C++ Basics and Compiling
1. Syntax
The syntax of C++ is simple. In PRO1, you will use the <iostream> library for most exercises to handle code input and output.
To include a library, we simply write #include <library> at the beginning of a .cpp or .cc file (C++ formats).
Besides the library, we also use a "namespace". A namespace acts like a function prefix; for example, if the standard library (std) has a cout function and a custom library paco has a function with the same definition paco, the prefix prevents conflicts. To avoid writing the prefix repeatedly (e.g., std::cout), we add the following line after the libraries (as per the course's standard structure): using namespace std;.
Every C++ program has a main function (main), which the compiler looks for to execute the code. (The example shows the declaration of the main function).
Example:
To add comments to a code (text that does not affect our code), we just need to write // (single-line comment) or /* */ (block comment).
#include <iostream>
using namespace std;
// This is a comment only for this line.
/*
This is a comment
until the block
is closed
*/
int main() {
// ...
}
2. Variable Types
We have different variable types:
* int (integers) -> (e.g., 124)
* string (words) -> (e.g., "Hello")
* char (characters) -> (e.g., 'a') Note: a character is stored using its ASCII value; for instance, '0' is 48.
* double (decimals) -> (e.g., 67.67)
* bool (true or false) -> (e.g., true) Note: it can also be declared with 1 (true) and 0 (false).
To declare variables, we use a keyword that assigns a value to that name.
Additionally, we can make a variable a constant, meaning its value will never change. We simply put const in front of the variable declaration.
Example of how to declare a variable:
#include <iostream>
using namespace std;
int main() {
// Structure: type variable_name = value;
int number = 124; // Every time we define a value, we must include the ;
string word = "Frieren";
char ch = 'a';
double sixseven = 67.67;
// Just as we can assign number = 124, we can do the following:
int aux = number; // Since number equals 124,
// we can assign this value to another variable.
// Declaration of a constant:
const int restricted_age = 18;
}
3. Input and Output
Now that we know how variables work, we will learn how to compile a .cpp file, input data through the terminal, and print data to the terminal.
First, we will do the classic exercise for any programming language:
1. HelloWorld.cpp
To see "Hello World!" in the terminal, we need to compile. How do we do it? Easy. In the Linux terminal, navigate to the file's directory:
You can name the executable whatever you want; out of habit, I always usex.x, while others create one executable per program.
Example of compilation and execution with HelloWorld:
After running./x.x, you will see in the terminal: Hello World!
NOTE: Remember to always save (
CTRL + SorCTRL + G) before compiling to avoid silly compilation errors.
2. HelloName.cpp
#include <iostream>
using namespace std;
int main() {
string name; // Declaration without assigning a value
// In C++, a string receives the empty value "" by default.
cout << "What is your name?" << endl;
cin >> name; // We type the name in the terminal with ONLY ONE word.
cout << "Hello " << name << '!' << endl;
// cin: Console INput
// >> extraction operator for cin (means: input the received value into the variable)
}
When using a cin, we input the requested values and press the Enter key to notify the code that we have provided the information.
IMPORTANT DIFFERENCES BETWEEN
cinANDcoutWITH STRINGS With acin, we can only input one word perstringvariable. Thecincommand treats a space as the end of the string. On the other hand, withcout, when we writecout << "phrase with spaces" << endl;, it prints the entire phrase to the console without cutting it off.
4. Logic Operators and Comparators
Arithmetic Operators
| Type | Declaration |
|---|---|
| Addition | + |
| Subtraction | - |
| Multiplication | * |
| Division | / |
| Modulo (Remainder) | % |
| Assignment | = |
NOTE: The division
/, when dividing integers, will return the integer part. Conversely, if we divide doubles, it will return the exact decimal division. Therefore, we can never dodouble % double, since decimal division itself inherently accounts for the remainder.
Compound and Increment Operators
// assume i is an integer
i++; // post-increment -> after working with i,
// we increment by 1, meaning: i = i + 1.
++i; // pre-increment -> before working with i,
// we increment by 1.
// post-decrement and pre-decrement can also be done
i += 20; // translation: i = i + 20
i /= 2; // translation: i = i / 2;
Example:
3. PostAndPreExample.cpp
#include <iostream>
using namespace std;
int main(){
// Post-Decrement Example
int i = 5;
int a = i--; // 'a' keeps the 5. Once assigned, 'i' drops to 4.
cout << "POST-DECREMENT -> a: " << a << ", i: " << i << endl;
// Pre-Decrement Example
int j = 5;
int b = --j; // 'j' drops to 4 immediately. Then it is assigned, so 'b' is also 4.
cout << "PRE-DECREMENT -> b: " << b << ", j: " << j << endl;
// post-increment and pre-increment work the same way:
i = 5;
a = i++; // 'a' keeps the 5. Once assigned, 'i' increases to 6.
cout << "POST-INCREMENT -> a: " << a << ", i: " << i << endl;
j = 5;
b = ++j; // 'j' increases to 6 immediately. Then it is assigned, so 'b' is also 6.
cout << "PRE-INCREMENT -> b: " << b << ", j: " << j << endl;
}
Logic Operators
| Type | Declaration |
|---|---|
| AND | && |
| OR | || |
| NOT | ! |
Logical Comparators
A comparator always yields a boolean result (true or false)
| Type | Declaration |
|---|---|
| Greater than | a > b |
| Less than | a < b |
| Greater than or equal to | a >= b |
| Less than or equal to | a <= b |
| Strictly equal | a == b |
| Not equal | a != b |
5. Conditionals
In programming, we use conditionals: if, else if, else.
We always declare the if first. If there is a second condition we want to handle after the first one fails, we use else if. For all other cases (the exceptions), we use else.
For structural clarity and elegance, we will always open a {} block after the conditional statement.
4. CanYouDrink.cpp
#include <iostream>
using namespace std;
int main() {
int age = 0; // everyone is at least 0 years old
cout << "How old are you? (type your age with a number (i.e. 24))" << endl;
cin >> age;
if (age >= 18) { // We check if the age is greater than or equal to 18.
cout << "You can drink alcohol" << endl;
} else {
cout << "You can't drink alcohol" << endl;
}
}
6. Final Exercise
To consolidate everything we have seen in this first topic (variables, cin/cout, operators, and conditionals), here is a complete program that implements a basic single-use calculator.
final.cpp
/*
* Final Exercise
* Example: Basic Single-Use Calculator.
* Made by: aso
*/
#include <iostream>
using namespace std;
int main(){
int a, b;
char op;
bool correct = true;
cout << "Type an integer operation with only one operator (i.e. 24 - 12), valid operators: +, -, *, /, %." << endl;
cin >> a >> op >> b; // read number a, operator, number b
int result = 0; // Whenever a number is printed to the screen, we initialize it to 0 for safety.
if (op == '+'){
result = a + b;
} else if (op == '-'){
result = a - b;
} else if (op == '*'){
result = a * b;
} else if (op == '/'){
if (b != 0){ // In PRO1 they always test for the weak points of the exercise.
result = a / b;
} else{ // b == 0
correct = false;
}
} else if (op == '%'){
if (b != 0){
result = a % b;
} else{ // b == 0
correct = false;
}
} else{ // invalid operator
correct = false;
}
if (correct){
cout << "The operation is equal to " << result << '.' << endl;
} else{
cout << "Error: Invalid operator or division by zero" << endl;
}
}