Loops, Conditions, and Lists

Check if an item is in the list

Sometimes you want to find out if some item is in the list. You can use the same pattern as before, but with a different change to the variable in the loop body.

Here’s an example:

# Does the list have string "Python" in it?
languages = ["Ruby", "JavaScript", "C", "Rust", "Smalltalk", "Clojure", "Python"]

has_match = False
for language in languages:
	if language == "Python":
		has_match = True

print(has_match)

It’s the same pattern as before, but with an if statement in Step 2.

• 🔑 Step 1: Create a variable called has_match, which starts as False
• 🔑 Step 2: Loop through the strings in the list. If the string matches, set has_match to True
• 🔑 Step 3: Print out the has_match variable

Filter a list

Sometimes, you want to find only the values in your list that meet a certain condition.

Again, we can use the three step pattern. The variable we are updating will be a new list, with just the filtered values.

numbers = [62, 60, 53, 53, 33, 3, 25, 61, 36, 1, 69, 55, 56, 39, 32, 76, 20, 62, 47]
# Write a program to find all the even numbers in the list
evens = []
for element in numbers:
  if element % 2 == 0:
		evens.append(element)

print(evens) # [62, 60, 36, 56, 32, 76, 20, 62]

We use an if statement and the modulo operator to find the even values, and add them to the list.

Practice: Sum of Odds

numbers = [62, 60, 53, 53, 33, 3, 25, 61, 36, 1, 69, 55, 56, 39, 32, 76, 20, 62, 47]

total = 0
for n in numbers:
  if n % 2 == 1:
    total += n

print(total)

Smallest Item

Finding the minimum value in a list uses the same pattern. Step through the elements of the list, and update a variable that is tracking the current minimum value.

Each time through the loop, check if the number is smaller than the current minimum value. If it is, then it’s the new minimum value.

We can use the first value in the list as the initial value of the minimum.

numbers =  [62, 60, -53, 53, 33, -3, 25, 81, 36, 1, 69, 55, 56, 39, -32, -76, 20, 62, 47]
minimum = numbers[0]
for number in numbers:
	if number < minimum:
		minimum = number

print(minimum) # -76

With one small change, this same code works to find the largest value instead.

Practice: Interactive Minimum

numbers = int(input("How many numbers will you enter? "))
i = 0
minimum = float('inf')

for i in range(numbers):
  x = int(input("Enter a number: "))
  # check whether x is smaller than minimum, and reassign minimum to it if so
  if x < minimum:
    minimum = x

print("The smallest number is:", minimum)