Computer Science · Class 11 · Lists Revisited

ListsWhole programs⏱️ 15 min read

Programs on a List You Read

Every list program so far has been handed its data by you, on the first line. Real questions do not say that — they say “accept a list from the user and…”. Nothing new is needed: the reading half is one of the two shapes from the last two pages, the working half is a loop you have already written, and the whole program is the two of them one after the other.

1Every program on this page has the same three parts

1 · Read

Ask the count and append that many times, or take the whole list with eval(). This half never changes.

2 · Work

The loop from the programming pages: a collector above it, a test inside it. This half is the question.

3 · Report

Print the list back as well as the answer, so the user can see what the program actually got.

Tip

Print the list back before the answer. Every program here does, and it is not decoration: when the answer looks wrong, the first question is always what did the program actually read? — and a stray letter or a missed value shows up immediately.

2Program 1 — total and average of marks the user types

📋 The problem

Ask how many marks there are, read them one at a time, and print the total and the average.

Input

what we ask the user for

the count, n
then n marks

Process

what we work out

append each mark to a list
add the list up in a second loop

Output

what we show

the list, the total and the average

read_average.py

# read a list of marks, then report the total and the average

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

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

total = 0

for m in marks:
    total = total + m

print('The marks are', marks)
print('Total:', total)
print('Average:', total / len(marks))

Output

How many marks? 4
Enter mark 1: 72
Enter mark 2: 65
Enter mark 3: 88
Enter mark 4: 91
The marks are [72, 65, 88, 91]
Total: 316
Average: 79.0

for i in range(n):

The reading loop. It ends before the working loop begins — the list has to be complete before anything can be asked about it.

for m in marks:

The working loop, over the list that now exists. Two loops one after the other, not one inside the other: the first fills the list, the second reads it.

Tip

The two loops can be merged, and usually should not be. You could add each mark to the total as it arrives and skip the second loop entirely. It works — but the moment the question also wants the largest, or the marks above average, you need the list anyway. Reading first and working afterwards is the shape that survives the next sentence of the question.

read_average.py

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

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

total = 0

for m in marks:
    total = total + m

print('The marks are', marks)
print('Total:', total)
print('Average:', total / len(marks))

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

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

total = 0

for m in marks:
    total = total + m

print('The marks are', marks)
print('Total:', total)
print('Average:', total / len(marks))

3Program 2 — largest and smallest, from a list typed in one go

📋 The problem

Take a whole list with eval() and report its largest and smallest values, without max() or min().

read_largest.py

# read a whole list at once, then find the largest and the smallest

numbers = eval(input('Enter a list of numbers: '))

largest = numbers[0]
smallest = numbers[0]

for n in numbers:
    if n > largest:
        largest = n

    if n < smallest:
        smallest = n

print('Largest:', largest)
print('Smallest:', smallest)

Output

Enter a list of numbers: [45, 88, 12, 91, 67]
Largest: 91
Smallest: 12

Three lines of reading became one. The champions still start at numbers[0] — and now that the list came from a user, that line is also the program's first assumption: there is at least one item. Section 7 is about what happens when there is not.

4Program 3 — split what the user typed into evens and odds

📋 The problem

Read a list of numbers and print the even ones and the odd ones as two separate lists.

read_split.py

# read a list, then split it into evens and odds

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

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

evens = []
odds = []

for value in numbers:
    if value % 2 == 0:
        evens.append(value)
    else:
        odds.append(value)

print('You entered:', numbers)
print('Evens:', evens)
print('Odds: ', odds)

Output

How many numbers? 5
Enter number 1: 12
Enter number 2: 7
Enter number 3: 30
Enter number 4: 45
Enter number 5: 8
You entered: [12, 7, 30, 45, 8]
Evens: [12, 30, 8]
Odds:  [7, 45]

Note the reading loop here appends in one line — numbers.append(int(input(...))) — rather than storing the value in value first. Both are correct; the two-line version is easier to read and easier to put a print() into when something goes wrong. Also note that the loop variable is called value in the second loop and i in the first: one is an item, the other is a position, and the names should say so.

5Program 4 — search the list the user typed

📋 The problem

Read a list and a value to look for, and report the position — or that it is not there.

read_search.py

# read a list, then search it for a value the user asks for

numbers = eval(input('Enter a list: '))
wanted = int(input('Which number are you looking for? '))

for i in range(len(numbers)):
    if numbers[i] == wanted:
        print(wanted, 'found at position', i)
        break
else:
    print(wanted, 'is not in the list')

Output

Enter a list: [45, 88, 12, 91]
Which number are you looking for? 12
12 found at position 2

read_search.py — a value that is not in the list

Output

Enter a list: [45, 88, 12, 91]
Which number are you looking for? 50
50 is not in the list

Two inputs of different kinds, which is the only new thing here: eval() for the list, plain int(input()) for the single value. Using eval() for both would work and is worth avoiding — the second answer is one number, and int() is the honest way to read one number.

