Computer Science · Class 11 · Strings Revisited

StringsThe short form⏱️ 13 min read

Traversal by Character

for ch in word: — the form to reach for unless you have a reason not to. The loop variable holds the character itself, so there is nothing to look up and nothing to get out of range. Nearly every string program you will be set is one of the three shapes on this page.

1Three shapes, and that is most of it

Every program here starts a variable above the loop and does something to it inside — the collector idea from the for loop practice, with characters instead of numbers. What changes is only what the collector is:

Count

count = 0

A number that goes up when the character passes a test.

Total

total = 0

A number that adds something worked out from the character.

Build

result = ''

A string that grows by one character at a time.

2Program 1 — count the capital letters

📋 The problem

Count how many capital letters a sentence contains.

capitals.py

# count the capital letters in a sentence

sentence = 'Learn Python With LambdaLab'
count = 0

for ch in sentence:
    if ch.isupper():
        count = count + 1

print('Capital letters:', count)

Output

Capital letters: 5

for ch in sentence:

One round per character — every character, spaces included. The name ch is not special; letter or c would do exactly the same. It is called the loop variable and it is created by this line.

if ch.isupper():

ch is a one-character string, so every string method works on it. isupper() answers True or False, which is all an if needs.

count = count + 1

Eight spaces: inside the if, inside the loop. Pull it back to four and every character is counted, capital or not.

Key Takeaway

ch is a string, so it has all the string methods. ch.isupper(), ch.isdigit(), ch.lower(), even ch in 'aeiou' — a one-character string is not a special kind of value, it is just a short string.

3Program 2 — add up the digits inside a string

📋 The problem

A string holds letters and digits mixed together. Add up the digits.

digit_total.py

# add up the digits that appear inside a string

data = 'a1b2c3d4'
total = 0

for ch in data:
    if ch.isdigit():
        total = total + int(ch)

print('The digits add up to', total)

Output

The digits add up to 10

Watch Out

int(ch) is doing real work here. ch is the text '1', not the number 1. Leave int() out and Python raises TypeError: unsupported operand type(s) for +: 'int' and 'str' — the same trap as the missing int() around input(), in a new place.

4Program 3 — keep only the digits

📋 The problem

A phone number has been typed with spaces and dashes in it. Produce the digits on their own.

digits_only.py

# keep only the digits of a phone number

messy = 'Ph: 98-765-43210'
digits_only = ''

for ch in messy:
    if ch.isdigit():
        digits_only = digits_only + ch

print('Digits only:', digits_only)

Output

Digits only: 9876543210

The collector starts as '' — the empty string, which is to joining what 0 is to adding. Each round either adds one character or does not, and after the last round the answer is complete. A string is never edited in place: each + builds a new string and the name is pointed at it.

digits_only.py

messy = 'Ph: 98-765-43210'
digits_only = ''

for ch in messy:
    if ch.isdigit():
        digits_only = digits_only + ch

print('Digits only:', digits_only)

messy = 'Ph: 98-765-43210'
digits_only = ''

for ch in messy:
    if ch.isdigit():
        digits_only = digits_only + ch

print('Digits only:', digits_only)

5The mistake everybody makes: changing `ch`

This program looks like it upper-cases a word. It does nothing at all:

no_change.py

# changing the loop variable does not change the string

word = 'cat'

for ch in word:
    ch = ch.upper()

print(word)

Output

cat

Key Takeaway

ch is a box of its own, refilled each round. It is handed the next character; it is not the character's place inside the string. Writing to it points that box somewhere new and the string never hears about it. Two lessons meet here: strings are immutable, so nothing can change one in place, and upper() returns a new value rather than editing anything.

The way to “change” a string is the way program 3 does it — build a new one and give it a name:

shouting.py

# the way to do what that program was trying to do

word = 'cat'
shouting = ''

for ch in word:
    shouting = shouting + ch.upper()

print(word)
print(shouting)

Output

cat
CAT

Both lines are worth reading: word is still cat. Nothing that has happened could have changed it.

6Program 4 — the longest run of one letter

📋 The problem

How many times in a row does the most-repeated letter appear? In aabbbccddddde the answer is 5.

longest_run.py

# how long is the longest run of the same letter?

word = 'aabbbccddddde'
best = 1
run = 1

for i in range(1, len(word)):
    if word[i] == word[i - 1]:
        run = run + 1
    else:
        run = 1

    if run > best:
        best = run

print('The longest run is', best, 'characters')

Output

The longest run is 5 characters

Watch Out

And there it is: this one could not be written with for ch in word. It has to compare each character with the one before it, and a loop variable holding a character cannot look backwards. That is the signal to switch forms — which is the next page. Note also that the loop starts at 1, not 0: there is no character before the first one.

7Recap

ch is a one-character string

So every string method works on it, and so does in. It is not a special character type — Python has none.

Collector above, work inside, print after

count = 0, total = 0 or result = '' before the loop; the print at the margin once the loop is done.

Changing ch changes nothing

The loop variable is a box that is refilled each round, not the character's place in the string. Build a new string instead.

It cannot see the neighbours

No position means no word[i + 1] and no word[i - 1]. The moment a problem needs those, switch to the index form.

✍️ Now write these yourself

1
Count the spaces in a sentence.
Hint · The test is ch == ' ' — a space in quotes is a perfectly ordinary character.
2
Count how many characters are not letters.
Hint · not ch.isalpha(), and remember a space is not a letter.
3
Build a new string with every vowel turned into a star.
Hint · An if…else inside the loop: add '*' in one branch and ch in the other.
4
Add up the lengths of nothing — count the characters without len().
Hint · A counter that goes up once per round, with no if at all.
5
Print the vowels of a word on one line, separated by spaces.
Hint · Build the line as a string with + ch + ' ' and print it once, after the loop.

Quick Check

word = 'cat'; for ch in word: ch = ch.upper(). What does print(word) show afterwards?

Quick Check

Why does total = total + ch fail when ch is a digit character?

← Previous

🔀 Two Ways to Walk a String

Traversal by Index 🔢

1Three shapes, and that is most of it

2Program 1 — count the capital letters

3Program 2 — add up the digits inside a string

4Program 3 — keep only the digits

5The mistake everybody makes: changing ch

6Program 4 — the longest run of one letter

7Recap

5The mistake everybody makes: changing `ch`