C++ Variables, Data Types and Input

Variables are named containers that hold data, like a name, a price or a score.

Declaring Variables

Write the type, the name and (usually) a starting value:

Example

cpp

#include <iostream>
#include <string>
using namespace std;

int main() {
  string name = "Bilal";
  int age = 22;
  double salary = 55000.75;
  char grade = 'B';
  bool isStudent = true;

  cout << name << " is " << age << " years old\n";
  cout << "Salary: " << salary << "\n";
  cout << "Grade: " << grade << "\n";
  cout << "Student: " << isStudent << "\n";
  return 0;
}

Output

Bilal is 22 years old
Salary: 55000.8
Grade: B
Student: 1

Two things to notice:

cout shows about 6 significant digits by default, so 55000.75 is shown as 55000.8
bool values print as 1 (true) and 0 (false). Add cout << boolalpha; to print true and false

Data Types

Type	Stores	Example
`int`	Whole numbers	`int rooms = 3;`
`double`	Decimal numbers	`double kg = 2.5;`
`float`	Decimals, less precise	`float r = 1.5f;`
`char`	One character	`char c = 'Z';`
`bool`	`true` or `false`	`bool done = false;`
`string`	Text (needs `<string>`)	`string city = "Hyderabad";`

Constants and auto

Example

cpp

const double GST = 0.18;   // cannot be changed later
auto count = 10;           // the compiler decides: int
auto rate = 2.75;          // the compiler decides: double

Use const for values that should never change, like tax rates or the number of days in a week. The compiler will stop you if you try to change them by mistake.

Type Conversion

Dividing two int values drops the decimal part. Use static_cast to convert:

Example

cpp

#include <iostream>
using namespace std;

int main() {
  int total = 7;
  int parts = 2;
  cout << total / parts << "\n";
  cout << static_cast<double>(total) / parts << "\n";
  return 0;
}

Output

3
3.5

Formatting Decimals

Include <iomanip> to control how many digits are shown:

Example

cpp

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
  double bill = 1234.5;
  cout << fixed << setprecision(2) << bill << "\n";
  return 0;
}

Output

1234.50

Reading Input with cin

cin >> reads a value typed by the user:

Example

cpp

#include <iostream>
#include <string>
using namespace std;

int main() {
  string name;
  int year;
  cout << "Your name: ";
  cin >> name;
  cout << "Birth year: ";
  cin >> year;
  cout << "Hi " << name << ", you are about " << 2026 - year << " years old.\n";
  return 0;
}

Output

Your name: Ayesha
Birth year: 2004
Hi Ayesha, you are about 22 years old.

cin >> stops reading at a space. To read a full line such as "Ayesha Khan", use getline(cin, name);.

Exercise

Which type should you use to store the text "Karachi"?

charintstringbool