Working with strings

Introduction

A string is a sequence of characters. You can pick out one character, take a slice, loop over every character, and use built-in methods that return new strings.

Most of the data a program touches is text: names, messages, addresses, search terms. Knowing how to pull text apart and tidy it up is one of the most practical skills in programming.

How it works

Indexing and slicing

word = "Dhaka"
print(word[0], word[-1])
print(word[1:4])
print(len(word))

Positions start at 0: in "Dhaka", word[0] is 'D' and word[4] is 'a'. Negative positions count from the end, so word[-1] is the last character.

A slice word[1:4] takes positions 1, 2 and 3; it stops before 4, just like range. len(word) is the number of characters, 5.

String methods

name = "  rahim uddin "
clean = name.strip().title()
print(clean)
print(clean.upper())
print(clean.replace("Uddin", "Ahmed"))

• strip() removes spaces at both ends; title() capitalises each word.
• upper() and lower() change the case.
• replace(old, new) swaps one piece of text for another.

Methods never change the original string, because strings cannot be changed. Each method returns a new string, so clean holds the result while name keeps its spaces.

Syntax

word[0]            # first character
word[-1]           # last character
word[1:4]          # positions 1, 2 and 3
word[::-1]         # reversed
word.strip().lower()
"a" in word

Example

text = "programming"
vowels = 0
for ch in text:
    if ch in "aeiou":
        vowels += 1
print(vowels)

for ch in text: gives one character per round. ch in "aeiou" checks whether the character is a vowel. Step through and watch vowels reach 3 for the letters o, a and i.

Line by line

1. text = "programming" is the string to examine.
2. vowels = 0 starts the count.
3. for ch in text: gives one character on each pass.
4. ch in "aeiou" tests whether that character is a vowel.
5. The count goes up only when the test passes.
6. After the loop, print shows the total.

Expected result

3

In the real world

Sign-up forms trim and lower-case email addresses, so that Mitu@Example.com and mitu@example.com count as the same person. Search engines, spell checkers and chat filters all begin by going through text character by character.

Common mistakes

• word[5] on a 5-letter word is an IndexError: the last position is 4.
• word[0] = "d" is a TypeError. Build a new string instead: "d" + word[1:].
• Calling a method without keeping its result: name.upper() alone changes nothing; write name = name.upper().

Best practices

• Clean text as soon as you read it, with strip and a consistent case.
• Keep the result of a method, because the original does not change.
• Use in rather than writing your own search.

Summary

A string is a sequence of characters that you can index, slice and loop over. Strings cannot change, so methods such as upper, strip and replace return new strings.

Practice

Read a word and print it backwards using the slice word[::-1]. Then print YES if the word reads the same both ways, otherwise NO.

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