6Program 5 — the names, numbered, in reverse

📋 The problem

Read a list of names and print them numbered from 1, last one first.

read_reverse.py

# read a list of names and print them numbered, in reverse order

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

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

print('In reverse:')

for i in range(len(names) - 1, -1, -1):
    print(len(names) - i, names[i])

Output

How many names? 3
Enter name 1: Asha
Enter name 2: Ravi
Enter name 3: Meera
In reverse:
1 Meera
2 Ravi
3 Asha

Key Takeaway

No int() anywhere in the reading loop. A name is text and must stay text — casting it would raise ValueError on the first name typed. The only int() in the program is round the count, which really is a number.

The printing loop walks backwards — len(names) - 1 down to 0 — while the label counts forwards: len(names) - i is 1 when i is 2, and 3 when i is 0. Two counters running in opposite directions off one loop variable, which is a thing only the index form can do.

7Program 6 — how many of the user's marks beat the average?

📋 The problem

Read the marks, work out the average, and count how many are above it.

read_above_average.py

# read a list and report how many marks are above the average

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

for i in range(n):
    marks.append(int(input('Enter mark ' + str(i + 1) + ': ')))

total = 0

for m in marks:
    total = total + m

average = total / len(marks)
above = 0

for m in marks:
    if m > average:
        above = above + 1

print('Average:', average)
print('Above average:', above)

Output

How many marks? 4
Enter mark 1: 72
Enter mark 2: 65
Enter mark 3: 88
Enter mark 4: 91
Average: 79.0
Above average: 2

Three loops, and each one has to finish before the next can start. Read them all; only then can the total be complete; only then does the average exist; only then can anything be compared with it. This is the two-pass program from the counting page with a reading pass in front of it.

8The answer that breaks every program here

The user types 0 for the count, or presses Enter on []. The reading loop runs no rounds, the list is empty, and every program on this page then fails in its own way:

total / len(marks)

ZeroDivisionError: division by zero — there is nothing to divide by.

largest = numbers[0]

IndexError: list index out of range — there is no first item to make a champion of.

max(marks)

ValueError: max() iterable argument is empty — even the built-in cannot answer.

Key Takeaway

Guard it with one if. if len(marks) == 0: before the working part, printing something honest like “no marks were entered”, and the rest of the program under the else. It is one line of thought that separates a program that works from a program that works when the user cooperates.

guarded.py

# the same program, with the empty list dealt with

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

for i in range(n):
    marks.append(int(input('Enter mark ' + str(i + 1) + ': ')))

if len(marks) == 0:
    print('No marks were entered, so there is nothing to work out')
else:
    total = 0

    for m in marks:
        total = total + m

    print('Average:', total / len(marks))

Output

How many marks? 0
No marks were entered, so there is nothing to work out

9Recap

Read first, work afterwards

The reading loop finishes before the working loop starts. Two loops one after the other, never one inside the other.

The reading half never changes

Count-then-append, or one eval(). Whatever the question asks about the list, that part is the same program.

Print the list back

When the answer looks wrong, the first thing to check is what the program actually read.

An empty list breaks all of it

ZeroDivisionError, IndexError or ValueError, depending on what you asked. One if len(...) == 0 guard is the fix.

✍️ Now write these yourself

1
Read a list of marks and print how many passed (33 or above) and how many failed.
Hint · Reading loop, then the two-counter loop. Print the list back before the answer.
2
Read a list with eval() and print it with every item doubled.
Hint · Either change it in place with the index form, or build a second list — the question decides which.
3
Read two lists from the user and print the items they have in common.
Hint · Two eval() lines, then the nested loop from the searching page.
4
Read a list of names and print only those starting with a letter the user chooses.
Hint · Two kinds of input again: the names with a loop, the letter with a plain input().
5
Read a list of numbers and report the largest, with a guard for the empty list.
Hint · The champion program under an else, with if len(numbers) == 0: above it.

Quick Check

Why does the working loop come after the reading loop rather than inside it?

Quick Check

A program reads names with names.append(int(input(...))). What happens?

Quick Check

The user answers 0 to 'How many marks?'. What does total / len(marks) do?

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🪄 Reading a List with eval()

Counting & Totals 🔢