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Cheatsheets / C++ for Programmers

Basic Syntax in C++

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Program Structure

C++ programs run line by line and generally follow the same program structure:

  • #include statements at the beginning of the program, which allow access to library functionalities.
  • main() function, which is run when the program is executed.
  • return 0 at the end of the main() function, which indicates that the program ran without issues.
#include <iostream>
using namespace std;

int main() {
  cout << "Hello, world!";
  return 0;
}

Comments

Comments are notes left by the programmer that explain complex code. Comments do not affect the performance of a program because they are ignored by the compiler.

In C++, there are two types of comments:

  • Single-line: begin with //.
  • Multi-line: begin with /* and end with */.

As a rule of thumb, comments should always go above the code they are commenting on.

// I am a single-line comment

/* 
I am a
multi-line
comment
*/

Input and Output

Input and output make C++ programs more interactive.

  • #include <iostream> must be placed at the beginning of the program to access input and output.
  • std::cout is the “character output” and it is used together with << to print to the terminal.
  • std::cin is the “character input” and it is used together with >> to read user input.
  • std::endl or \n can be used to insert a new line.
#include <iostream>

int main() {
  int age;
  std::cout << "How old are you? ";
  std::cin >> age;
  std::cout << "You are " << age << " years old.";
  
  return 0;
}

Variables

Variables are used to store and retrieve data. When declaring a variable, it must be given a data type and a name.

Multiple variables of the same type can be declared in a single statement using a comma-separated list.

Variables can be declared with the const keyword, which prevents their value from being changed later.

int number = 100;

char letter;
letter = 'c';

const int pi = 3.14;

References and Pointers

C++ provides two powerful features for memory manipulation:

  • References: aliases to existing variables
  • Pointers: store memory address as its value

Reference variables are created using the & symbol. & is also used to access the memory address of a variable.

Pointer variables are created using the * symbol. * is also used to obtain the value pointed to by a pointer variable.

int year = 2021;

int &ref = year;

int *ptr = &year;

std::cout << &year << "\n";

std::cout << *ptr << "\n";

User Input

std::cin, which stands for “character input”, reads user input from the keyboard.

Here, the user can enter a number, press enter, and that number will get stored in tip.

int tip = 0;

std::cout << "Enter amount: ";
std::cin >> tip;

Variables

A variable refers to a storage location in the computer’s memory that one can set aside to save, retrieve, and manipulate data.

// Declare a variable
int score;

// Initialize a variable
score = 0;

Arithmetic Operators

C++ supports different types of arithmetic operators that can perform common mathematical operations:

  • + addition
  • - subtraction
  • * multiplication
  • / division
  • % modulo (yields the remainder)
int x = 0;

x = 4 + 2;  // x is now 6
x = 4 - 2;  // x is now 2
x = 4 * 2;  // x is now 8
x = 4 / 2;  // x is now 2
x = 4 % 2;  // x is now 0

double Type

double is a type for storing floating point (decimal) numbers. Double variables typically require 8 bytes of memory space.

double price = 8.99;
double pi = 3.14159;

Chaining the Output

std::cout can output multiple values by chaining them using the output operator <<.

Here, the output would be I'm 28.

int age = 28;

std::cout << "I'm " << age << ".\n";

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