• In all our programs there has been one fixed sequence of events which can occur even though different values may be input, calculated, or output
• Most programs need the ability to execute different sequences of instructions in different conditions
• Example: if the user types in an incorrect value then print an error message, otherwise proceed with the calculations
• Example: in determining all the factors of a number, repeatedly find one factor at a time until all factors have been found
• Example: in our "change making" problem from the practice quiz, many people wanted the algorithm to include statements like "if the amount is at least .... then do ...."

• Control structures determine which code segments are executed, and Boolean expressions are used to describe and evaluate conditions during execution of the program
• Control structures are things like if and repeat, whereas Boolean expressions are statements (or propositions) which evaluate to true or false based on current conditions, such as
  • the user typed in an incorrect value
  • all the factors have been found
  • the input value entered was greater than 0 and less than 256

• Boolean expressions are simply statements which must evaluate to true or false given an existing set of facts
• They are named for George Boole, who developed an algebraic system for dealing with two-valued systems
• Boolean expressions can involve comparisons,
  e.g. (X < Y) is a Boolean expression which evaluates to true if X is less than Y, and evaluates to false otherwise
• Two or more Boolean expressions can also be combined together to create a single, more complex Boolean expression.

  This is done using a variety of logical operators (AND, OR, NOT,...). For example

  • the symbol && means logical AND.
  • given simple Boolean expressions such as X < Y and Y < Z we can create a combined expression using the && operator: (X < Y) && (Y < Z)
  • this evaluates to true if Y is between X and Z, and evaluates to false otherwise
• Almost all our control structures are based on the the evaluation of Boolean expressions, for example

    if (Boolean expression evaluates to true) 
    {
       // do something
    } else 
    {
       // do something else
    }
  

  Suppose we wanted to see if the user had entered the character C to continue, or Q to quit. We could use something like this:

  // prompt the user to choose between quitting/continuing,
  //    read and store their response
  printf( "Enter C to continue or Q to quit:\n");
  char userChoice;
  scanf("%c", &userChoice);
  
  // act on the user's choice
  if (userChoice == 'Q') {
     // here, insert whatever code is needed
     //   in order to shutdown/quit
  } else if (userChoice == 'C') {
     // here, insert whatever code is needed
     //   in order to continue
  } else {
     // here, insert code to handle the fact
     //   that the user made an invalid choice
  }
  

• A Boolean value is one which evaluates to true or false
• The bool data type is used for Booleans in C++
• By convention, and for backwards compatibility with the C programming language, the integer value 0 corresponds to a logical value of false, and the integer value 1 is corresponds to the logical value of true
• In the program we can declare variables to be of type bool and make use of the identifiers true and false

  #include <cstdio>
  
  int main()
  {
     bool bVar1, bVar2;
     bVar1 = true;
     bVar2 = false;
     ...
  }
  

• A relational operator compares two numeric values and returns true or false based on the comparison
• E.g. X < Y would return true if X is less than Y, and would return false if X is greater than or equal to Y
• There are six relational operators in C++:
  • Equal to: (X == Y)
  • Not equal to: (X != Y)
  • Less than: (X < Y)
  • Greater than: (X > Y)
  • Less than or equal to: (X <= Y)
  • Greater than or equal to: (X >= Y)
• COMMON ERROR WARNING: often when programmers want to check if X is equal to Y they type (X = Y) instead of the correct (X == Y)
• In terms of operator precedence, relational operators are evaluated after all the other (arithmetic) operators we've examined

• Since each Boolean expression evaluates to true or false, most control structures basically say: evaluate the expression, if it evaluates to true do one thing, if it evaluates to false do something else
• This is most clear with the if statement

     // normal instructions
     if (<Boolean expression>) 
     {
        // set of statements to execute
        // if expression evaluates to true
     } 
     else 
     {
        // set of statements to execute
        // if expression evaluates to false
     }
     // more normal instructions
  

• Example: checking user input:

     printf( "Please enter a positive number\n");
     scanf("%f", &iInputValue);
     if (iInputValue <= 0) 
     {
        printf( "Sorry, but %f", iInputValue);
        printf( " is not a positive number");
     } 
     else 
     {
        printf( "The square root of ";
        printf("%f is %f", iInputValue, sqrt(iInputValue));
     }
     printf("\n");
  

• So far we have seen the six relational operators but often we want to form more complex expressions
• Example: perform a calculation if X is between 3 and 17: i.e. if X is greater than 3 and X is less than 17
• Example: we want to generate an error message if Y is less than zero or Y is greater than 23
• Example: we want to do some set of actions when a particular Boolean expression is not true

• The three logical operators are AND, OR, NOT, and the symbols used for them are &&, ||, !, respectively
• See the table in your text for the precedence of these operators
• <expression> && <expression> evaluates to true if and only if both expressions evaluate to true

  For instance, the expression (iData < 3) && (iData > 1) is true if and only if iData = 2

• <expression> || <expression> evaluates to true if either or both expressions evaluate to true

  For instance, the expression (iData < 1) || (iData > 3) is true as long as iData is not one in the range 1..3

• !<expression> evaluates to true if and only if the expression evaluates to false

  For instance, the expression !(iData == 1) is true if and only if the expression iData == 1 is false

• Functions (such as built-in library routines) can also return Boolean values, some examples from the <cctype> library are:
  • islower(cData) returns true if cData is a lower case letter, and returns false otherwise
  • isspace(cData) returns true if cData is a white space (tab, endl, blank etc) and returns false otherwise
• We can create functions to return Boolean values, e.g.

