Why use pseudocode?
- Once pseudocode is created, it is simple to translate it into real
- Opportunity to detect any logic error prior to actual coding, which
is a lot more expensive and time consuming.
- Used for planning the programming.
Common words and key words:
- What can your computer do? Calculate, Read, Print, Convert.....
- How do you instruct the computer to do what you intend?
- Keywords: The words that are predefined and reserved to a computer
to execute certain instruction. Example: Print, Read, and Compute. Keywords
are reserved by a computer programming language, and they contain some
particular definition and instruction. Examples in Visual Basic are
Private, Sub, End, and Print.
- Common words: Any other English words to complete the instruction.
They include names of variables, constants, file names and so on. Common
words are created and defined by a user.
|Six basic computer operations:
1. A computer can receive information.
- When a computer is required to receive information or input from a
particular source, whether it be a keyboard, a disk or any other device,
the verbs Read and Get are used in pseudocode. Read is usually used
when the algorithm is to receive input from a record on a file, while
Get is used when the algorithm is to receive input from the keyboard.
- Read student name (from the student master file).
- Get system date (from the computer system).
- Read student ID number.
- Get order.
2. A computer can put out information.
- When a computer is requested to supply information or output to a
device, the verbs Print, Write, Put, Output or Display are used in pseudocode.
Print is used to send the output to the printer, while Write is used
for writing to a file. For putting the information to a screen, the
common keywords are Put, Display or Output.
- Print "End of the Output"
- Write student record to master file
- Put out name, address and post code
- Output grade; Display "an input error occurred, please re-enter."
- In each example, the data to be put out is described concisely using
mostly lower-case letters. (Why?)
3. A computer can perform arithmetic operation.
- A programmer may use actual mathematical symbols or the words for
- For example: "Add score to total_score" is same as "total_score
= total_score + score" (programming languages use mathematical
equations to assign values to memory locations. In this case, a new
value of total_score is assigned to a memory location named total_score
by adding score to the current (not new) value of total_score)
- The following symbols can be used in pseudocode: + for Add, - for
Subtract, * for Multiply, / for Divide, ( ) for Parentheses
- For more example:
- Divide total_score by student_count
- class_average = total_score / student_count
- Compute C = (F - 32) * 5 / 9
- When writing mathematical calculation for the computer, the order
of operation should be considered; otherwise you may end up with incorrect
- The order of operators are following:
- ( ) : Values within parentheses are always evaluated first. Ex:
- ^ : Exponentiation (raising a number to a power) is second Ex:
- - : Negation (creating a negative number) is third. Ex:
- * and / : Multiplication or division is fourth. Ex:
- \ : Integer division (a.k.a. Div) is fifth. Ex:
- Mod : Remainder division is sixth. Ex:
- + or - : Addition and subtraction are last.
- Example: For an expression Total = 10 + 15 * 2 / 4 ^ 2 -(2 + 3) ,
the order of computation is following:
- Total = 10 + 15 * 2 / 4 ^ 2 -(2 + 3)
- Total = 10 + 15 * 2 / 4 ^ 2
- Total = 10 + 15 * 2 / 16 -(5)
( 5 as a negative value)
- Total = 10 + 15 * 2 / 16 -
- Total = 10 + 30 / 16 - 5
- Total = 10 + 1.875 - 5
- Total = 11.875 - 5
- Total = 6.875
4. A computer can assign a value to a variable or memory location.
- There are three cases where you may write pseudocode to assign a value
to a variable or memory locations
- To give data an initial value. The verbs Initialize or Set are
- To assign a value as a result of some processing, the symbol " =
" is used.
- To keep a piece of information for later use, the verbs Save or
Store is used.
- Initialize total_score to 0: total_score = 0
- Set student_count to 0: student_count = 0
- total_score = total_score + score1
- student_count = student_count + 1
- class_average = total_score / student_count
- Store class_average in class_average_quiz1
5. A computer can compare two variables and select one of two alternative
- For this operation, special keywords are used: If..Then or
- The comparison of data is established in the If clause, and the choice
of alternatives is determined by the Then or Else options. Only one
of these alternatives will be performed.
If student is part_time Then
add 1 to part_time_count
add 1 to full_time_count
|If the student is a part-time student, then "add 1 to part_time_count"
is performed. Otherwise, the computer skips to the Else clause
to perform "add 1 to full_time_count" instruction.
6. A computer can repeat a group of actions.
- When there is a sequence of processing steps that need to be repeated,
keywords Do While and EndDo, are used. And processing stems in between
those keywords should be repeated as long as the condition for initiation
Do While student_total < 30
Read student record
Print student name, address
Add 1 to student_total
| In this example, the set of instruction in the Do While loop
will be performed repeatedly as long as student_total is less
than 30. And each time computer goes through the loop, the value
of student_total will be incremented by 1, in which eventually
will make the student_total to be equal to 30, and terminate the
|The Structure Theorem: How does a computer
know what to do first and to do next? Go to www.wiley.com
for graphical samples.
- The sequence structure controls the straightforward execution
of one processing step after another. For example, within a module,
a computer executes the first line of instruction first before the
second. And it processes the following instruction in the order
of appearance unless it encounters a selection (If..Then) or a loop
Statement 3 ….
- The selection structure (If .. Then) represents the decision-making
abilities, and allows a programmer to set what will be the next
step depends on the condition.
As a result, only one step, either x or y, will be performed and
the other step will be skipped.
If condition k is true Then
step x is the next
step y is the next
- A loop (Do While) executes a same set of instruction as long as
the initial condition for the loop is met. At the end of a loop,
a computer goes back to the beginning of the loop and tests the
condition. If the condition is met, it goes through the loop one
more time. If the condition is not met, then it terminates the loop
and continues to execute instructions from the end of the loop.
- Usually, a loop contains a counter which increments or decrements
each time the loop is executed. And the counter with the new value
is used in determining whether the look should be repeated or terminated.
|The variable student_data acts as a counter.
student_total = 0
Do While student_total < 25
student_total = student_total + 1
- Question: Based on the above codes, if there were 25 students
in the file, would this algorithm print all 25 students? Count how
many time the loop occure.
student_total = 1
DoWhile student_total <= 25 And student_record <> " " Read
Print student.address student_total = student_total +
Convert the following flowchart to pseudocode.
This is the pseudocode for a Game of Monopoly, including one person's
move as a procedure:
Monopoly_Game Hand out each player's initial money. Decide which player
goes first. Repeat Call Procedure Monopoly_Move for next player. Decide
if this player must drop out. Until all players except one have dropped
out. Declare the surviving player to be the winner.
Begin one's move. Throw the dice. Move the number of spaces on the
board shown on the dice. If the token landed on "Go to Jail," then go
there immediately. Else if the token landed on "Chance" or "Community
Chest," then draw a card and follow its instructions. Else follow the
usual rules for the square (buying property, paying rent, collecting
$200 for passing "Go", etc.). End one's move.