Lists and list operations

“There are 25 students — do we need 25 separate variables for their scores?” No: a list keeps all the values under one name. Lists appeared in every DİM entrance exam in informatics in 2025–2026: a program changes elements inside a loop, ends with print(a), and the five answer options often differ by a single element. The written part also likes the sentence “n integers are entered from the keyboard into an empty list…”. In this lesson you will learn lists well enough to follow every element step by step.

List, element and index

Remember three rules. len(a) gives the number of elements, so the last index is len(a) - 1; writing a[len(a)] raises an IndexError. The assignment a[i] = x replaces an element in place — the length of the list does not change. The expression x in a checks whether the value x is in the list and gives True or False.

a = [7, 3, 9, 3, 5]
print(len(a), a[0], a[-1])
a[1] = a[1] * 10
a[-1] = a[0] + a[2]
print(a)
print(3 in a, 5 in a)

5 7 5
[7, 30, 9, 3, 16]
True False

In the exam a list is usually filled from the keyboard. Two forms are common. First: n, then n numbers each on its own line — you create an empty list and add each number to its end with append. Second: all values on one line separated by spaces — then input().split(' ') cuts the line into pieces. Careful: split returns a list of strings; convert them to numbers with int before calculating.

# form 1: n, then n numbers, each on its own line
n = int(input())
a = []
for i in range(0, n):
    a.append(int(input()))

# form 2: all numbers on one line, e.g. 12 5 8
b = input().split(' ')
for i in range(0, len(b)):
    b[i] = int(b[i])

lines = ['3', '4', '-2', '7']    # what the keyboard would give
n = int(lines[0])
a = []
for i in range(1, n + 1):
    a.append(int(lines[i]))
print(a)

b = '12 5 8'.split(' ')
print(b, b[0] + b[1])
for i in range(0, len(b)):
    b[i] = int(b[i])
print(b, b[0] + b[1])

[4, -2, 7]
['12', '5', '8'] 125
[12, 5, 8] 17

Walking through a list: by value and by index

In for x in a: the variable x takes the values of the elements one by one — handy for reading, but x = … does not change the list, because x is only a copy. In for i in range(0, len(a)): the variable i takes the indexes: a[i] is the element itself, so you can change it, add its index to a sum and look at its neighbours. Most DİM list tasks are written in the second form.

a = [4, 7, 1, 8]
for x in a:
    x = x * 2
print(a)
for i in range(0, len(a)):
    a[i] = a[i] * 2
print(a)
s = 0
for i in range(0, len(a)):
    if a[i] > 5:
        s = s + i
print(s)

[4, 7, 1, 8]
[8, 14, 2, 16]
4

When you work with neighbouring elements, watch the loop limits: a[i + 1] exists only when i ≤ len(a) − 2, and a[i - 1] makes sense only when i ≥ 1. At i = 0, a[i - 1] gives no error — it is a[-1], the last element. The table shows which loop header you need for which task.

a = [2, 5, 3, 1, 4, 9, 1, 7]
s = 0
for i in range(1, len(a) - 1):
    if a[i] > a[i - 1] and a[i] > a[i + 1]:
        s = s + a[i]
        a[i] = 0
print(s, a)

17 [2, 0, 0, 1, 4, 0, 1, 7]

List methods and built-in functions

A method is written after the list name with a dot: a.append(5). A function takes the list in parentheses: len(a). The methods append, insert, remove, pop, reverse, sort change the list itself; count and index only give information. Every row of the table starts again from the list a = [5, 2, 8, 2].

a = [5, 2, 8, 2]
a.append(4)
a.insert(1, 9)
a.remove(2)
x = a.pop()
print(a, x)
a.sort()
print(a, a.count(2), a.index(8))
print(len(a), sum(a), min(a), max(a))

[5, 9, 8, 2] 4
[2, 5, 8, 9] 1 2
4 24 2 9

A new list by a condition. Lists and strings

1. 1
   Empty list

   Create b = [] for the result before the loop.

2. 2
   Loop

   Walk through the original list by value (for x in a) or by index (for i in range(0, len(a))).

3. 3
   Condition

   Check the condition for each element, for example if x % 2 == 0:.

4. 4
   Add

   If the condition holds, add the element (or a value made from it) with b.append(...).

5. 5
   Print

   After the loop write print(b) — Python prints the list as [6, 15, 9], with square brackets and commas.

a = [12, 7, 30, 5, 18, 7]
b = []
for x in a:
    if x % 2 == 0:
        b.append(x // 2)
print(b)
c = []
for x in a:
    if c.count(x) == 0:
        c.append(x)
print(c, len(c))

[6, 15, 9]
[12, 7, 30, 5, 18] 5

Switching between lists and strings is often needed in DİM programs. list('2026') turns a string into a list of characters: ['2', '0', '2', '6']. ''.join(c) glues a list of strings into one string (the separator is the string before the dot). s.split(' ') cuts a string at the spaces. To reverse a number use the chain str(n) → list → reverse() → join → int.

n = 1720
c = list(str(n))
print(c)
c.reverse()
r = int(''.join(c))
print(r, n + r)
w = '12 5 40'.split(' ')
print(w, len(w))
print('-'.join(w))

['1', '7', '2', '0']
271 1991
['12', '5', '40'] 3
12-5-40

a = [6, 1, 4, 9, 4, 2, 7, 3]
b = [4, 7, 1]
for i in range(0, len(a) - 1):
    if b.count(a[i]) > 0:
        a[i + 1] = a[i + 1] + a[i]
print(a)

[6, 1, 5, 9, 4, 6, 7, 10]

a = [3, 5, 7, 8, 10, 12, 15, 17, 19]
s = 0
i = 1
while i < len(a) - 1:
    if a[i] - a[i - 1] == a[i + 1] - a[i]:
        s = s + i
    i = i + 1
print(s)

12

a = [3, 9, 4, 4, 7, 2, 8, 1]
b = []
s = 0
# your loop here

[9, 7, 8]
11

a = [5, 3, 5, 2, 3, 5, 9]
u = []
k = 0

[5, 3, 2, 9]
2

Indexes, len and in work the same way for strings — you saw this in the lesson “Strings and string operations”. In the next lesson, “Functions: def, parameters and return”, we will read programs that pass every element of a list to a function (a[i] = k(a[i])). Extra tools such as slices (a[1:4]) and list comprehensions are in the Python course lesson “Lists”.