  #include <cstdio>
  
  bool IsIntegerEven(int iData);
  
  int main()
  {
     int iUserInput;
     printf( "Please enter an integer\n");
     scanf("%d", &iUserInput);
     if (IsXEven(userint)) 
     {
        printf( "%d is even", iUserInput);
     } 
     else 
     {
        printf( "%d is odd", iUserInput);
     }
     printf("\n");
  }
  
  bool IsIntegerEven(int iData)
  {
     if ((iData % 2) == 0) 
     {
        return true;
     } 
     else 
     {
        return false;
     }
  }
  

• The && and || (and, or) logical operators have one other special feature: they return a value as soon as they know what an expression must evaluate to
• Example: suppose the value of count is currently 0, and we have the following expression

     if ((iCount > 0) && (iCount < 100)) 
     {
        iCount = iCount + 1;
     } 
     else 
     {
        printf( "count is %d", iCount);
     }
  

  First we look at count > 0, and we realise this evaluates to false, so the whole expression must evaluate to false regardless of the results of (count < 100)
• This is the notion of short-circuiting: figuring out the value for an entire expression without having to look at the entire expression

int iDataX = 13;
int iDataY = 7;
int iDataZ = -3;
char cData = 'd';

• (iDataX <= iDataY) && (iDataY > iDataZ))
• (isdigit('9') || (iDataY == -3))
• (iDataX > iDataY) && (iDataY > iDataZ) && (iDataX != iDataZ)
• (iDataX % iDataY) > iDataZ
• (cData != 'e') && (int(cData) > iDataY)
• pow(iDataY * iDataZ, 2) < pow(iDataX, 2)
• (iDataX < iDataY) || (iDataY < iDataZ) || (isspace(cData) || true

#include <cstdio>

int main()
{
   int iUserInput;

   printf( "Please enter an integer\n");
   scanf("%d", &iUserInput);

   if (iUserInput < 0) 
   {
       printf( "You entered a negative number\n");
   }
   
   if (iUserInput > 0)
   {
       printf( "You entered a positive number\n");
   }

   if (iUserInput == 0)
   {
       printf( "You entered 0\n");
   }
}

Just in case it comes in handy, here's a similar program that will keep prompting a user until they enter a positive number

#include <cstdio>

int Get_positive();

int main()
{
   int iEnteredNumber;
   iEnteredNumber = Get_positive();
   printf( "The positive value you entered");
   printf( " was %d\n", iEnteredNumber);
}

int Get_positive()
{
   int iEnteredNumber;  // user's entry value

   printf( "Please enter a positive integer\n");
   scanf("%d", &iEnteredNumber);

   if (iEnteredNumber >= 0) {
      printf( "Sorry, that was an invalid value\n");
      iEnteredNumber = Get_positive();
      return iEnteredNumber ;
   } else {
      return iEnteredNumber;
   }
}

• The basic syntax for a complete if-else statement is

    if (<Boolean expression>) 
    {
       // statement set A
    } 
    else 
    {
       // statement set B
    }
  

• If the Boolean expression evaluates to true then the set of statements A is executed
• If the Boolean expression evaluates to false then the set of statements B is executed
• Example:

     scanf("%d", &iData);
     if ((iData > 0) && (iData < MAXINT)) {
        printf( "sqrt(%d) = ", iData);
        printf("%f\n", sqrt(iData));
     } else {
        printf( "Sorry, cannot take sqrt(");
        printf( "%d)\n", iData);
     }
  

• In fact, the "else" portion of the if-else is not strictly necessary
• The statement below is perfectly valid

  scanf("%c", &cData);
  if ((cData == 'd') || (cData == 'D')) 
  {
     printf( "Did you know that such great names as Dave");
     printf( " begin with %c\n", cData);
  }
  

• Note, however, that you cannot have an else without an if

• The standard notation is useful when there are a series of statements you wish executed in the "if part" or the "else part"
• However, sometimes you only have a single statement to execute, in which case the following syntax is also valid:

     if (iData < iMaxAllowable) 
        iData  = iData + 1;
     else 
        iData = iMaxAllowable;
  

• COMMON ERROR: often people put a semi-colon right after the Boolean expression, e.g.

    if (iData < iMaxAllowable); 
       iData = iData + 1;
  

  You can get very strange behaviour from this kind of error, as it will get translated the same as:

    if (iData < iMaxAllowable) {
    } 
    iData = iData + 1;
  

