Additional Session 1 Exercises

Swapping the contents of variables

Explain what the overall effect of this code is:

left = 'L'
right = 'R'

temp = left
left = right
right = temp

Compare it to:

left, right = right, left

Do they always do the same thing? Which do you find easier to read?

Solution

Both examples exchange the values of left and right. In the first case we used a temporary variable temp to keep the value of left before we overwrite it with the value of right. In the second case, right and left are packed into a tuple and then unpacked into left and right.

In-Place Operators

Python (and most other languages in the C family) provides in-place operators that work like this:

x = 1  # original value
x += 1 # add one to x, assigning result back to x
x *= 3 # multiply x by 3
print(x)

6

Write some code that sums the positive and negative numbers in a list separately, using in-place operators. Do you think the result is more or less readable than writing the same without in-place operators?

Solution

positive_sum = 0
negative_sum = 0
test_list = [3, 4, 6, 1, -1, -5, 0, 7, -8]
for num in test_list:
if num > 0:
    positive_sum += num
elif num == 0:
    pass
else:
    negative_sum += num
print(positive_sum, negative_sum)

Here pass means “don’t do anything”. In this particular case, it’s not actually needed, since if num == 0 neither sum needs to change, but it illustrates the use of elif and pass.

Turn a String into a List

Use a for-loop to convert the string “hello” into a list of letters: [“h”, “e”, “l”, “l”, “o”]

Solution

my_list = []
for char in "hello":
    my_list.append(char)
print(my_list)

Reverse a String

Knowing that two strings can be concatenated using the + operator, write a loop that takes a string and produces a new string with the characters in reverse order, so 'Newton' becomes 'notweN'.

Solution

newstring = ''
oldstring = 'Newton'
for char in oldstring:
    newstring = char + newstring
print(newstring)