C++ Control Structures

After discussing algorithms and pseudocode, let us move beyond simple programs, and learn to build more complex programs, of which control structures are an integral part.

Flow of Program

•Sequential execution

–Statements are normally executed in the order in which they are written

•Transfer of control

–You can specify, for example, Next statement to be executed IS NOT next one in sequence

Control Structures

•          There are 3 control structures;

–        Sequence structure

•          Programs executed sequentially by default

–        Selection structures

•          if, if/else, switch

–        Repetition structures

•          while, do/while, for

•          C++ keywords

–        Cannot be used as identifiers or variable names

There are only two ways of combining control structures;

•          Control Statement Stacking

–        One control structure is followed by another control structure

–        Control Statement Nesting

–        One control structure is contained in another control structure

Sequence Structures

•Simplest of all

•This one is in place by default if no other control structure has been defined to alter the flow of your program

•Key Rule : Execute instructions one by one in the order they appear in the source code

Selection Structures

Key Rule :Choose among alternative courses of actions

•Three selection statements available in C++

–Single Selection Statement :if

•Selects/ignores a single action

–Double Selection Statement :if/else

•Selects 1 out of 2 actions

–Multiple Selection Statement : switch

•Selects some out of many actions

'if' Selection Structure

              •      Pseudocode example:

                    If student’s grade is greater than or equal to 60

                    Print “Passed”

–        If the condition is true

•          Print statement executed, program continues to next statement

–        If the condition is false

•          Print statement ignored, program continues

                                

                             Remember:  Select OR ignore a single action

•          Translation into C++

                     If student’s grade is greater than or equal to 60

                     Print “Passed”

                      if ( grade >= 60 ) 
                    cout << "Passed";

•          Diamond symbol (decision symbol)

–        Indicates decision is to be made

–        Contains an expression that can be true or false

•          Test the condition and follow one of the two paths (True/False)

'if/else' Selection Structure

•          Pseudocode Example:

              if student’s grade is greater than or equal to 60
              print “Passed”

              else

              print “Failed”

             Remember:  Select 1 out of 2 actions

•          C++ code

                if ( grade >= 60 )
               cout << "Passed";
               else
               cout << "Failed";

•          Conditional operator (?:)

–        The only ternary operator in C++

–        Operates on 3 operands

–        Three Operands are (condition, value if true, value if false)

•          Nested if/else structures

–        One inside another, test for multiple cases

–        Once condition met, other statements skipped

               if student’s grade is greater than or equal to 90
               Print “A”

               else
               if student’s grade is greater than or equal to 80
               Print “B”
                else
               if student’s grade is greater than or equal to 70
              Print “C”
              else
              if student’s grade is greater than or equal to 60
              Print “D”
              else

              Print “F”

•          Example              

                if ( grade >= 90 )       // 90 and above
              cout << "A";
              else if ( grade >= 80 )  // 80-89
              cout << "B";
              else if ( grade >= 70 )  // 70-79
              cout << "C"; 
              else if ( grade >= 60 )  // 60-69
              cout << "D";
              else                     // less than 60
              cout << "F";

•          Compound statement/ Block

–        Set of statements within a pair of braces

               if ( grade >= 60 ) {
              cout << "Passed.\n";      
              else {
              cout << "Failed.\n";
              cout << "You must take this course again.\n";
                }

–        Without braces,

              cout << "You must take this course again.\n";

              Will always be executed

Dangling 'else' Problem

Remember : C++ compiler always associates an ‘else’ with the immediately preceding ‘if’