• Often you have a whole series of different cases to handle, e.g. do something if X is 1, something else if X is 2, something else if X is 3, etc
• The most effective construct for handling this is:

     if (iData == 1) 
     {
  
        // statement set 1
  
     } 
     else if (iData == 2) 
     {
  
        // statement set 2
  
     } 
     else if (iData == 3) 
     {
  
        // statement set 3
  
     } 
     else if (iData == 4) 
     {
  
        // statement set 4
  
     } 
     else 
     {
  
        // default statements
  
     }
  

• The "default" statements cover all the cases not specifically mentioned

• In other cases, after dividing the flow of control with the first if statements, you might want to subdivided it further with nested if statements

    if (iDataX < 0) 
    {
       // do this stuff if iDataX is negative
       if (iDataY < 0) 
       {
          // iDataX and iDataY are both negative
          total = iDataX * iDataY;
       } 
       else
       { 
          // iDataX is negative, iDataY is positive
          total = - (iDataX * iDataY);
       }
  
    } 
    else 
    {
       // do this stuff if iDataX is positive
       if (iDataY < 0) 
       {
          // iDataX is positive, iDataY is negative
          total = - (iDataX * iDataY);
       } 
       else 
       {
          // iDataX and iDataY are both positive
          total = iDataX * iDataY;
       }
    }
  

• note the use of indentation to try and clarify which statements are part of which if/else statements

• Example: preventing for divide-by-zero

       // check that we won't perform divide-by-zero
       if (iDenominator == 0) 
       {
  
         printf( "Cannot divide %d", iNumerator);
         printf( " by %d\n", iDenominator);
  
       } 
       else 
          result = iNumerator / iDenominator;
  

• Example: checking user input

  int get_1_to_10()
  {
      int iUserValue;
      printf( "Please enter a value from 1 to 10\n");
      scanf("%d", &iUserValue);
  
      if ((iUserValue < 1) || (iUserValue > 10)) 
      {
         printf( "%d is invalid", iUserValue);
         iUserValue = get_1_to_10();
      }
   
      return iUserValue ;
  }
  

void get_command()
{
  bool bQuit = false;
  char cCmd;

  // prompt user and get command
  printf( "Enter S to calculate square roots\n");
  printf( "      X to translate hexadecimal values\n");
  printf( "   or Q to quit\n");
  scanf("%c", &cCmd);

  // handle the current command
  if (cCmd == 'S') {
     square_root_routine();
  } else if (cCmd == 'X') {
     translate_routine();
  } else if (cCmd == 'Q') {
     bQuit = true;
  } else {
     printf( "Invalid command\n"); 
  }
  
  // if user hasn't asked to quit,
  // go through whole cycle again
  if (!quit) {
     get_command();
  }
}

• The switch statement can be used to replace a series of if-else statements when you are checking for a series of discrete values
• The syntax is as follows

  switch (<selector>) 
  {
      case  <label 1>:   <statements 1>
                         break;
      case  <label 2>:   <statements 2>
                         break;
      ...
      case  <label n>:   <statements n>
                         break;
      default: <default statements>
  }
  

• Example:

  switch (iData % 3) 
  {
      case 0:  printf("%d is divisible by 3", iData);
               break;
      case 1:  printf("%d-1 is divisible by 3", iData);
               break;
      case 2:  printf("%d-2 is divisible by 3", iData);
               break;
      default: printf( "ERROR: it should not be possible)";
               printf( " for (%d %% 3) to be anything but ", iData);
               printf( " 0, 1, or 2");
  }
  

• The selector statement can be any variable, function call, or expression that evaluates to a short, int, long, or char
• The selector is compared one by one to the label values until a match is found,
• then the corresponding block of statements is executed
• Example:

  // course numbers from JCU's old CS program
  switch (iCourseNumber)
  {
     case 1010: printf( "Intro to Multimedia");
                break;
     case 1030: printf( "Intro to Information Technology");
                break;
     case 1200: printf( "Intro to Computer Science I");
                break;
     case 1300: printf( "Intro to Computer Science II");
                break;
     case 1500: printf( "Information Systems");
                break;
     default: printf( "Invalid subject number");
  }
  

• Switch statements only work for integers and characters
• If the select statement doesn't match any of the labels, and no default is given, then the program will crash Always give a default
• When a match has been found, the corresponding set of statements is executed, however it will continue executing statements until a break is hit
• E.g. suppose we forget our break statements and x == 2

  switch (iData) {
     case 1: printf( "the value entered is one\n");
     case 2: printf( "the value entered is two\n");
     case 3: printf( "the value entered is three\n");
     default: printf( "the value entered is not in 1..3");
  }
  

  The output will be:

  the value entered is two
  the value entered is three
  the value entered is not in 1..3
  

bool bQuit = false;
char cCmd;

printf( "Please enter your next command\n");     
printf( "    A to add new entry\n");     
printf( "    L to look up an entry\n");     
printf( "    Q to quit\n");     
scanf("%c", &cCmd);

switch (cCmd)
{
   case 'a':
   case 'A':  add_entry();
              break;
   case 'l':
   case 'L':  lookup_entry();
              break;
   case 'q':
   case 'Q':  bQuit = true;
              break;
   default:   printf( "Invalid command entered");
}