02. Your first C++ program — Homework Solutions

The solution .cpp files are in exercises/02/homework/solutions/.

Open this page after trying on your own. How to use it: compare your line of reasoning with the How to think about it part, not the code line by line. Two pieces of code with different shapes can both be correct, and what you need to copy is the way of getting there.

Running one of the solutions:

g++ -std=c++17 -O2 -o jawab exercises/02/homework/solutions/01-biodata.cpp
./jawab

Problem 1 — Personal details

Problem. Print four lines of personal details, one cout per line.

How to think about it. Each output line is one cout statement ending with endl. No variables, no logic — the goal is to get your hands used to the cout << ... << endl; shape and the semicolon.

Walkthrough.

#include <iostream>
using namespace std;

int main() {
    cout << "Nama: Budi Santoso" << endl;
    cout << "Kelas: 9A" << endl;
    cout << "Sekolah: SMP Negeri 1 Bandung" << endl;
    cout << "Saya ingin ikut OSN karena senang memecahkan teka-teki." << endl;
    return 0;
}

Tests. This program reads no input; its output must be four lines, each containing one detail. Check one thing that is easy to miss: there are no blank lines between them, and no spaces at the end of a line.

Common mistakes.

• Forgetting the semicolon at the end of one of the lines.
• Writing cout << "Nama: Budi" << endl without the closing quote, so the compiler complains missing terminating " character.

Problem 2 — Name card

Problem. Print a box of +, -, and | with a name in the middle.

How to think about it. The picture has three lines, so the code has three couts. The important part: the top, middle, and bottom lines must have the same length, otherwise the box is crooked. First count the width of the content (here 16 characters), then write +, 16 - signs, then +.

Walkthrough.

cout << "+----------------+" << endl;
cout << "|   Budi Santoso |" << endl;
cout << "+----------------+" << endl;

Tests. Count the length of the three lines by hand — all three must be exactly the same:

If your name is longer or shorter than Budi Santoso, the number of - signs must be adjusted along with it. This is why this problem forbids loops for now: so you feel how tedious it is to match widths by hand, and how easy it is after Chapter 05.

Common mistakes.

• The number of - on the top and bottom lines differs.
• Counting the length of the middle line without accounting for spaces.

Problem 3 — Table with tabs

Problem. Print a table of the names and scores of three friends with \t as the separator.

How to think about it. \t jumps to the next tab position, rather than adding a fixed number of spaces. That is why the columns look aligned as long as the names are the same length or differ by no more than one tab jump.

Walkthrough.

cout << "Nama\tNilai\n";
cout << "Budi\t90\n";
cout << "Siti\t85\n";
cout << "Andi\t78\n";

Tests. The output:

Nama    Nilai
Budi    90
Siti    85
Andi    78

Tab positions in a terminal are usually multiples of 8 characters. Because Nama, Budi, Siti, and Andi are all four letters, one \t is enough to jump to the same column. Try changing one name to Muhammad and see the column shift by one jump.

Common mistakes.

• Writing \t outside the quotes.
• Using names whose lengths differ a lot (Muhammad Rizki and Ani), then wondering why the columns do not line up. That case needs setw from <iomanip>, which is not used in this chapter yet.

Problem 4 — Fix the errors

Problem. A file deliberately broken with three mistakes.

How to think about it. Fix one mistake, the topmost, then compile again. One mistake often produces many lines of messages, so fixing everything at once based on guesses usually adds more problems.

The three mistakes:

1. using namespace std without a semicolon.
2. Cout should be cout — C++ distinguishes upper and lower case.
3. No semicolon after endl.

The first compile gives two messages at once:

04-perbaiki-error.cpp:11:20: error: expected ';' before 'int'
   11 | using namespace std
      |                    ^
      |                    ;
04-perbaiki-error.cpp: In function 'int main()':
04-perbaiki-error.cpp:14:5: error: 'Cout' was not declared in this scope
   14 |     Cout << "Semua error sudah diperbaiki." << endl
      |     ^~~~

Notice that the third mistake has not appeared at all yet. The compiler stops understanding the body of main as soon as it meets the unknown Cout, so it has not yet got as far as checking the semicolon after endl. Only after the first two mistakes are fixed and the code is compiled again does this appear:

04-perbaiki-error.cpp:14:52: error: expected ';' before 'return'

This is the correct way to work: fix the topmost one, compile again, read the new message. Trying to guess all three at once from the first message is impossible, because the third message does not exist yet.

