1.5 Strings

Big ideas

• A string is a sequence of characters. Index from 0: s[0] is the first character, s[-1] the last, len(s) the length.
• Slices copy a piece: s[a:b] is characters a up to but not including b; leave out an end to go to the edge; s[::-1] reverses.
• Strings are immutable: methods give back a new string. upper() lower() strip() replace(a, b) split() find(x) count(x) startswith(x) isdigit(). Use ", ".join(list) to glue pieces.
• + joins, * repeats, in tests for a substring, and a for ch in s loop visits each character.
• Escape sequences: \n newline, \t tab, \" a quote. ord("A") is 65 and chr(66) is "B".

Code you should know

s = "Bobcats"
print(s[0], s[-1], len(s))
print(s[0:3], s[3:], s[:3], s[::-1])
print(s[1:6:2])

B s 7
Bob cats Bob stacboB
obas

line = "  Go Cats, go!  "
clean = line.strip()
print(clean.upper())
print(clean.lower().count("go"))
print(clean.replace("Cats", "Bobcats"))
words = clean.split()
print(words, len(words))
print("-".join(words))
print(line)   # unchanged

GO CATS, GO!
2
Go Bobcats, go!
['Go', 'Cats,', 'go!'] 3
Go-Cats,-go!
  Go Cats, go!  

text = input("Text: ")
vowels = 0
for ch in text.lower():
    if ch in "aeiou":
        vowels += 1
print(f"{vowels} vowels in {len(text)} characters")

5 vowels in 15 characters

Worked example

Common mistakes

• Expecting s.upper() to change s. Assign the result: s = s.upper().
• Off-by-one in slices: s[1:3] has two characters, not three.
• s[len(s)] is an IndexError; the last index is len(s) - 1.
• Comparing a digit string to a number: "7" == 7 is False.