a value from the user. Write a function get_pos_int(prompt) that takes a string prompt and performs the following tasks:

   • gets an initial integer value from the user using the input function and the specified prompt

   • if the user does not enter a positive integer, keeps asking for additional inputs until the user enters a positive value; these additional inputs (if any) should use the following prompt:

     The input must be positive. Try again:
     

   • once the user enters a positive value, returns the value.

   You may assume that the user always enters an integer.

   For example:

   >>> get_pos_int('Enter your age: ')
   Enter your age: -2
   The input must be positive. Try again: -5
   The input must be positive. Try again: 0
   The input must be positive. Try again: 20
   result: 20
   

   Hints:

   • This problem lends itself to an indefinite loop (i.e., a while loop). Why?

   • Don’t forget that input always returns a string. As a result, you will need to use the int function to convert the input to an integer as discussed in lecture.

Midterm review

As time permits, the course staff will be giving you a chance to do some review for midterm 1.

Important: You will not be allowed to use loops on this midterm exam!

Optional: Additional practice on loops

Complete any of the problems listed under the Warmup-2 collection of problems on CodingBat. Rather than solving them using recursion, use a loop instead!

Last updated on October 20, 2025.