PROGRAMMING CONSTRUCTS

* Sequential – all statements are executed in the order they arewritten.

* Selection – a statement/set of instructions that is/are executed if a condition is satisfied

* Repetition/Iteration – a statement/set of instructions thatis/are executed until a condition is satisfied a specific number of times.

SELECTION  STRUCTURES

Selection (decision)- This construct is used when no action needs to be performed if the expression returns false.

The IF…THEN   selection statement

Syntax:

 IF (expression) THEN

                        {Statement (s)}            Execute statement(s) if logical expression is TRUE

Eg.1) Write an algorithm to read the score of a student in an examination and determine whether the student has passed. If the score is greater than or equal to 50, the student has passed. Print whether the student has passed.

Answer

  Start                                                                                                     

     Print (‘Please enter the score’)

     Input Score

     IF Score >= 50 THEN

              Print ‘PASS’

Stop

The IF … THEN … ELSE construct syntax:

IF (expression) THEN

                        {Statements}    executed only if condition is TRUE

            ELSE                                                                                                                                     

                        {Statements}    executed only if condition is FALSE

ENDIF

Only one group of statements could be executed each time the program is executed

Eg.2) Using the scenario in Example1, determine whether the student has passed or failed and print the result.

Answer 

Start

     Print (‘Please enter the score’)

     Input Score

     IF Score >= 50 THEN

              Print ‘PASS’

     ELSE

              Print ‘FAIL’

Stop

Eg.3) Write an algorithm which accepts a number and determines whether the number is ‘odd’ or ‘even’.  Print the result.

Answer

Start

     Print(‘Please enter a number’)

     Input num

     IF (num MOD 2 = 0) THEN

               Print(num,’  is even’)

     ELSE

               Print(num,’ is odd’)

Stop

Eg.4) Write an algorithm which reads the age of a person and determine whether he/she is a student. The person is considered a student if he/she is between the ages of 5 and 18. If the person is under the age of 5 then he/she is a ‘toddler’. Otherwise, the person is an adult.  Print the age of the person.

Answer

Start

     Print(‘Please enter the person’s age’)

     Input age

     IF (age <= 18) AND (age >=5) THEN

                        Print(‘You are a student’)

     ELSE  IF (age < 5) THEN

                        Print(‘You are a toddler’)

     ELSE

                        Print(‘You are an adult’)

Stop