Computer Science · Class 11 · Strings Revisited

ProgramsWords⏱️ 15 min read

Working Word by Word

Everything so far has walked a sentence one character at a time. Half the questions you will be set are about its words, and there is one line that changes the unit: sentence.split() hands back a list of words, and a for loop over a list works exactly as it does over a string.

1One line changes what the loop walks

split_demo.py

# the same sentence, two different units

sentence = 'learn python at lambdalab'

print(sentence.split())
print(len(sentence), 'characters')
print(len(sentence.split()), 'words')

Output

['learn', 'python', 'at', 'lambdalab']
25 characters
4 words

Key Takeaway

split() gives a list, and each item is a whole word. So for w in sentence.split(): hands out 'learn', then 'python' — each one an ordinary string you can measure with len(), index with w[0] and test with w.isupper(). The spaces are gone: they were the dividers, and they are not part of any word.

Called with nothing, split() splits on runs of whitespace — one space or five, a tab or a newline — and throws away the empty pieces. That is why it counts the words of a badly typed sentence correctly when a hand-written space-counter does not.

2Program 1 — count the words and measure each

📋 The problem

Ask for a sentence, say how many words it has, and print each word with its length.

word_lengths.py

# how many words, and how long is each?

sentence = input('Enter a sentence: ')
words = sentence.split()

print('Words:', len(words))

for w in words:
    print(w, 'has', len(w), 'letters')

Output

Enter a sentence: learn python at lambdalab
Words: 4
learn has 5 letters
python has 6 letters
at has 2 letters
lambdalab has 9 letters

words = sentence.split()

Split once, into a name. Writing sentence.split() again inside the loop would do the same work on every round, and it is the kind of thing that gets you a comment rather than a mark.

for w in words:

The same loop shape as for ch in word — only the list is a list of words, so w holds a whole word. Nothing new is being learnt here except what is in the box.

word_lengths.py

sentence = input('Enter a sentence: ')
words = sentence.split()

print('Words:', len(words))

for w in words:
    print(w, 'has', len(w), 'letters')

sentence = input('Enter a sentence: ')
words = sentence.split()

print('Words:', len(words))

for w in words:
    print(w, 'has', len(w), 'letters')

3Program 2 — the longest word

📋 The problem

Find the longest word in a sentence, and say how long it is.

longest_word.py

# the longest word in a sentence

sentence = 'practice makes a programmer perfect'
words = sentence.split()
longest = words[0]

for w in words:
    if len(w) > len(longest):
        longest = w

print('The longest word is', longest)
print('It has', len(longest), 'letters')

Output

The longest word is programmer
It has 10 letters

Key Takeaway

The champion starts as words[0], not as ''. An empty string would work here by luck — every real word is longer than nothing — but the honest starting value for “the best so far” is a real member of the list. It is the same rule as starting a highest-mark search at marks[0], and it is the one that keeps working when the comparison is not about length.

Ties go to the first word, because the test is > and not >= — a later word of equal length never displaces the champion. Worth knowing, because “longest word” questions with two equal answers are common and both answers cannot be right.

4Program 3 — the words in the opposite order

📋 The problem

Turn python makes strings easy into easy strings makes python — the words reversed, but each word still spelt forwards.

reverse_words.py

# the words in the opposite order

sentence = 'python makes strings easy'
words = sentence.split()
result = ''

for i in range(len(words) - 1, -1, -1):
    result = result + words[i]

    if i > 0:
        result = result + ' '

print(result)

Output

easy strings makes python

The backwards walk from the index page, over a list instead of a string — range(len(words) - 1, -1, -1) counts 3, 2, 1, 0. The if i > 0 is the fence-post rule again: four words need three spaces between them, so the space is skipped on the last round, which here is the round where i is 0.

Tip

Reversing the words is not reversing the sentence. sentence[::-1] gives 'ysae sgnirts sekam nohtyp' — every character backwards, so the words are spelt backwards too. Two different questions, and papers ask both.

5Program 4 — initials from a name

📋 The problem

Turn a full name into its initials: a p j abdul kalam becomes A.P.J.A.K.

initials.py

# the initials of a name

name = input('Enter a full name: ')
words = name.split()
initials = ''

for w in words:
    initials = initials + w[0].upper() + '.'

print('Initials:', initials)

Output

Enter a full name: a p j abdul kalam
Initials: A.P.J.A.K.

w[0] is the first character of the word — indexing a string that came out of a list, which is the ordinary thing it looks like. Two additions per round, the letter and the dot, so no fence-post problem: every initial gets a dot, including the last.

Watch Out

w[0] would fail on an empty word. split() never produces one, which is why this program is safe — but a program that split on a fixed character, like name.split(','), can get an empty piece from 'a,,b', and ''[0] raises IndexError: string index out of range.

6Program 5 — the words that start with a given letter

📋 The problem

Print every word of a sentence that begins with a chosen letter, and count them.

starts_with.py

# the words that begin with a chosen letter

sentence = 'sam sold seven small silver shells on sunday'
letter = 's'
count = 0

for w in sentence.split():
    if w[0] == letter:
        count = count + 1
        print(w)

print(count, 'words begin with', letter)

Output

sam
sold
seven
small
silver
shells
sunday
7 words begin with s

The loop runs over sentence.split() directly, without a name in between — fine when the list is used once. w[0] is the first letter and w[-1] would be the last, if the question asked for words ending in something.

7Program 6 — the average word length

📋 The problem

Work out the average number of letters per word.

average_length.py

# the average length of the words in a sentence

sentence = 'a programmer writes readable programs'
words = sentence.split()
total = 0

for w in words:
    total = total + len(w)

print('Words:', len(words))
print('Letters:', total)
print('Average word length:', total / len(words))

Output

Words: 5
Letters: 33
Average word length: 6.6

Watch Out

Divide by len(words), not by len(sentence). The two are 5 and 37 here, and the second would give an answer under 1 that still looks like a number. And the division goes after the loop: inside it, you would be dividing a part-finished total by the full count.

8Recap

split() changes the unit

It hands back a list of words. A for loop over that list gives whole words, and each one is an ordinary string.

Split once, into a name

words = sentence.split() above the loop. Calling it again inside repeats the same work every round.

The champion starts as words[0]

A real member of the list, not '' or 0. Ties go to the first word, because the test is > and not >=.

Words backwards is not the sentence backwards

Reversing the list keeps each word spelt forwards; sentence[::-1] spells every word backwards too.

✍️ Now write these yourself

1
Print the shortest word in a sentence.
Hint · Program 2 with < instead of >. The champion still starts at words[0].
2
Count the words that have more than five letters.
Hint · A counter above the loop and len(w) > 5 inside it.
3
Print each word with its first letter capitalised, on one line.
Hint · Build the line with w[0].upper() + w[1:] + ' ' — a slice from 1 is the rest of the word.
4
Count how many words are palindromes.
Hint · The palindrome test from the searching page, applied to w inside this loop.
5
Report the word that contains the most vowels.
Hint · A loop inside a loop: words on the outside, characters of w on the inside, and a champion for the best count so far.

Quick Check

What does sentence.split() hand back?

Quick Check

A longest-word program starts longest = ''. Two words tie for longest. Which is reported?

Quick Check

Which gives 'easy strings makes python' from 'python makes strings easy'?

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