syntax

Syntax is the most essential to grasp to write a c++ program. Here is an overview.

structure

#include <iostream>
using namespace std;
int main() { cout << "Hello World"; }

This is a basic C++ program which prints "Hello World" to the console.

It is made up of three essential parts:

1. #include: includes the necessary header files for functions used in the program.
2. using namespace std: includes the standard library of c++ functions.
3. int main() {}: the main function, where all driver code should go.

cout is for standard output (i.e. "printing"), and cin is for receiving input to a variable defined earlier.

datatypes

variables can be thought of as 'containers' for values, that can be changed and reused throughout a program. in c++, variables take one of many datatypes, shown below.

Basic Data Types

Data Type	Size (in bytes)	Stores	Example
int	$4$	Numbers ($-2147483648$ to $2147483647$)	$12442$
float	$4$	Decimal numbers (up to $7$ d.p.)	$5.63562$
double	$8$	Decimal numbers (up to $15$ d.p.)	$3.1415926535879732$
bool	$1$	true or false values	true, false
char	$1$	A single character	'D', '$'
string	variable	Multi-character text	'hi mom'

Data Type Modifiers

these are most commonly used for numeric datatypes (int and double), to resize their limits according to the program's requirements.

the default modified datatype is int.

Modified Datatype	Size (in bytes)	Range
short	$2$	$-32768$ to $32767$
unsigned short	$2$	$0$ to $65535$
unsigned	$4$	$0$ to $4294967295$
long	$4$	$-2147483648$ to $2147483647$
unsigned long	$4$	$0$ to $4294967295$
long long	$8$	$-2^{63}$ to $2^{63}-1$
unsigned long long	$8$	$0$ to $18446744073709551615$
long double	$12$	-

control flow

a program can include logical statements, that control how the program behaves in differing conditions.

if statements

these can be thought of as asking yes/no questions to the computer.

if the answer is 'yes', something would happen; if 'no', something else would happen.

below is code that invites the user to a drink if 18 and over, or else denies access.

#include <iostream>
using namespace std;
int main() {
  int a;
  cout << "enter your age: ";
  cin >> a;

  if (a < 18) {
    cout << "access denied";
  } else {
    cout << "have a drink!";
  }
}

conditions in if-statements are surrounded by parentheses and the following code to be executed is in curly braces.

multiple conditions can be expressed with else if:

#include <iostream>
using namespace std;
int main() {
  int a;
  cout << "enter your age: ";
  cin >> a;

  if (a < 18) {
    cout << "you cannot drink nor smoke.";
  } else if (a < 21) {
    cout << "you can drink, but not smoke.";
  } else {
    cout << "you can do both! welcome to adulthood!";
  }
}

Logical Operators

if multiple conditions are to be satisfied, the following can be used.

Op.	Means	True Example	False Example
&&	"and"	(5 > 3 && 2 < 4)	(3 > 5 && 2 < 4)
||	"or"	('s' == 's' or 't' == 'u')	('s' == 't' or 't' == 'u')
!	"not"	(!(5 < 3))	(!('a' == 'a'))

Comparison Operators

these are used inside conditions to compare values to one another, resulting in true/false return values.

Op.	Means	True Example
==	is equal to	(5 == 5)
!=	is not equal to	(5 != 3)
>	is greater than	(5 > 3)
>=	is greater than or equal to	(5 >= 5)
<	is smaller than	(3 < 5)
<=	is smaller than or equal to	(5 <= 5)

Loops

For Loop

say you wanted to repeat a block of code some number of times. writing the same block of code that many times would be tiresome, and it gets worse if the number of times to repeat it is determined by the user.

we can use for loops instead! a for-loop contains these three parts

1. initialisation: initialise the counter variable to be used
2. condition: loop as long as the condition is true
3. increment/decrement: increment/decrement variables with every run of the loop

any of these parts can be ommited.

#include <iostream>
using namespace std;
int main() {
  //  initialize, cond., inc./dec.
  //  output: 1 2 3 4 5
  for (int i = 1; i <= 5; i++) {
    cout << i << ' ';
  }
  //  output: 4 3 2 1
  for (int i = 4; i > 0; i--) {
    cout << i << ' ';
  }
}

While Loop

if you didn't have a certain number of times the loop should run, a loop can still run as long as certain condition(s) hold true.

the block of code inside the loop will repeat as long as the condition is true.

#include <iostream>
using namespace std;
int main() {
  int i = 10;
  while (i > 0) { // put condition in parens.
    cout << i << ' ';
    i--;
  }
  // output: 10 9 8 7 6 5 4 3 2 1
}

Functions

functions define a block of code to be reused throughout the program at different junctures.

you can think of these as variables but for blocks of code.

#include <bits/stdc++.h>
using namespace std;

// function name, (parameters)
int sum(int a, int b, int c) { return a + b + c; }

int main() {
  cout << sum(1, 2, 3);
  // output: 6
}

normally, a function is expected to return a certain value after the operations inside have been completed. these values take the c++ datatypes we've seen before:

1. int function

int add(int a, int b, int c) { return a + b + c; }

2. void function

this function returns nothing, and is normally used to modify existing variables.

void addAndPrint(int a, int b, int c) { cout << a + b + c; }

int main() { addAndPrint(1, 2, 3); }

3. string function

returns a string.

string add(string a, string b, string c) { return a + b + c; }

int main() {
  cout << add("ab", "cd", "ef");
  // output: abcdef
}

4. bool function

returns a boolean value: true or false.

bool check(int a, int b, int c) {
  if (a + b > c)
    return true;
  return false;
}

note: the else keyword isn't needed here, because when return is called, the function ends, ignoring everything else after it.

alternatively,

int add(int a, int b, int c) { return a + b > c; }

note: this function can still produce true or false results depending on the values of a, b and c.