Program Design Tools

A computer does exactly what it is told, in exactly the order it is told, and nothing else. It cannot guess what you meant. So before anyone writes a program, they plan the solution on paper: what goes in, what must be worked out, what comes out, and in which order. This lesson teaches the three planning tools every programmer uses — the algorithm, the flowchart and pseudocode — and the three patterns from which every plan is built: sequence, selection and repetition. Lessons 19 and 20 then turn these plans into real programs in QBASIC, and grade 9 does the same in Python.

18.1 Program and programming; steps of program development

The calculator on your phone, the eSewa and Khalti apps, the game you play and the operating system that runs them are all programs. Each one was planned before it was written, in the same way that a house in Bhaktapur is drawn as a naksa (building plan) and approved before a single brick is laid. Planning first is cheaper: a mistake found on paper costs a pencil line, while a mistake found after the program is in use can cost a bank or a school real money.

The steps of program development

Writing a program is done in a fixed order of steps, called the program development life cycle (PDLC). Different books number them a little differently; the six below cover everything they include.

1. Problem analysis. Understand the problem completely before trying to solve it. Find out what input the program will receive, what output it must give, and what processing turns one into the other. Many programmers write this as an IPO chart (Input–Process–Output).
2. Program design. Plan the solution step by step, using the design tools of this lesson: an algorithm, a flowchart or pseudocode. A good design is independent of any programming language.
3. Coding. Write the design as a program in a programming language such as QBASIC, Python or C. The program as typed by the programmer is called the source code.
4. Testing and debugging. Run the program with test data whose correct answers are already known, and compare. An error in a program is called a bug; finding and removing it is debugging. (Lesson 19 explains the three kinds of error.)
5. Documentation. Write explanations: comments inside the program, and a user guide that tells people how to install and use it. Good documentation lets another programmer — or you, a year later — understand and change the program.
6. Implementation and maintenance. Install the program where it will be used, train the users, and keep it working: fix bugs that appear later, and update it when rules change. A billing program must be changed when the VAT rate changes, and a school result program when the grading system changes.

18.2 Algorithm: meaning, features and writing algorithms

The word comes from the name of Muhammad ibn Musa al-Khwarizmi, a Persian mathematician of the ninth century who worked in Baghdad. When his book on Hindu–Arabic numerals was translated into Latin, his name was written Algoritmi, and step-by-step methods of calculation came to be called after him.

You follow algorithms every day without calling them that. Here is one for making a cup of milk tea:

1. Start.
2. Pour one cup of water and half a cup of milk into a pan.
3. Heat the pan until the mixture boils.
4. Add one spoon of tea leaves and two spoons of sugar.
5. Boil for two more minutes.
6. Strain the tea into a cup.
7. Stop.

Notice that every step is a single, clear action, the order matters (you cannot strain the tea before boiling it), and the list comes to an end.

Features of a good algorithm

Advantages of algorithms. They are written in ordinary language, so they are easy to write and to understand; they are independent of any programming language; and they let a programmer find mistakes in the logic before any code is typed. Disadvantages. A long algorithm for a big problem is tiring to read, and branches and loops are harder to follow in words than in a picture — which is why flowcharts are used beside them.

Writing an algorithm

Number the steps. Begin with Start and end with Stop. Use the same few action words every time: Read (or Input) for input, Calculate or a formula for processing, Display (or Print) for output, If … then … else for decisions, and Go to step n or Repeat for loops. Give every value a short name — a variable such as P, T, R or SUM — and use the same name all the way through.

18.3 Flowchart: symbols, rules and drawing flowcharts

Flowcharts are older than computers. The engineers Frank and Lillian Gilbreth drew "flow process charts" of factory work in 1921, and in 1947 Herman Goldstine and John von Neumann used flow diagrams to plan programs for one of the first electronic computers. The symbols were later fixed by standards (ANSI in 1970, then ISO 5807 in 1985), so a flowchart drawn in Kathmandu means the same thing to a programmer anywhere in the world.

Rules for drawing a flowchart

1. A flowchart has one Start and (normally) one Stop terminal.
2. Use only the standard symbols, each for its own job.
3. The flow goes from top to bottom and from left to right; every flow line has an arrowhead.
4. A process or input/output symbol has one entry and one exit. A decision symbol has one entry and two exits, labelled Yes and No (or True and False).
5. Flow lines should not cross. If a flowchart is too big, use connectors instead of long lines.
6. Write short, clear statements inside the symbols, using the same variable names as the algorithm.
7. Every path through the flowchart must lead to Stop.

18.4 Sequence, selection and repetition in algorithms and flowcharts

In 1966 two Italian computer scientists, Corrado Böhm and Giuseppe Jacopini, showed that every algorithm can be built from only three patterns, called control structures. Learn to recognise them, because every exam algorithm is made of them.

