Decisions, Lists and Loops

Making decisions with if

score = 67

if score >= 70:
    grade = "A"
elif score >= 60:
    grade = "B"
elif score >= 50:
    grade = "C"
else:
    grade = "F"

print(f"Score {score} is grade {grade}")

Two rules that catch every beginner:

• The line before a block ends with a colon :.
• The block is indented (4 spaces). Indentation is how Python knows what belongs inside the if.

Comparison operators: == equal, != not equal, >, <, >=, <=. Combine conditions with and, or and not.

Lists

A list stores many values in order, inside square brackets:

courses = ["ECO 201", "STA 211", "GST 201"]
scores = [67, 81, 54]

print(courses[0])      # first item: ECO 201 (counting starts at 0)
print(len(scores))     # how many items: 3
courses.append("CSC 201")  # add one to the end
print(max(scores), min(scores), sum(scores))

Loops

A for loop runs the same code for every item in a list:

for course in courses:
    print("Registered:", course)

Loop over two lists together with zip:

for course, score in zip(courses, scores):
    if score >= 50:
        print(f"{course}: {score} - pass")
    else:
        print(f"{course}: {score} - fail")

Repeat something a set number of times with range:

for week in range(1, 5):
    print("Week", week)

This prints weeks 1 to 4. range stops before the second number.

Try it

1. Make a list of your scores in five courses (real or made up).
2. Loop through them and print a grade for each using if, elif and else.
3. Print the average score: sum(scores) / len(scores).
4. Count how many scores are 70 or above. Hint: start with count = 0 and add 1 inside the loop.