# Control Structures or Logical Structures

The key to better algorithm design and thus to programming lies in limiting the control structure to only three constructs. These are illustrated below:

The sequence structure

The first type of control structures is called the sequence structure. This structure is the most elementary structure. The sequence structure is a case where the steps in an algorithm are constructed in such a way that, no condition step is required. The sequence structure is the logical equivalent of a straight line.

For example, suppose you are required to design an algorithm for finding the average of six numbers, and the sum of the numbers is given. The pseudocode will be as follows.

Start
Get the sum
Average = sum / 6
Output the average
Stop

The corresponding flowchart will appear as follows.

Example 1: This is the pseudo-code required to input three numbers from the keyboard and output the result.

Use variables: sum, number1, number2, number3 of type integer
Accept number1, number2, number3
Sum = number1 + number2 + number3
Print sum
End program

Example 2: The following pseudo-code describes an algorithm which will accept two numbers from the keyboard and calculate the sum and product displaying the answer on the monitor screen.

Use variables sum, product, number1, number2 of type real
display “Input two numbers”
accept number1, number2
sum = number1 + number2
print “The sum is “, sum
product = number1 * number2
print “The Product is “, product
end program