Unit 4: Text, Grids & Input

Build something real.

Unit 4 of 8 in C++ for kids. Its 5 lessons are String Tools, Walking Through Text, Vector Tools, Grids & Asking the User and Quiz Machine — below is everything each one explains, and a question or two from it to try.

Every sample on this page was compiled and run with g++ before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

🔧 String Tools

Taking a piece out

substr cuts a piece: where to start, and how many letters to take.

C++

string word = "dragonfly";
cout << word.substr(0, 6) << endl;
cout << word.substr(6, 3) << endl;

It prints

dragon
fly

Asking a string about itself

length() counts the letters, empty() says whether there are none, and at() fetches one — like [ ] but it checks the number is sensible first.

C++

string w = "code";
cout << w.length() << endl;
cout << w.at(1) << endl;
string nothing = "";
cout << boolalpha << nothing.empty() << endl;

It prints

4
o
true

Try it yourself

What does this print?

C++

string w = "birthday";
cout << w.substr(0, 5) << endl;

What does this print?

C++

string a = "cat";
string b = "cat";
cout << (a == b) << endl;

Answer them in the app

🚶 Walking Through Text

A string is loopable

A range-for walks a string one letter at a time, exactly as it walks a vector.

C++

for (char c : string("cat")) {
    cout << c << endl;
}

It prints

c
a
t

Building text up

Start with an empty string and add to it as you go — the usual way to build a word letter by letter.

C++

string out = "";
for (char c : string("abc")) {
    out = c + out;
}
cout << out << endl;

It prints

cba

Try it yourself

What does this print?

C++

string word = "banana";
int count = 0;
for (char c : word) {
    if (c == 'a') {
        count = count + 1;
    }
}
cout << count << endl;

What does this print?

C++

string out = "";
for (int i = 0; i < 3; i++) {
    out = out + "ab";
}
cout << out << endl;

Answer them in the app

🧺 Vector Tools

Shrinking as well as growing

pop_back removes the last item, clear empties the whole thing, and empty() reports whether anything is left.

C++

vector<int> nums = {1, 2, 3};
nums.pop_back();
cout << nums.size() << endl;
nums.clear();
cout << boolalpha << nums.empty() << endl;

It prints

2
true

Changing what is inside

A range-for hands you a copy of each item, so writing to it changes nothing. To edit the vector itself, loop by index.

C++

vector<int> nums = {1, 2, 3};
for (int i = 0; i < nums.size(); i++) {
    nums[i] = nums[i] * 10;
}
cout << nums[0] << " " << nums[2] << endl;

It prints

10 30

Try it yourself

What does this print?

C++

vector<int> v = {5};
v.push_back(6);
v.push_back(7);
v.pop_back();
cout << v.size() << " " << v[v.size() - 1] << endl;

Why does for (int n : nums) { n = n * 2; } leave the vector unchanged?

The vector is const
n is a copy of each item, so doubling it changes only the copy
You cannot multiply in a loop
It does change it

Answer them in the app

⌨️ Grids & Asking the User

A vector of vectors

Put vectors inside a vector and you have a grid — a game board, a maze, a picture. Two numbers reach a square: row first, then column.

C++

vector<vector<int>> grid = {{1, 2}, {3, 4}};
cout << grid[0][1] << endl;
cout << grid[1][0] << endl;

It prints

2
3

Reading what the user types

cin >> reads one thing the player typed and puts it in a box. It waits until they press enter.

C++

int n;
cin >> n;
cout << n * 2 << endl;

It prints

If the player types 4, this prints 8

Try it yourself

What does this print?

C++

vector<vector<int>> g = {{1, 2, 3}, {4, 5, 6}};
cout << g.size() << " " << g[0].size() << endl;

Which way do the arrows point for reading, compared with printing?

The same way for both
cout << sends out; cin >> brings in — the arrows point the way the data travels
cin uses <<
cin needs no arrows

Answer them in the app

🏆 Quiz Machine

Try it yourself

What does this print?

C++

vector<string> words = {"a", "bb", "ccc"};
int total = 0;
for (string w : words) {
    total = total + w.length();
}
cout << total << endl;

Answer it in the app