Loops - Answer Key

Exercise 1

1. Create a for loop that goes through a list that contains True, 4 and pretzel. For each element in that list have it print out the element followed by the datatype of the element.

# Iterate through the list, assigning a new element to "element" each time through
for element in [True, 4, "pretzel"]:
    # Print out the element followed by the data type of the element
    print(element, type(element))

True <class 'bool'>
4 <class 'int'>
pretzel <class 'str'>

Exercise 2

1. Use list comprehension to sort through a list ["DNA", "RNA", "Protein"] and only retain the elements “DNA” or “RNA”.

# 
[molecule for molecule in ["DNA", "RNA", "Protein"] if molecule == "DNA" or molecule == "RNA"]

['DNA', 'RNA']

Exercise 3

1. The Collatz conjecture is a big open question in mathematics. It states that any positive integer will form a sequence that ends with 1, if:

• On even numbers you halve the value
• On odd numbers you triple the value and add 1.

Let’s create a while loop to evaluates 17 to see each step in the path that it takes on its way to 1. With each iteration, we want:

• Print the current integer being tested
• If it is an even integer, divide it in half
• If it is in an odd integer, multiply it by 3 and add 1
• Exit the while loop when it reach 1

Hint: You will likely you want to use a modulo operator, %. Modulo operators tell you the remainder following division. For example, 4 mod 2 would return 0 because the remainder of 4 divided by 2 is zero, while 5 mod 2 would returns 1 because the remainder of 5 divided by 2 is 1.

# Initialize the number variable
number = 17

# While number is NOT equal to one
while number != 1:
    print(number)
    # If the value is even
    if number % 2 == 0:
        # Halve the number
        number = number / 2
    # If the value is odd
    else:
        # Multiple number by 3 and add 1
        number = 3 * number + 1

# Print the final number
print(number)

17
52
26.0
13.0
40.0
20.0
10.0
5.0
16.0
8.0
4.0
2.0
1.0