Working with strings

Letters have positions

• A string is a row of characters.
• Each character has a position, called an index.
• Indexes start at 0. Use s[-1] for the last character.

word = "Python"
print(word[0])
print(word[-1])

Take a slice

• A slice takes part of a string: s[start:end].
• The character at end is not included.
• Leave a side empty for "from the start" or "to the end": s[:3], s[2:].

msg = "hello"
print(msg[:3])
print(msg[2:])

Worked example: positions and slices

• In "banana" the positions are b=0, a=1, n=2, a=3, n=4, a=5.
• word[0] is the first letter; word[-1] counts back from the end.
• word[:3] takes positions 0, 1 and 2, and stops before 3.

word = "banana"
print(word[0])     # b
print(word[-1])    # a   (the last letter)
print(word[:3])    # ban (positions 0, 1, 2)
print(word[2:5])   # nan (positions 2, 3, 4)

Length and changing case

• len(s) gives the number of characters.
• s.upper() makes a CAPITAL copy; s.lower() makes a lowercase copy.
• These make a new string; the original does not change.

city = "Tokyo"
print(len(city))
print(city.upper())
print(city.lower())

f-strings build text

• An f-string starts with f before the quote.
• Put a value inside { } and Python drops it into the text.
• It is the neat way to mix words and values.

fruit = "apple"
count = 3
print(f"I have {count} {fruit}s")

Common mistakes

• Indexing starts at 0; s[-1] is the last character.
• Strings cannot be changed in place — methods like .upper() return a NEW string, so store the result.
• You can only join a string to a string with +.

Now you try

• Read the task to see which variable to fill or what to print.
• Press Check answer to test your code.