Selection and Repetition Control Structures

PRG 210
March 30, 2011
John Rogers

Selection and Repetition Control Structures
Introduction
This paper reviews and explains selection control structure and repetition control structure. Selection and repetition control structures increase the ability and power of a computer programs. Selection control structure allows statement execution if a condition is true and another statement execution if a condition is false. Repetition control structure or looping is the process of repeating execution of one or more steps of an algorithm or computer program. Essentially in programming, many performed calculations have repetitious tasks. Examples of both a selection control structure statement and a repetition control structure statement identifies and helps understand the importance of available computer operations to the programmer to compare variables, select one of two actions and repeat tasks as necessary.
Selection Control Structure
Statement executions in programming are under the control of one statement at a time. When statements execute, control is turned over to the next statement similar to running a relay race; one runner passes the baton to the next runner until the result of the baton across the finish line. When one statement finishes executing, control passes to the next statement until the desired output calculates. Computer programs use selection control structures to choose between alternative actions. An assumption is made based on whether it is true or false. If the statement is true it executes on one statement and if it is false it executes another statement. Computers can solve practical problems, make decisions and execute different sequences and instructions in such a way that it produces results faster and more accurate than human calculations. IF statements based on comparing two items usually expressed with one of the following relational operators.
|< |Less than | |Greater than |>= |Greater than or equal to | |
|= |Equal to | |Not equal to | |

Simple selections make a choice between two alternative paths, depending on the result of a condition being true or false. The structure represented in pseudo code using the keywords IF, THEN, ELSE, and ENDIF. For example, a teacher needs to provide to students whether or not he or she passed his or her class. A simple selection structure IF statement calculates the result (output).

The IF statement in pseudo code would look like this:

IF student _grade >= 60 THEN passed ELSE Failed ENDIF

Only one of the THEN or ELSE paths follows, depending on the result of the condition in the IF clause.
When a performed task and the particular condition is true, and no processing takes place with a false condition this is a null ELSE structure of the IF statement or a simple selection with a null false branch. Combined selection structure of the IF statement uses “AND” and “OR” connectors, which represents two conditions that have to be met before determining true or false.
In an IF statement and the statement contains the word IF more than once it is a nested selection. Nested selection IF statements can be linear or non-linear statements. A linear nested IF statement occurs when a field tests for various values and a different action is to be taken for each value. A linear statement contains ELSE immediately following the IF condition to which it corresponds. Non-linear nested IF statements occur when a number of different conditions are met before a particular action can occur. In a defined non-linear statement, the ELSE statement is separated from the IF statement with which it is paired. Repetition Control Structures
Many programs require the same repeated logic for several sets of data. A good way to accomplish this is to write code that can repeat the same set of logic over and over. Repetition control structure is known as the looping or iteration control structure. Executing one or more steps of an algorithm or program and repeating it is the process of looping. Repetition control structure is necessary in programming because most programs perform tasks that repeat several times. Every loop contains three parts; 1) the loop termination decision, 2) the body of the loop, and 3) Transfer back to the beginning of the loop. The loop termination decision determines when or under what condition the loop terminates or stops. The loop terminates otherwise the condition would allow the computer to continue the task over and over without stopping. The body of the loop is the step or steps of the algorithm that repeats within the loop, essentially all the tasks that need repeated within the pseudo code statement. Transfer back to the beginning of the loop returns control to the beginning of the loop so that a new repetition (iteration) can start.
Instructions repeat in three ways, each way determines where to place the repeated decision: beginning of the loop (leading decision loop), end of the loop (trailing decision loop), and (counted loop) counted number of times. In a leading decision loop the DOWHILE construct tests the condition before statement execution. In the following example of pseudo code the following flowchart identifies a DOWHILE loop,

Print_course_scores Read name, course_score DOWHILE course_score not >=61 Print name, course_score Read name, course_score ENDDO

Two considerations are taken when performing a lead decision loops. First perform some initial processing to set up the condition before testing and make sure a condition to render a false result is achievable in which ends the loop. A trailing decision loop uses the REPEAT and UNTIL structure. This type of loop tests the condition at the end of the loop. Trailing decision executed when the condition is false; it is only when the condition becomes true that the loop stops. Counted repetition happens when the exact number of loop iterations is known before the loop starts. The loop execution controlled by a loop index and the usages of a simple keyword DO. The DO loop can, 1) start the loop index to the required start value, 2) increase the loop index by one for each time the loop is run, 3) test the value of the loop index when the loop starts to make sure it stays within the range of values, and 4) terminate the loop once the loop index reaches the specified final value. Basically, a counted repetition automatically performs the initializing, incrementing, and testing of the loop. It also terminates the loop once the required amount of loop times is reached.
Conclusion
The paper reviewed and explained selection control structure and repetition control structure. Selection and repetition control structures increase the ability and power of a computer programs. Selection control structure allows a statement(s) to execute if a condition is true and another statement(s) executes if a condition is false. Repetition control structure or looping is the process of repeating execution of one or more steps of an algorithm or computer program. Essentially in programming, many performed calculations have repetitious tasks. Examples of both a selection control structure statement and a repetition control structure statement identifies and helps understand the importance of available computer operations to the programmer to compare variables, select one of two actions and repeat tasks as necessary. In programming the use of both selection and repetition control structure can reduce time for comparative statements and repeated conditions of tasks and provide computers with faster abilities to analyze and process data, which increases the usefulness and power of a computer program.

References
Conditions, Logical Expressions, and Selection Control Structures, Chapter 6 http://computerscience.jbpub.com/vbNet/pdfs/McMillan06.pdf
Robertson, Lesley Ann, Simple Program Design a step-by-step approach; http://instructor.mstc.edu/instructor/hkronhol/CourseInformation/152- 101ProLogicBeg/Unit5Decision/Robertson5ePP_Ch04_files/frame.htm www.thevbprogrammer.com; http://www.thevbprogrammer.com/Ch05/05-04- RepetitionStructure.htm