control flow~12 minRoute 01 3 of 12

Strings

Text is a sequence of characters — and almost every language has its own quirks about it.

Story — text is just a row of characters

A string looks like one thing — "hello" — but inside, the computer sees a row of slots, each holding one character. Slot 0 holds h, slot 1 holds e, and so on. Same indexed-row idea as an array — the values just happen to be characters.

Strings are also immutable in most modern languages. Once made, you can't change a character in place; every "edit" builds a fresh string. That sounds wasteful and is sometimes a real performance trap — but it's the property that makes strings safe to share between threads and use as dictionary keys.

Wordy says

Concatenation glues with +. Interpolation embeds variables right in the literal. Both produce the same string — interpolation is easier to read once you have more than one variable.

See it — index, slice, transform

Most useful operations on a string fall into a small set: index (one character), slice (a range), length, contains, upper/lower, split/join, replace, format. Master these eight and you'll handle 90% of text problems.

✎ Sharpen your pencil

In one sentence, what is the difference between concatenation and interpolation?

Wordy says

I'm immutable in most languages — every change builds a fresh copy. That's why string-building inside a loop can quietly become slow.

Try it — predict the slice

Exercise. In Python, s = "codeflow". What is s[0:4]? What is s[-4:]? What does s.replace("flow", "wave") return?

Reveal answers

s[0:4] = "code" (slots 0,1,2,3 — end is exclusive). s[-4:] = "flow" (last 4 — negative indexes count from the end). s.replace(...) returns "codewave" — note "returns": the original s is unchanged because strings are immutable.

Code it — five languages, one row of characters

The core operations have nearly the same names everywhere — only the syntax differs. Watch for length: Go reports bytes, while most others report characters.

name = "Ada"
greeting = "Hello, " + name + "!"     # concatenation
greeting2 = f"Hello, {name}!"          # interpolation
print(len(name))                       # 3
print(name.upper())                    # "ADA"
print(name[0])                         # "A"

Quiz it — make it stick

Predict — Question 1
What does this print in Python?
```
s = "abc"
s = s + "d"
print(s)
```
Your guess
Question 2
Why is repeatedly building a string with `+=` inside a loop sometimes slow?
Strings are limited in sizeEach += creates a new copy of the whole stringThe garbage collector pauses the programThere is no good reason — it is fast
True or false — Question 3
In Python, "abc" == "abc" is True regardless of how the two strings were constructed.

No Dumb Questions

Real questions other learners asked on this page.

Why are strings immutable?
It makes them safe to share between threads and use as hash-table keys (the hash never goes stale). The cost is that "edits" are really new strings — for heavy editing, use a StringBuilder, list of chars, or bytes buffer.
What is a Unicode code point?
A number assigned to each character in the world's writing systems. "A" is U+0041, "🐱" is U+1F431. Modern languages count code points, not bytes — but encoding (UTF-8, UTF-16) is what gets stored on disk.
Why does len("café") sometimes give 4 and sometimes 5?
Depends on whether the runtime counts characters (4) or UTF-8 bytes (5 — é is two bytes). Python 3 counts characters. Older languages and lower-level APIs count bytes.