Strings and string operations

'Bakı 2026' is a string of 9 characters: 4 letters, 1 space and 4 digits. To Python a string is a chain of numbered characters: you can take any character by its number, cut out a part, count characters and turn the string around. In both 2025 DİM papers one of the written programs was about strings (the number of distinct characters; processing items typed on one line and separated by spaces), and in 2026 string operations appeared inside the list and function tasks. This lesson continues input() and the str type from «Programming languages and Python: variables, input and output».

Strings, indexes and slices

s = 'programming'
print(len(s), s[0], s[-1], s[3])
print(s[3:7], s[:3], s[7:])
print(s[::2], s[::-1])
print(s[-4:], s[1:8:3])

11 p g g
gram pro ming
pormig gnimmargorp
ming rrm

Operations and methods

Operations on a string return a new string or a number, and the original string does not change: strings are immutable, so s[0] = 'P' gives an error. If you need a change, build a new string and assign it again: s = s.upper().

s = 'banana'
print(s.count('a'), s.count('ana'), s.find('n'), s.find('z'))
print(s.replace('a', 'o'), s.upper(), 'nan' in s)
print('2026'.isdigit(), '20a'.isdigit())
print('ab' * 3 + '!', len(s * 2))

3 1 2 -1
bonono BANANA True
True False
ababab! 12

Capital and small letters are different characters: 'A' == 'a' is false. To count a letter regardless of case, first turn the string into small letters: s.lower().count('a'). upper() and lower() leave digits and signs unchanged.

s = 'abracadabra'
k = s.count('a') + s.find('c')
t = s[k:] + s[:2]
print(t)

raab

Looping over characters and building a new string

A string can be walked in two ways: over the characters themselves (for c in s:) and over the indexes (for i in range(len(s)):). The second is chosen when you need the position or neighbouring characters (s[i] and s[i + 1]). A new string starts empty (k = '') and grows with k = k + c; k = c + k collects the characters in reverse order.

1. 1
   An empty string

   k = '' — the characters already seen will be collected here.

2. 2
   Loop

   for c in s: — look at every character.

3. 3
   Is it new?

   if k.count(c) == 0: (or if c not in k:) — add only a character that is not there yet, with k = k + c.

4. 4
   Result

   print(len(k)) — the number of distinct characters. A space is a character too.

s = 'mississippi-2026'
v = 0
d = ''
k = ''
for c in s:
    if c in 'aeiou':
        v = v + 1
    if c.isdigit():
        d = d + c
    if k.count(c) == 0:
        k = k + c
print(v, d, k, len(k))

4 2026 misp-206 8

s = 'olympiad'
k = ''
for i in range(len(s)):
    if i % 2 == 0:
        k = k + s[i].upper()
    elif s[i] in 'aeiou':
        k = k + '*'
    else:
        k = k + s[i]
print(k, len(k))

OlYmP*Ad 8

Numbers and strings: str(n) and int(s)

str(n) turns a number into a string of digits, and int(s) turns a string of digits into a number. This is the second way of working with digits (the first is n % 10 and n // 10, see «Working with numbers: digits, divisors and primes»): len(str(n)) is the number of digits, str(n)[::-1] the reversed notation, str(n).count('7') how many times the digit 7 occurs.

n = 40718
t = str(n)
print(len(t), t[0], t[-1], t.count('0'))
print(int(t[::-1]), t == t[::-1])
s = 0
for c in t:
    s = s + int(c)
print(s, t + '5', n + 5)

5 4 8 1
81704 False
20 407185 40723

a = [707, 17, 1234, 77, 470, 5]
m = 0
for x in a:
    t = str(x)
    if t.count('7') == 1:
        m = m + x
    else:
        m = m - x
print(m)

-1536

split and join: items given on one line

In written tasks the items are often typed on one line, separated by spaces: b = input(), then a = b.split(' '). The result is a list of strings: the first character of item i is a[i][0], the last is a[i][-1] (or a[i][len(a[i]) - 1]), its length is len(a[i]); if it is needed as a number, write int(a[i]). If a sum of indexes is asked, loop over the indexes.

b = 'ada 7x7 radar 45 noon ab'
a = b.split(' ')
k = 0
s = 0
for i in range(len(a)):
    if a[i][0] == a[i][-1]:
        k = k + 1
        s = s + i
print(a)
print(k, s)
print('-'.join(a[:3]))

['ada', '7x7', 'radar', '45', 'noon', 'ab']
4 7
ada-7x7-radar

s = 'informatics'
k = ''
# collect the distinct characters in k

print(len(k), k)

10 informatcs

line = '31 405 7 1234 56 800'
a = line.split(' ')
total = 0
# loop over a here

print(total)

831