Working with Lists

If we need to store the grades of 30 students, creating 30 separate variables is inconvenient. A list (list) solves this: it keeps items in square brackets separated by commas, for example marks = [5, 4, 5, 3, 4]. A list can also hold values of different types.

Items are numbered starting from zero, and this number is called an index. marks[0] is the first item (5), marks[2] is the third. A negative index counts from the end: marks[-1] is the last item. The len(marks) function returns the length, and changing an item is simple: marks[1] = 5.

There are methods for modifying a list. marks.append(4) adds an item at the end, marks.insert(0, 3) inserts at a given position, marks.remove(3) deletes the first 3 found, and marks.sort() sorts in ascending order. The functions sum(marks), min(marks) and max(marks) find the total, the smallest and the largest value.

You can go through the items of a list with a for loop: for m in marks: and print(m) inside. To calculate the average grade, it is enough to write print(sum(marks) / len(marks)). To check whether a value is present, use the condition if 5 in marks:.

Key takeaways

• A list stores several values in one variable: [1, 2, 3].
• Indexes start at 0; len() returns the length.
• append, insert, remove and sort modify a list.
• A for loop goes through the items of a list one by one.