Module 2 · Making the program decide Lesson 6 of 15
Lists — one name, many values
A list holds an ordered set of values: marks = [72, 88, 65]. Indexing starts at 0, and slicing gets a piece: marks[0:2] is the first two.
The concept
A list keeps several values in order under one name: marks = [72, 88, 65]. Position one is written marks[0], because counting starts at zero, and a slice gets a piece: marks[0:2] is the first two values. len(), sum(), sorted(), append() and remove() cover almost everything a first job asks for.
What makes a list powerful is that the computer does the repetitive work: sum(marks) / len(marks) is the average in one line, and it still works when the list has ten thousand values instead of three. Looping over a list — for mark in marks: — is the pattern behind every marksheet, attendance sheet and report you will ever write.
marks = [72, 88, 65, 90]
print(marks[0]) # 72 — counting starts at 0
print(marks[0:2]) # [72, 88]
print(max(marks), min(marks), round(sum(marks) / len(marks), 1))
marks.append(77) # add one
marks.sort(reverse=True) # highest first
print(marks)Common mistakes
The mistakes are small and constant: using marks[1] when you meant the second value and getting the first, trying to sum() a list of text because the numbers came from an input file, and editing a list while looping over it. Convert first with [int(x) for x in marks], then calculate.
Worked example
sum(marks) / len(marks) gives the average in one line — the same three lines of manual addition a beginner usually writes first.
Your practice task
Store ten marks in a list and print the highest, the lowest and the average without writing a single if statement by hand.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
A list keeps several values in order under one name: marks = [72, 88, 65].
Keep your place in this course
A free account saves which lessons you have finished and unlocks the certificate. The lesson itself stays free to read.
Create free account or sign inCourse complete
You finished every lesson.
Your certificate is ready, and the projects you built are the proof to attach to an application.
Almost there
15 lessons left of 15
Finish every lesson to unlock the certificate. Each one ends in a practice task — do them and the final project brief writes itself.