Lists, Strings and Files in Python

Lists

A list holds many values under one name, each found by its index, counted from 0.

scores = [14, 9, 21, 6, 18]
print(scores[0])
print(scores[-1])
scores[3] = 11
print(len(scores))

scores[0] is 14 and scores[-1] is the last item. len(scores) is 5, and the last index is 4.

Working through a list

total = 0
for score in scores:
    total = total + score
print("Average:", total / len(scores))

Loop over the items when you only need the values. Loop over range(len(scores)) when you need the index as well, for example to change items in place.

Useful list operations

scores.append(25)
scores.insert(0, 3)
scores.remove(9)
value = scores.pop()
scores.sort()
print(max(scores), min(scores), sum(scores))

Two dimensional lists

A list of lists, useful for anything grid shaped.

board = [[1, 0, 0],
         [0, 1, 0],
         [0, 0, 1]]

print(board[1][2])

for row in board:
    for cell in row:
        print(cell, end=" ")
    print()

The first index selects the row, the second the position within it.

Strings

A string is a sequence of characters, indexed from 0 just like a list.

word = "Computing"
print(len(word))
print(word[0])
print(word[0:3])
print(word[-1])
print(word.upper())
print(word.lower())
print(word.find("put"))
print(word.replace("ing", "er"))
print("put" in word)

word[0:3] is a slice, and it gives characters 0, 1 and 2. Like range, a slice stops before the second number.

Splitting and joining

line = "Amira,15,Year 10"
parts = line.split(",")
print(parts[0])
joined = "-".join(parts)

split is how you take a line read from a file apart, and it is needed in almost every Paper 2 task that involves a data file.

Characters as numbers

print(ord("A"))
print(chr(66))

ord gives the character set code and chr converts back, which is how case conversion and simple ciphers are done.

Files

A program that forgets everything when it closes is of limited use. Writing to a file makes data persist between runs.

Reading

file = open("scores.txt", "r")
for line in file:
    print(line.strip())
file.close()

strip() removes the newline character at the end of each line, and forgetting it is why comparisons against read values so often fail.

The safer form closes the file for you even if something goes wrong:

with open("scores.txt", "r") as file:
    lines = file.readlines()

Writing

with open("scores.txt", "w") as file:
    file.write("Amira,18\n")
    file.write("Ben,14\n")

Putting it together

totals = {}
with open("scores.txt", "r") as file:
    for line in file:
        parts = line.strip().split(",")
        name = parts[0]
        score = int(parts[1])
        totals[name] = score

for name in totals:
    print(name, totals[name])

Note the int() on the score. Everything read from a text file is a string, every time.

Key terms you must be able to define

List

A Python data structure holding many values under one name, accessed by index.

Index

The position of an item, counted from zero.

Two dimensional list

A list whose items are themselves lists, used for grids and tables.

Slice

A section of a list or string taken between two indexes, stopping before the second.

split

A string method that divides a string into a list at a given separator.

strip

A string method that removes whitespace, including the newline, from both ends.

append

A list method that adds an item to the end.

Append mode

Opening a file with "a" so that new data is added to the end rather than replacing it.

Flashcards

Say the definition out loud before you flip the card. Retrieving it from memory is what builds recall, reading it again is not.

Reach the top grade on this topic

Paper 2 tasks are almost always list or file tasks, so the details here are worth more than anywhere else.

Strip and cast every line you read. int(line.strip()) handles both the newline and the type in one step, and skipping either produces a fault that is hard to see because the value looks correct when printed.

Use len(list) - 1 when you need the last index. Write it deliberately rather than by habit, and check it against a small example.

Choose the loop that matches what you need. for item in list when you only need values, for i in range(len(list)) when you need to change items in place.

Open files in the right mode. Reading is "r", appending is "a", and "w" empties the file first. Say why you chose the mode if a question asks.

Mistakes that cost marks here