Walkthrough.

#include <iostream>
using namespace std;

int main() {
    cout << "Semua error sudah diperbaiki." << endl;
    return 0;
}

Common mistakes.

• Panicking at a long error message. The first message is almost always the real one; the rest are consequences.
• Deleting the line with the error instead of fixing it.

Challenge — Nested quotes

Problem. Print Guru berkata: "tulis \n untuk pindah baris" as-is.

How to think about it. There are two kinds of characters that need to be escaped: a double quote becomes \", and a backslash becomes \\. The trap is the \n inside the sentence: if written as-is, it turns into a line break. For it to appear as the two characters \ and n, write \\n.

Walkthrough.

cout << "Guru berkata: \"tulis \\n untuk pindah baris\"" << endl;

Tests. The output is exactly one line:

Guru berkata: "tulis \n untuk pindah baris"

If your output is split into two lines, the \n inside the sentence is still being read as a line break command, not as two ordinary characters.

Common mistakes.

• Writing \n instead of \\n, so the sentence is cut into two lines.
• Forgetting the \ on a quote, so the compiler thinks the text has ended and complains expected ';'.

OSN-style exercises — walkthrough

The solution files are in exercises/02/osn/solutions/.

Problem 1 — Chessboard

How to think about it. An 8 × 8 board has only two row shapes: #.#.#.#. and .#.#.#.#. They alternate, starting with the first shape. Without loops, the answer is eight cout statements.

Walkthrough.

cout << "#.#.#.#.\n";
cout << ".#.#.#.#\n";
cout << "#.#.#.#.\n";
cout << ".#.#.#.#\n";
cout << "#.#.#.#.\n";
cout << ".#.#.#.#\n";
cout << "#.#.#.#.\n";
cout << ".#.#.#.#\n";

Tests. There must be exactly eight lines and each line exactly eight characters. A quick way to check this in the terminal:

./catur | wc -l
./catur | awk '{ print length($0) }'

The first command must print 8, the second eight 8s.

Common mistakes.

• Swapping the order of the rows, so the top-left corner becomes ..
• Writing nine lines because the last line gets an extra endl.

Problem 2 — Staircase

How to think about it. Line i consists of 5 - i spaces followed by i # signs, so the length of every line is always five. Counting the spaces is easier from the bottom: the last line has no spaces at all.

Walkthrough.

cout << "    #\n";
cout << "   ##\n";
cout << "  ###\n";
cout << " ####\n";
cout << "#####\n";

Tests. Every line must have length five:

./tangga | awk '{ print length($0) }'

If any number other than 5 appears, the number of spaces is wrong — or a space has slipped in at the end of a line.

Common mistakes.

• Writing a left-aligned staircase (#, ##, ###) when the problem asks for a right-aligned one.
• Adding spaces at the end of lines so it "looks tidy" in the editor.

Problem 3 — Shop Receipt

How to think about it. Each of the three troublesome characters has its own escape form: \t for a tab, \" for a quote, \\ for a single backslash. The path C:\struk\2026.txt contains two backslashes, so in the code it is written C:\\struk\\2026.txt.

Walkthrough.

cout << "Toko\t\"Sumber Rejeki\"\n";
cout << "Berkas\tC:\\struk\\2026.txt\n";
cout << "Pensil\t3\t6000\n";
cout << "Buku\t2\t14000\n";

Tests. Tabs are not visible on screen, so check with cat -A:

./struk | cat -A

Every tab appears as ^I and every line end as $. The first line must read Toko^I"Sumber Rejeki"$.

Common mistakes.

• Replacing the tab with several spaces because they look the same on screen.
• Writing C:\struk in the code: \s is not a recognized escape, and the compiler complains unknown escape sequence.
• Forgetting one \ on the closing quote, so the whole rest of the line is treated as part of the text.

If you are still stuck

• Cannot read an error message → Chapter 02, section Reading error messages.
• Confused about when to use endl and when "\n" → Chapter 02, section Printing with cout.
• Quotes and backslashes are confusing → Chapter 02, the Special characters table.

Before moving on to Chapter 03, make sure you can write the complete program skeleton from memory, without copying, in less than one minute.