Modifying the value of a variable:

1. Input statement
• cin >> length;
• cin >> length >> width;
• Semantics
• wait for the user to enter a value
• place the value into the memory location associated with the value
• Potential error : not enough room allocated for variable (user may enter incorrect type)
2. Assignment statement
• General format: Variable = Expression
• perimeter = length + length + width + width;
• Semantics
• evaluate the right hand side (rhs)
• use operator precedence
• use type rules for operators
• possibly typecast result to the lhs type
• place result in memory for variable
• Potential error
• Incorrect types for operators
• Division by 0
• Typecasting results in loss of precision
• Not enough room in memory (type mismatch)

More Assignment Statements

• Initializing variables within declarations
  int count = 1;
• Unitialized variables have no value. Whatever is left over in memory (RAM)
• Arithmetic Operator Precedence
  • Parentheses ()
  • Multiplication *, Division /, Modulus %
  • Addition +, Subtraction -
• Shorthand assignments
  • General Format: variable operator= Expression
  • +=, -=, *=, /=, %=
  • a += b; //same as a = a + b;
• Assignment Compatibility
  Assigned value (rvalue) must be the same type as the variable on the left hand side (lvalue)

  CORRECT int counter; counter = 20;

  INCORRECT int counter; counter = 12.2;

• Literal is a name for one specific value. For example the number two is the literal 2

More Programming Basics

• Floating point notation
  • mantissa
  • e+2 is the same as 10²
  • 0.35e+2 is 35
  • 0.35e-2 is 0.0035
• Escape Sequences
  • \" : to print a "
  • \\ : to print a \
• Constants
  • const is key word.
  • Value cannot be changed
  • Permitted (by instructor) to be defined prior to main
  • const PI = 3.14;
• Integer and Floating-Point Division

  int num=20, num2=3; float x; x = num/num2; //division is integer cout << x; // 6 is printed

  float num=20, num2=3; float x; x = num/num2; //division is floating cout << x; // 6.33 is printed

• Type casting
  • can specifically state type to be used
  • place the name of the type in ( )
  • float num=20, num2=3;
    float x;
  • x = static_cast<int> num/num2; //division is integer
    cout << x; // 6.0 is printed
  • To see the UNICODE value of a character try:

        char inChar;
        cin >> inChar;
        cout << inChar << " has the value "<< static_cast<int>(inChar);
    

• Increment and Decrement Operators
  • ++ and --
  • i++; // is the same as i+=1 and i=i+1
  • Pre and post increment/decrement permitted.
  • a=b++; // a is assigned b and then b is incremented
  • a = ++b; // b is incremented and then a is assigned new b

Formatting Output

• iomanip library contains formatting methods to modify the output produced by a cout statement
• include iomanip as a library after iostream
• some of the manipulators only control the next item output
• We will focus on formatting numbers with decimal points
• the setw method sets the width of the "print field" of the next item
• to print x in a field of size 8 and y in a field of 6:
  cout << setw(6)<< x << setw(8)<< y;
• other manipulators include:
  • fixed shows number with decimal point instead of scientific notation
  cout << fixed << 12346.897
  displays 12346.897 instead of 1.2346897e+04
  • showpoint is used to display the decimal point for numbers such as 123.000
  • setprecision sets the number of decimals places
    cout << setprecision(3) << number << endl;
  • left and right manipulators. Used to right and left justify numbers
  cout << right;
  cout << << setw(8) << 1.23 << endl;
  cout << setw(8) << 351.34 << endl;
  displays as:

      1.23
    351.34
  

• Try this example

• Single letters include escape characters: '\n' and '\t'
• cin reads until the first blank space
  string name;
  cout <<Enter your name ";
  cin >> name;

  • If you only enter one name such as Abdullah, it operates as expected
  • But if you enter a first and last name such as Tori Gardener, only Tori is read in and placed into name
  • What happens to the second string (i.e., Gardener)? It is buffered
• Use the getline function to enter strings with spaces
  getline(cin, name);
• The get function can be used to enter a single character
  char ch;
  cin.get(ch);

  Warning... mixing getline and get in your program requires knowledge of the input format and MUCH testing

  See program 3-22 in your text and the discussion that follows it
• the cin.ignore function is a much clearer way to solve the problem
  cin.ignore (n, c) tells cin to SKIP n characters or until the character c is encountered
  cin.ignore(5,'\n'); //skips 5 characters or until a newline occurs (which ever comes first)
  cin.ignore(); // skips the next character
• There are numerous string functions in the string library
• if name is a string and Sz is an integer,
  size=name.length();
  Places the length of the string in size
• Try this example of getline, length and concatenation

• By including the math library, you have access to numerous mathematical function
• include <cmath>
• there is a list of some of the more often used function in Table 3-13
• sqrt is the square root function
  double x=sqrt (25.0); // x is assigned 5.0
• pow is the function to raise a number to a power
  double x=pow(3.0, 3.0); // x is assigned 81
• One of the most useful functions for programmers is the random function (the function is named rand)
  • You need to first seed (the seed function is named srand) the random number generator, otherwise your program will always generate the same numbers
  • A good seed is the current clock time of the computer which is accessed with the time(0) function
  • To use the time function, you need to include the ctime library
  • To put the number into the range you want, you will need to use modulus
  • So, the include directives are:
    include <iostream>
    include <cstdlib>
  • The seeding code comes next:
    unsigned mySeed=time(0);
  • Next, request a random number to be generated into x
    int x=rand();
  • Now, put x into the correct range (e.g., Min of 1 and Max of 10)
    cout << x % 11 + 1;
• Try this code using random numbers