C4AP1

Chapter 4 Assignment Page 1

Counter Controlled Loop

NOTE: The problems in the previous chapters perform the processing steps for only one input record. If you need to perform the processing steps multiple times for more than one input record, then you need to use a loop. A loop performs the steps multiple times. A computer will perform the loop steps until it is told to stop. There are four methods that can be used to tell the computer when to stop performing the loop steps. Two of the methods are covered in Chapter 4: Counter Controlled loop and Header Record Logic. The loops in Chapter 4 can only be used if the number of times the loop will be performed is known. Two of the methods are covered in Chapter 5: Trailer Record logic and Automatic End of File. The loops in Chapter 5 are used when the data is large or volatile. It is used when there are many input records and the number of input records are constantly changing. For example when dealing with student files, the number of enrolled students is constantly increasing and decreasing.

1. Characteristics of a loop

Purpose of a loop – a program loop is defined as a sequence of processing steps that may be repeated. The computer may re-execute the instructions. Once the computer has performed the processing steps once, the computer must decide if it needs to perform the processing steps again. A decision needs to be made. The computer can either check before the loop steps are performed or check after the loop steps are performed. A DOWHILE loop makes a decision before the steps are performed. A DOUNTIL loop makes a decision after the loop steps are performed. NOTE: Solve the problems using DOWHILE loops. The majority of the languages used today will use the DOWHILE loop as the main processing loop. This type of loop does not assume you want to process at least one input record.

Characteristics of a DOWHILE loop – the DOWHILE pattern is a leading-decision program loop. The test to determine whether the loop should be executed or exited is encountered immediately upon entering the loop. In other words, the test immediately follows the connector symbol. Whether the additional steps inside the loop are executed depends on the outcome of the test. Notice the significance of the fact that, if the tested condition is not true, the loop is exited immediately thereafter. Thus if the tested condition is not true the first time it is tested, the remaining steps in the loop are not executed at all. The DOWHILE loop is entered on the true path of a decision and exited on the false path of a decision.

To create a properly formed DOWHILE loop you must:

Place the loop test before any other step within the loop (leading decision).
Place the loop steps in the "yes" path of the loop test.
Indicate that the loop will exit in the "no" path of the loop test.

Pseudocode Example:

NOTE: The keywords for the DOWHILE loop in pseudocode are DOWHILE, which marks the beginning of the loop, and ENDDO, which marks the end of the loop. All statements between the words DOWHILE and ENDDO are on the true path of the loop and are performed when there are more records to process. All statements on the true path are indented. All statements on the false path of the loop are placed after the keyword ENDDO. These statements are processed when there are no more records to process and the loop is exited. The statements on the false path of the loop are not indented. The students will work on problems that have statements placed on the false path of the loop in Chapter 5 when totals need to be output.

Flowchart Example:

Characteristics of a DOUNTIL loop – the DOUNTIL pattern is a trailing decision program loop. The test to determine whether the loop should be executed or exited is encountered after processing the loop steps are performed. This means that no matter what the outcome of the test within the loop, the processing steps that precede the test will always be performed at least once. So before using DOUNTIL loops we must be sure that we want to perform the functions within a trailing-decision loop at least once. The execution of the processing steps in the loop and subsequent condition testing continue until the tested condition is known to be true. Then the loop is exited. The DOUNTIL loop is entered on the false path of a decision and exited on the true path of a decision.

To create a properly formed DOUNTIL loop you must:

Place the loop test after all steps within the loop (trailing decision).
Place the loop steps in the "no" path of the loop test.
Indicate that the loop will exit in the "yes" path of the loop test.

Pseudocode Example:

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Flowchart example:

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Week 12 Course Outline

2. Counter controlled loop

Accumulator and counter

NOTE: Accumulators and counters are created using variables. The variables need to be initialized before they can be used. If they are used to hold a total, they are usually initialized to 0. This gives the variable location a starting value, otherwise you are adding to a preexisting value and will not receive the correct result. Some programming languages initialize variables to 0 automatically, but as a rule you should always initialize the variables.

Accumulator – a variable that is used to hold the sum of a group of values. This sum is computed by gradually adding each value to the variable each time the loop is executed.

Counter - a special type of accumulator. A counter adds or accumulates by a constant amount, usually 1. The value of such a counter increases by 1 each time the loop if executed.

Purpose of the preparation symbol –

The preparation symbol is used to initialize a variable and place a limit on a loop-control variable.

