Computer Science · Class 11 · Lists Revisited

ListsInput⏱️ 13 min read

Reading a List with a Loop

Every list so far has been typed into the program by you. A real program is given its data by whoever is using it — and input() hands back one string, not a list. The standard way round that is the one on this page: ask how many values there will be, then go round that many times, appending one each round.

1Why this needs a loop

Because you do not know how many values there will be when you write the program — and a program cannot have a different number of input() lines depending on the answer. A loop can run a different number of times on every run, which is exactly the shape of the problem. The count is asked first, so that the loop knows how many rounds to take.

Input

what we ask the user for

how many numbers, n
then n numbers, one at a time

Process

what we work out

start with an empty list
cast each value and append() it

Output

what we show

the finished list, and how many items it holds

2The program

read_list.py

# read a list of numbers, asking how many there will be first

n = int(input('How many numbers? '))
numbers = []

for i in range(n):
    value = int(input('Enter number ' + str(i + 1) + ': '))
    numbers.append(value)

print('The list is', numbers)
print('It has', len(numbers), 'items')

Output

How many numbers? 3
Enter number 1: 10
Enter number 2: 20
Enter number 3: 30
The list is [10, 20, 30]
It has 3 items

n = int(input('How many numbers? '))

The count, cast to a whole number because range() will not take text. This one question is what turns an unknown number of values into a known number of rounds.

numbers = []

An empty list, made BEFORE the loop. It has to exist before anything can be appended to it, and it has to be outside the loop or each round would throw the previous value away.

for i in range(n):

n rounds. The loop variable i is not used to index anything here — it is only there to number the prompts, which is the one job it has.

value = int(input('Enter number ' + str(i + 1) + ': '))

Three things at once: build the prompt, read the answer, cast it. str(i + 1) is what lets the number be joined to the text — + refuses to mix a string with a number.

numbers.append(value)

One item on the end of the list, once per round. After n rounds the list holds n items, which is what len() confirms on the last line.

Key Takeaway

numbers = [] goes above the loop, always. Put it inside and every round starts with an empty list again, so the finished list holds exactly one item — the last one typed. It is the same fault as total = 0 inside a loop, and it looks just as reasonable.

read_list.py — with numbers = [] moved inside the loop

Output

How many numbers? 3
Enter number 1: 10
Enter number 2: 20
Enter number 3: 30
The list is [30]
It has 1 items

3Numbering the prompts

'Enter number ' + str(i + 1) + ': ' is worth taking apart, because it is the first time a prompt has been built rather than written out. i counts 0, 1, 2, so i + 1 counts 1, 2, 3 — people number things from one. str() is what makes the join legal:

joining.py

# text and numbers do not add

i = 0

print('Enter number ' + str(i + 1) + ': ')
print('Enter number', i + 1)

Output

Enter number 1:
Enter number 1

Watch Out

'Enter number ' + (i + 1) raises TypeError. can only concatenate str (not "int") to str — the same refusal as anywhere else. Inside input() the prompt has to be one finished string, so str() is not optional there. In a print() you can use commas instead and let print() do the joining, which is why the second line above needs no str().

4The same shape, for names

Nothing about the shape is about numbers. Drop the int() and the same program reads a list of names — and dropping it is not an oversight, it is the point: a name is text and must stay text.

read_names.py

# the same shape, reading names instead of numbers

n = int(input('How many names? '))
names = []

for i in range(n):
    name = input('Enter name ' + str(i + 1) + ': ')
    names.append(name)

print('The list is', names)

Output

How many names? 2
Enter name 1: Asha
Enter name 2: Ravi
The list is ['Asha', 'Ravi']

Notice the quotes in the output. print() shows a list the way Python would write it, so the strings keep their quotes — which is how you can tell at a glance that these are text and the numbers in the first program were not.

read_list.py

n = int(input('How many numbers? '))
numbers = []

for i in range(n):
    value = int(input('Enter number ' + str(i + 1) + ': '))
    numbers.append(value)

print('The list is', numbers)
print('It has', len(numbers), 'items')

n = int(input('How many numbers? '))
numbers = []

for i in range(n):
    value = int(input('Enter number ' + str(i + 1) + ': '))
    numbers.append(value)

print('The list is', numbers)
print('It has', len(numbers), 'items')

5What can go wrong

The user types a word where a number was wanted

int('ten') raises ValueError: invalid literal for int() with base 10: 'ten' and the program stops there, halfway through filling the list. Nothing on this course fixes that yet — try/except is Class 12 — but you should know why it happens.

The count and the values disagree

Type 5 and then get bored after three: the program keeps asking, because the loop was told 5 rounds. The count is a promise the user has to keep.

0 is a perfectly good answer

range(0) runs no rounds at all, so the list stays empty and prints as []. No error — and any program that then does numbers[0] or divides by len(numbers) will fail on it.

6Recap

Ask the count first

It is what turns an unknown number of values into a known number of rounds — and it is why this is a for loop and not a while.

The empty list goes above the loop

numbers = [] inside the loop leaves you with a one-item list holding whatever was typed last.

Cast each value as it arrives

int() for numbers, nothing at all for text. Each value is read and cast on its own, one round at a time.

str() to build a prompt

A prompt is one finished string, so a number joined into it must be converted. print() takes commas instead and needs no str().

✍️ Now write these yourself

1
Read a list of marks and print the total and the average.
Hint · This program, then the total program from two pages back. Nothing new is needed.
2
Read a list of numbers and print the largest, without max().
Hint · The champion starts at numbers[0] — which exists only after the reading loop has finished.
3
Read n numbers and build two lists as you go: the evens and the odds.
Hint · Three empty lists above the loop, and the if inside the reading loop rather than in a second one.
4
Read a list of names and print them numbered, one per line.
Hint · Read with one loop, print with another — over range(len(names)), so you have the number.
5
Read numbers until the user types -1, without asking the count first.
Hint · A while loop and a sentinel — the count is unknown, so for is the wrong tool.

Quick Check

Where must numbers = [] be written?

Quick Check

Why does the prompt need str(i + 1) rather than just i + 1?

Quick Check

The user answers 0 to 'How many numbers?'. What happens?

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Reading a List with eval() 🪄