Part 2: Core Programming Concepts

8. Strings & Text Processing

Working with text

A string is a sequence of characters — letters, digits, spaces, punctuation — treated as a single value. Text is everywhere in software: names, messages, file contents, web pages, user input. Knowing how to pick strings apart and reassemble them is one of the most practical everyday skills.

Characters and indexing

A string behaves much like the array from the previous chapter: each character sits at a zero-based index, and the string has a length. You can read a single character by its position, and reach the last character at index length − 1.

word = "HELLO"
print(word[0])      # 'H'  (first character)
print(word[4])      # 'O'  (last)
print(len(word))    # 5

Concatenation: joining strings

Putting strings together end-to-end is called concatenation, usually written with +. Remember that + means "add" for numbers but "join" for strings — and you cannot mix the two without converting, because "Age: " + 30 is an error, while "Age: " + str(30) works.

first = "Ada"
last = "Lovelace"
full = first + " " + last     # "Ada Lovelace"
label = "Age: " + str(30)     # convert the number first

Slicing: taking a piece

A slice extracts a substring by giving a start and end position. As with ranges, the start is included and the end is excluded, so word[0:3] gives the first three characters.

word = "programming"
print(word[0:4])    # "prog"  (indices 0,1,2,3)
print(word[4:])     # "ramming" (from 4 to the end)

Common operations

Languages provide a rich toolkit for text. The names vary slightly, but the ideas are universal:

Operation	Meaning	Example	Result
length	Number of characters	len("cat")	3
upper / lower	Change case	"cat".upper()	"CAT"
replace	Swap one piece for another	"a-b".replace("-", ":")	"a:b"
split	Break into a list on a separator	"a,b,c".split(",")	["a","b","c"]
strip	Trim surrounding whitespace	" hi ".strip()	"hi"
find / index	Locate a substring	"hello".find("ll")	2

Immutability

In many languages strings are immutable: operations like upper() or replace() do not change the original string — they return a new one. If you want to keep the result, assign it to a variable. Beginners often call a method and wonder why "nothing happened"; the answer is that the new string was thrown away because it was never stored.

name = "ada"
name.upper()          # makes "ADA" but discards it
print(name)           # still "ada"
name = name.upper()   # now keep it
print(name)           # "ADA"

Splitting and joining: a everyday pattern

A huge amount of real text work is "split a line into parts, process the parts, join them back." Reading a CSV line, parsing a date, or cleaning user input all follow this shape.

line = "Ada,36,London"
parts = line.split(",")     # ["Ada", "36", "London"]
name = parts[0]             # "Ada"
age = int(parts[1])         # 36

Key idea: strings are indexed sequences of characters; join them with concatenation, carve them with slicing, transform them with built-in operations, and remember that many string operations return new strings rather than changing the original.