Preparation Symbol -

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Problems using counter controlled loops–

A counter controlled loop uses a counter to tell the computer system when it is time to exit the loop. With a counter controlled loop, a counter is created using a variable. The counter is either initialized to 0 or to the total of times to perform the loop. If the counter is initialized to 0, each time through the loop, the counter is incremented by 1and compared to the total of times the loop should be performed. Once the counter value equals the total number of time the loop should be performed, the loop is exited. If the counter is initialized to the total number of time to perform the loop, each time through the loop, the counter is decremented by 1 and compared to 0. Once the counter value equals 0, the loop is exited. This type of loop is used when the number of input records or times to perform the loop in known and will not change. If the number of input records changes the programming statements must be changed. NOTE: The problem below, walks through an example of a counter controlled loop that will simply read a record and print the output. This problem will allow you to focus on the loop and not processing the problem. Walk through the steps the computer will perform using a memory diagram.

Input File

Kendall Boulton
Charleen Thompson
Carol Knox
Michael Worthington
Warren Smith

Pseudocode Example:

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Flowchart Example:

Exercises:

Problem 9 – This problem walks through a flowchart that is already designed. Create a memory diagram and walk through the problem to show the correct processing steps. The problem then asks you what would happen if something were changed in the design. The are many solutions to this problem depending upon where the new statements are placed. For example one of the steps asks what would happen if something were placed inside the loop. There are many locations that are considered inside the loop and each location would produce a different result. You need to indicate to me where you placed the statement.

Problems 10, 11, 12 – These problems may be difficult to solve because they do not use input from a file.

Problem 19 - This is the best problem to start with because it is similar to the problems from Chapter 3. It asks you to solve the problem using modules.

Modules

Concept of modules

Module – takes a problem and breaks it into smaller manageable pieces. Each of the pieces is called a module. This allows you to concentrate on the overall processing the program needs to perform, instead of the being overwhelmed by the details early on. To solve a program using modules you will initially decide on the pieces of the program and solve of the pieces of the program one at a time.

Benefits of modules in design –

Complex program can be organized into smaller more manageable pieces.

Programs can be modified easier than with non modular programs. It is relatively easy to maintain, or change, the program because modules can be changed or corrected individually without requiring revisions to other portions of the program.

A collection of library modules can be created that can be used in other programs.

Errors in logic can be quickly isolated and fixed.

Modules can be developed and tested individually. A programmer can focus on one module at a time, without having to consider the big picture.

Rules of modularization – NOTE: The rules are just guidelines for you to follow.

Each module should have a single function.

Each module must be a manageable size; it should have less than 50 program instructions.

Each module has a single entry point and a single exit point.

Structure chart – to show the overall modular structure, a structure chart (hierarchy chart) is used. A structure chart shows the flow of control within a program. It shows the relationship between all modules within a program. A hierarchy chart portrays a module as a rectangular box in which the module name is placed. The structure chart is created from a general level to a more specific lever. It shows the order and structures in which these modules are executed.

Calling and the called module – a main routine (Overall control), or calling module should be created. This routine executes all other modules and contains the entry and exit points for the program. The other modules are called the called modules. They represent modules that may be given control during program execution. When the steps in the called modules are finished, then control is returned back to the calling module. It is possible for a module to be both the calling module and the called module. The modules are named and can also be labeled to indicate level they are on in the structure chart and the order to perform them in.

NOTE: In this example, the Overall control module is the calling module. The module calls the Initialization, Process a Record and Termination modules. The Initialization, Process a Record and Termination modules are the called modules. Once the computer have finished processing the modules, processing returns to the Overall Control module.

Problems using modules

Initially the problem can be broken down into three parts: Initialization, Process a record and Termination. Almost all programs that the students will solve contains a main processing section of the program that separates the main processing of the program from the initialization and termination sections.
Initialization – these steps are performed before records are processed. Some of the steps that can be performed are:

Set accumulator to 0

Set counter to 0

Initialize switches

Write headings

Process a record – these steps perform nearly identical processing for each input record. This module contains all the processing steps that are necessary to process one input record. These steps are performed inside the loop.
Termination – these steps are performed after the records are processed. Some of the steps that can be performed are:

Write total lines

Process overall averages

Once the problem is broken down into these three steps, each of these steps may be broken into smaller modules. For example: in the termination module There is processing to calculate totals and averages and also output totals. These functions can be broken into two smaller modules.

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