Sequence

p = float(input("Principal (Rs): "))
t = float(input("Time (years): "))
r = float(input("Rate (% per year): "))
si = p * t * r / 100
print("Simple interest = Rs", si)

Principal (Rs): 50000
Time (years): 2
Rate (% per year): 8
Simple interest = Rs 8000.0

Selection

a = int(input("First number: "))
b = int(input("Second number: "))
if a > b:
    print(a, "is greater")
else:
    print(b, "is greater")

First number: 45
Second number: 72
72 is greater

A decision can test any condition that is either true or false. The comparison signs used in algorithms are > (greater than), < (less than), = (equal to), ≥ (greater than or equal to), ≤ (less than or equal to) and ≠ (not equal to). To test whether a number is even, divide it by 2 and look at the remainder: the remainder is 0 for an even number and 1 for an odd one. Most programming languages call the remainder operation MOD, so "N MOD 2 = 0" means "N is even".

A decision can also lead to another decision. This is called a nested decision, and it is how a plan chooses among three or more possibilities.

Repetition

A loop repeats a group of steps. Every loop has the same three parts, and leaving out any one of them is a classic error:

1. Initialisation — give the loop variables their starting values before the loop (I = 1, SUM = 0).
2. Condition — a test, checked every time round, that decides whether to repeat (Is I ≤ N?).
3. Update — a step inside the loop that changes the loop variable, so that the condition eventually becomes false (I = I + 1).

A variable such as I that counts the repetitions is called a counter. A variable such as SUM that collects a running total is called an accumulator.

n = int(input("Enter N: "))
i = 1
total = 0
while i <= n:
    total = total + i
    i = i + 1
print("Sum =", total)

Enter N: 10
Sum = 55

18.5 Pseudocode

Pseudocode sits between an algorithm and a program. An algorithm is written in sentences; a program must obey the exact grammar (syntax) of its language; pseudocode uses a small set of keywords in capital letters and indentation to show which statements belong inside a decision or a loop.

Rules. Write one statement on each line. Put keywords in capitals. Indent the statements inside IF, WHILE and FOR, and close every block with its matching END word. Use meaningful variable names, and keep the pseudocode free of the special punctuation of any one language.

(a)  BEGIN
         INPUT A, B
         IF A > B THEN
             PRINT "A is greater"
         ELSE
             PRINT "B is greater"
         ENDIF
     END

(b)  BEGIN
         INPUT N
         SET I = 1
         SET SUM = 0
         WHILE I <= N
             SUM = SUM + I
             I = I + 1
         ENDWHILE
         PRINT SUM
     END

BEGIN
    INPUT N
    SET F = 1
    FOR I = 1 TO N
        F = F × I
    NEXT I
    PRINT F
END

n = int(input("Enter a number: "))
f = 1
i = 1
while i <= n:
    f = f * i
    i = i + 1
print("Factorial of", n, "is", f)

Enter a number: 5
Factorial of 5 is 120

Practice questions

Group A — Objective (1 mark each)

1. A finite set of step-by-step instructions for solving a problem is called [1]aa flowchartban algorithmca compilerda bug
2. Which flowchart symbol is used for a decision? [1]arectanglebparallelogramcdiamonddoval
3. Which flowchart symbol is used for "Read N" or "Print SUM"? [1]aparallelogrambrectangleccircleddiamond
4. The feature that an algorithm must end after a limited number of steps is [1]adefinitenessbfinitenessceffectivenessdoutput
5. Which is the first step of program development? [1]acodingbtestingcproblem analysisddocumentation
6. A small circle in a flowchart is a [1]aterminalbconnectorcdecisiondprocess
7. Structured English statements such as IF … THEN … ENDIF that cannot be run on a computer are called [1]amachine codebpseudocodecsource codedobject code
8. Finding and removing errors from a program is called [1]acodingbdebuggingcdocumentationdmaintenance
9. "Keep stirring until the sugar dissolves" is an example of [1]asequencebselectioncrepetitionddocumentation
10. An algorithm sets S = 0, then adds I to S for I = 1, 2, 3 and 4, then displays S. The output is [1]a4b6c10d24
11. How many exits does a decision symbol have? [1]aonebtwocthreedfour
12. Which of these can be run directly on a computer? [1]aan algorithmba flowchartcpseudocodeda program

Group B — Short answer (2 marks each)

1. Define program and programming. [2]
2. What is an algorithm? Write any two features of a good algorithm. [2]
3. Draw the symbols for terminal, input/output, process and decision, and write the use of each. [2]
4. Write any two differences between an algorithm and a flowchart. [2]
5. What is pseudocode? Write one advantage of it. [2]
6. Write an algorithm to find the area of a rectangle. [2]
7. Write an algorithm to convert a temperature in degrees Celsius into degrees Fahrenheit. Test it with 37 °C. [2]
8. What is selection? Give one example from daily life. [2]
9. Define debugging and documentation. [2]
10. What is a loop? What is an infinite loop? [2]

Group C — Long answer (4 marks each)

1. Explain the steps of program development. [4]
2. Write an algorithm and draw a flowchart to find the area and circumference of a circle of radius R. [2+2]
3. Write an algorithm and draw a flowchart to check whether a person is eligible to vote in Nepal (age 18 years or more). [2+2]
4. Write an algorithm and draw a flowchart to find the factorial of a number. Trace it for N = 4. [2+2]
5. What is a flowchart? Write any four rules for drawing a flowchart and any two of its advantages. [1+2+1]