Lecture 7 - Exercises

7A - While Loops

1. What does the following program print?

   count = 10
   while count < 21:
       print(count, end=" ")
       count += 3

2. Consider the following program:

   n = 12345
   m = 0
   while n != 0:
       m = (10 * m) + (n % 10)
       n //= 10

   1. How many times is the body of the while loop executed?
   2. What are the values of m and n after the code is executed?

3. Consider the following code, which has two missing snippets, marked [[1]] and [[2]].

   i = [[1]]
   while [[2]]:
     i += 1
     print(i)

   Consider the following candidates to replace the missing snippet [[1]]:

   1. 0

   2. 1

   3. 10

   and the following candidates to replace missing snipped [[2]]:

   1. i < 9

   2. i <= 9

   3. i < 10

   4. i <= 10

   To form a complete program, we need to pick a pair of snippets, one to replace [[1]] and one to replace [[2]].

   1. Which pair or pairs of snippets will cause the program to print the numbers from 1 to 10, inclusive?
   2. Which pair or pairs of snippets will cause the program to print exactly 10 numbers?

7B - Nested while loops

4. Consider the following program:

   i = 0
   while i < 4:
     j = 1
     while j < 11:
       print('*', end='')
       j += 1
     i += 1
     print()

   1. How many asterisks (*) are printed by the program?
   2. What are the values of i and j at the end of the program?

Problems

1. Write a program that prompts the user for a positive integer and prints the powers of 2 up to .

   Enter a power: 4
   2^0: 1
   2^1: 2
   2^2: 4
   2^3: 8
   2^4: 16

2. Write a program that prompts the user for a positive integer and prints the powers of 2 less than .

   Enter a number: 9
   2^0: 1
   2^1: 2
   2^2: 4
   2^3: 8

3. Write a program that prompts the user until they enter the a secret password correctly. For this problem, the secret password can be whatever you want and should be hard-coded into the program. For example, if I chose "banana" as the secret password, a run of the program might look like this:

   Enter the password: algebra
   Incorrect, try again: bookend
   Incorrect, try again: banana
   You're in!

4. Write a program that repeatedly prompts a user for positive numbers until a negative number is entered, then print the sum of the positive numbers entered (be sure not to include the negative number in the sum).

   Enter a number: 4
   Enter a number: 8
   Enter a number: 3
   Enter a number: -9
   The positive numbers entered sum to 15

5. Write a program that prints a multiplication table for all possible combinations of the numbers 1 through 6. Ideally, print spaces as needed to pad out single-digit numbers so the table’s columns line up neatly.

   1  2  3  4  5  6
   2  4  6  8 10 12
   3  6  9 12 15 18
   4  8 12 16 20 24
   5 10 15 20 25 30
   6 12 18 24 30 36

6. (Challenge Problem!) Write a program that prompts the user for a positive integer and prints the binary representation of that integer, with no leading zeros.

   Enter a positive integer: 65
   65 in binary is 1000001