Basics - sheerazwalid/COMP-I GitHub Wiki

• Used to store data in the memory.
• Referred to by its identifier (the name of a variable).
• The data stored in a variable can be changed using the assignment operator.

• To use a variable, you must declare it first.
• A declaration specifies a type, and contains a list of one or more variables of that type.

variable_type variable_name1, variable_name2;

int age, steps;
char c;
bool win;
double height, width, length;
double weight;

• Tells the computer how to interpret the data stored in a variable
• Determines the size of storage needed to store the data
• Some basic data types in C++:

Use sizeof() to determine the size of a variable in bytes.

cout << sizeof(int) << endl;  // normally prints 4

int k;
cout << sizeof(k) << endl;  // normally prints 4

An identifier must start with either:

• a letter (i.e., A to Z, and a to z), or
• the underscore symbol (i.e., _)
• The rest of the character may be _letters (i.e., A to Z, and a to z), digits (i.e., 0 to 9), or The underscore symbol (i.e., )
• C++ is case‐sensitive. So radius, RADIUS, Radius, etc., are different
• Cannot be a keyword in C++

There are various ways to initialize variables; the following shows one way and contrasts with an uninitialized variable.

#include <iostream>

using namespace std;

int main () {
  int k = 3;  // Declares an integer k and initializes it to 3.
  int n;      // Declares an integer n with an undetermined value.
  char c = ‘Y’ // A character constant is written as a character within single quotes

  cout << k;  // prints 3
  cout << n;  // output undetermined
  return 0;
}

Sometimes we want to assign a fixed value to a variable, and never change it.

• double PI = 3.1416;  
• constant double PI = 3.1416;

• Add in front of a variable declaration to declare the variable as a constant variable. Then, the complier will not allow the program to change its value.

A string is not a primitive datatype; it is a complex datatype. The dot operator is used with complex datatypes to access functionality of that datatype. The code below illustrates how to declare strings and work with them.

#include <iostream>

using namespace std;

int main () {
  string s = "hi"; // Declares a string s and initializes it to "hi".
  string t;        // Declares a string t and initializes it to empty.

  cout << s.size();  // Prints "2".
  cout << t.size();  // Prints "0".

  s += " bob";
  cout << s;  // Prints "hi bob".

  // Use substr(start_index, number_of_chars) to extract substrings.
  cout << s.substr(3, 0); // Prints "bo".
  return 0;
}

The type char is a primitive type; it represents a single byte that can be treated as a character (letter, number, punctuation). Strings are made up of sequences of chars.

Character literals are demarcated with apostrophes.

char c = 'x';

String literals are demarcated with quotes.

string s = "xyz";  

Special characters -- such as the new line character -- start with a backslash and are followed by one or more other characters.

string t = "Dear John,\nHow are you?\n"; // '\n' is a newline character.

To include a backslash in a string you need to escape it with a backslash.

cout << "\\"; // prints a single backslash

To include a quote in a string you need to escape it with a backslash.

cout << "\""; // prints a single quote character

String literals can be spread out into multiple chunks across multiple lines.

cout << "This is "  "a single "
        "line of characters\n";

• A variable may also be initialized or changed at a later time after its declaration using an assignment statement.
• An assignment statement consists of a variable on the left-hand side of an equal sign,  and a value or an expression on the right‐hand side.

variable_name = expression;

int x;
double y;

x = 7;                // initializes the value of x to 7
y = 20.5 + 13.22;    // 20.5 and 13.22 are examples of the expressions

• Expressions combine variables and constants to produce new values.
• Expressions are composed of operands (data) and operators (instructions).

• Might come in the form of variables or constants

• Specify what is being done to the operands

2 + x / y;    // 2, x and y are operands 
              // '+' and '/' are operators
              // 'x / y' is an expression 

Declare an integer k and assign a value of 3 to it.

int k;
k = 3;

Declare an integer n and assign the value represented by k to it.

int n;
n = k;

The following show several examples of addition and substraction.

int k = 3;
k = k + 1; // k becomes 4.
k += 2;    // k becomes 6.
k -= 2;    // k becomes 4
++k;       // k becomes 5.
k++;       // k becomes 6.
--k;       // k becomes 5.
k--;       // k becomes 4.

Here are examples of the pre-increment and post-increment operators.

cout << ++k;  // Prints 8 because increment is done before k is used.
cout << k++;  // Prints 8 because increment is done after k is used.
cout << k;    // Prints 9.

Here are some examples of multiplication and division.

int n = 2;
n = n * 2;  // n becomes 4
n *= 2;     // n becomes 8
n /= 2;     // n becomes 4

It's important to keep in mind the difference between floating point division and integer division. If either the left-hand side or the right-hand side of a division are floating point numbers, then you get floating point division. In floating point division, the fractional component is retained.

cout << 5 / 2.0; // Prints 2.5 

If the left-hand side and right-hand side of a division are both integers, then you get integer division. In integer division, the fractional component is discarded.

cout << 5 / 2;   // Prints 2 

Use the modulo operator % to get the remainder from integer division.

cout << 5 % 2;  // Prints 1 because 5 divided by 2 produces a remainder of 1.

See table in Operators for operator precedence.

• Variables and Types
• Operators

• 2016-06-29
• 2016-07-06