basic computer science and programming

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Basic Computer Science and Programming

CS 102

Asst. Prof. Dr. Mehmet İsmet Can Dede

Program Planning and DesignAlgorithms

The goal of the problem-solving as explained before is to develop an algorithm, or set of steps to solve the problem.

We use algorithms every day. Whenever we are doing a task, such as assembling a bookcase, the algorithm tells us each step in the proper order.

Write an algorithm to make butter-cheese sandwich. 1- Put the sliced bread, butter, sliced cheese, knife and plate onto the

workspace. 2- Place two slices of bread on the plate. 3- Using the knife, spread butter on one slice of bread. 4- If you want cheese, place the cheese slices on the buttered slice of bread. 5- Slap two slices of bread together. 6- Repeat steps 2 through 5 for each sandwich needed. 7- Eat sandwiches.


In the previous algorithm, step 1 is the input, steps 2 through 6 explain the process, and step 7 is the output.

It is clear from the algorithm that we should not do step 5 before step 3, or the sandwich would not be interesting.

Step 4 lets us make a choice whether to include cheese slices in the sandwich.

Clearly, a number of other “if” statements could be included for choosing lettuce, tomatoes, or any other ingredients.

Also, step 6 allows us the flexibility to repeat the same process exactly several times.


To be correct, an algorithm has some specific characteristics. Has input, performs a process, and gives output. Must be clear and unambiguous. Must correctly solve the problem. Can be followed with paper and pencil. Must execute in a finite number of steps.

When writing algorithms for more complex numeric problems, usually all the input is obtained in the first few steps and the output is given in the last few steps.

Many steps can be subdivided. Write the algorithm for a simple ATM machine.

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Program Planning and DesignAlgorithms 1. Get the password from the user. 2. If the password is not valid, construct an error message and skip to

step 6. 3. Get inputs.

3.1. Get the transaction type (deposit-withdrawal) and amount from the user. 3.2. Get the current balance from the bank.

4. If the transaction type is deposit, add the amount to the current balance.

5. If the transaction type is withdrawal, check the current balance. 5.1. If amount is greater than the current balance, construct an error message

and skip to step 6. 5.2. If amount is equal to or less than then current balance, subtarct the amount

from the current balance.

6. Output the error message or the cash, and the current balance. 7. Ask the user whether to repeat steps 3 through 6 for another

transaction.

Program Planning and DesignPseudocode and Flowcharting

The algorithms in the previous section have been written in a structured English method called pseudocode.

Notice that all the lines of the pseudocode contain a specific verb.

In pseudocode, the steps of the algorithm are numbered in such a way that one action is executed per line.

If a line requires two steps, it is subdivided in outline form to the next level.

Pseudocode is usually a good way to begin to design a solution to the problem, it lists the steps of the algorithm.

However, it is not always easy to begin coding the program from pseudocode, because the program flow is not clear.


The conditional statements and the statements to skip to another step do not provide enough of a picture of what should happen when.

In order to express the flow of processing in a much more lucid fashion, many programmers use a flowchart, which is a structured picture map showing the steps of the algorithm.

The shapes usually used for each part of the flowchart are described in the following slide.

The most frequently used ones are the rectangle, parallelogram, diamond, circle and arrow shapes.


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Get bread, butter, cheese slices, knife and plate

Put 2 slices of bread on the plate

Spread butter on one slice

Do you want cheese?

Put cheese on the bread

Slap two slices together

Eat the sandwich(es)

Do you want another?

no

yes

yes

no


In the flowchart, it is easy to identify the input and the output actions because of the shapes used.

The results of the decision on whether to use cheese or not is clear, either cheese is added or it is not.

Also, the arrow shows the flow of control back up to the top to repeat the sandwich-making process.

Program Planning and DesignBasic Control Structures

Computer programs only do what the programmer has told them to do, and in that specific order.

The basic control structures that are used are sequence, selection, and repetition.

In the previous flowchart, most steps are executed in sequence.

Step 4 allows a selection and step 6 provides for repetition.

A simple flowchart for 2 ATM machines could be constructed with each step, input, process and output is done in order, with no decisions or repetitions.


Get password

Get deposit amount

Add amount to current balance

Print the receipt

Get password

Get cash amount

Subtract amount from current balance

Dispense the cash

It is clear from flowcharts that some decisions are necessary. Usually we want one machine to handle both withdrawal and deposit

actions. Also, what if the password is not good? What if these is not enough

money in the account for withdrawal? The flowchart needs to be constructed carefully one section at a time.

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yes

yes

no

Get password

Get type and amount

Subtract from current balance

Is the password

good?

Deposit or withdrawal?

Is amount < balance?

Add amount to current balance

Give response or error message

withdrawaldeposit

no

Program Planning and DesignTop-down Design and Data Abstraction

The algorithms developed in the previous slides are fairly simple.

Most often problems cannot be solved in such a simple way. To attack complex problems, top-down design is used. In top-down design, the overall problem is broken into the

main tasks (usually input, process, output), and then each of these tasks is broken down into the subtasks until a simple solution can be seen for each subtask.

Usually hierarchy charts are used as a help in top-down design.

Hierarchy charts differ from flowcharts in that they indicate what should be done, not how the job will be accomplished.


Instead of indicating the flow of data, they illustrate the tasks, or modules, for each part of the solution.

The only flow of data important is what should go into and come out of each module.

Look back at the ATM program example, the overall task, or the module at the highest level, is to simulate an ATM machine.

The main subtasks would be to get the inputs, to perform the calculations and to give the outputs.

Get inputs Perform all calculations Give outputs

Simulate an ATM machine

First level ATM hierarchy chart


Each box represents a module of solution. Notice, like pseudocode and flowcharts, each task begins

with a verb to show the action that must be performed by that module.

Arrows coming from a box indicate data coming out from that module, and arrows going into the box denote data needed by that module.

Now that the overall task and the main subtasks are identified, each module can be subdivided even further into all the tasks that must be performed in that section.

The next level for the inputs and outputs is fairly straightforward.

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Get password Get transaction type

Get transaction amount

Get inputs

Second-level ATM hierarchy chart for ATM input

Print error message

Print balance Dispense charge

Give outputs

Second-level ATM hierarchy chart for ATM output


The calculation module is a little more complex; there are two main subtasks, one to handle deposits and one to handle withdrawals.

Each of these subtasks can be further broken down into other simpler tasks.

Handle deposits Handle withdrawals

Perform calculations

Second- and third-level ATM hierarchy chart for ATM calculations

Check balance Subtract amount from balance

Give error message

Add amount to balance


The “Handle withdrawals” module must check the balance to see if there is enough money in the account before adjusting the balance.

If there is not enough money, an error message is generated. An error message is needed in the module that gets password.



Second- and third-level ATM hierarchy chart for ATM calculations


Give error message





Complete hierarchy chart for ATM machine


Give error message


Get password Get transaction type

Get transaction amount

Get inputs Give outputs

Print error message Print balance Dispense

charge

Simulate an ATM machine

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