Pseudocode is a kind of structured english for describing algorithms. It allows the designer to focus on the logic of the algorithm without being distracted by details of language syntax.  At the same time, the pseudocode needs to be complete.  It describe the entire logic of the algorithm so that implementation becomes a rote mechanical task of translating line by line into source code.

In general the vocabulary used in the pseudocode should be the vocabulary of the problem domain, not of the implementation domain.  The pseudocode is a narrative for someone who knows the requirements (problem domain) and is trying to learn how the solution is organized.  E.g.,

    Extract the next word from the line (good)
    set word to get next token (poor)

    Append the file extension to the name (good)
    name = name + extension (poor)

    FOR all the characters in the name (good)
    FOR character = first to last (ok)

Note that the logic must be decomposed to the level of a single loop or decision. Thus "Search the list and find the customer with highest balance" is too vague because it takes a loop AND a nested decision to implement it. It's okay to use "Find" or "Lookup" if there's a predefined function for it such as String.indexOf().

Each textbook and each individual designer may have their own personal style of pseudocode. Pseudocode is not a rigorous notation, since it is read by other people, not by the computer. There is no universal "standard" for the industry, but for instructional purposes it is helpful if we all follow a similar style. The format below is recommended for expressing your solutions in our class.

The "structured" part of pseudocode is a notation for representing six specific structured programming constructs: SEQUENCE, WHILE, IF-THEN-ELSE, REPEAT-UNTIL, FOR, and CASE. Each of these constructs can be embedded inside any other construct. These constructs represent the logic, or flow of control in an algorithm.

It has been proven that three basic constructs for flow of control are sufficient to implement any "proper" algorithm.

    SEQUENCE is a linear progression where one task is performed sequentially after another.
    WHILE is a loop (repetition) with a simple conditional test at its beginning.
    IF-THEN-ELSE is a decision (selection) in which a choice is made between two alternative courses of action.

Although these constructs are sufficient, it is often useful to include three more constructs:
 
    REPEAT-UNTIL is a loop with a simple conditional test at the bottom.
    CASE is a multiway branch (decision) based on the value of an expression. CASE is a generalization of IF-THEN-ELSE.
    FOR is a "counting" loop.

SEQUENCE

Sequential control is indicated by writing one action after another, each action on a line by itself, and all actions aligned with the same indent. The actions are performed in the sequence (top to bottom) that they are written.

Example (non-computer)
    Brush teeth
    Wash face
    Comb hair
    Smile in mirror

Example
    READ height of rectangle
    READ width of rectangle
    COMPUTE area as height times width

Common Action Keywords
    Several keywords are often used to indicate common input, output, and processing operations.

        Input: READ, OBTAIN, GET
        Output: PRINT, DISPLAY, SHOW
        Compute: COMPUTE, CALCULATE, DETERMINE
        Initialize: SET, INIT
        Add one: INCREMENT, BUMP

IF-THEN-ELSE

Binary choice on a given Boolean condition is indicated by the use of four keywords: IF, THEN, ELSE, and ENDIF. The general form is:

    IF condition THEN
        sequence 1
    ELSE
        sequence 2
    ENDIF

The ELSE keyword and "sequence 2" are optional. If the condition is true, sequence 1 is performed, otherwise sequence 2 is performed.

Example
    IF HoursWorked > NormalMax THEN
        Display overtime message
    ELSE
        Display regular time message
    ENDIF

WHILE
The WHILE construct is used to specify a loop with a test at the top. The beginning and ending of the loop are indicated by two keywords WHILE and ENDWHILE. The general form is:

    WHILE condition
        sequence
    ENDWHILE

The loop is entered only if the condition is true. The "sequence" is performed for each iteration. At the conclusion of each iteration, the condition is evaluated and the loop continues as long as the condition is true.

Example
    WHILE Population < Limit
        Compute Population as Population + Births - Deaths
    ENDWHILE

Example

    WHILE employee.type NOT EQUAL manager AND personCount < numEmployees
        INCREMENT personCount
        CALL employeeList.getPerson with personCount RETURNING employee
    ENDWHILE

CASE

A CASE construct indicates a multiway branch based on conditions that are mutually exclusive. Four keywords, CASE, OF, OTHERS, and ENDCASE, and conditions are used to indicate the various alternatives. The general form is:

    CASE expression OF

        condition 1 : sequence 1
        condition 2 : sequence 2
        ...
        condition n : sequence n
        OTHERS:
        default sequence
    ENDCASE

    The OTHERS clause with its default sequence is optional. Conditions are normally numbers or characters

indicating the value of "expression", but they can be English statements or some other notation that specifies the condition under which the given sequence is to be performed. A certain sequence may be associated with more than one condition.

Example
        CASE  Title  OF
                Mr      : Print "Mister"
                Mrs     : Print "Missus"
                Miss    : Print "Miss"
                Ms      : Print "Mizz"
                Dr      : Print "Doctor"
        ENDCASE

Example
        CASE  grade  OF
                A       : points = 4
                B       : points = 3
                C       : points = 2
                D       : points = 1
                F       : points = 0
        ENDCASE

REPEAT-UNTIL

This loop is similar to the WHILE loop except that the test is performed at the bottom of the loop instead of at the top. Two keywords, REPEAT and UNTIL are used. The general form is:

    REPEAT
        sequence
    UNTIL condition

The "sequence" in this type of loop is always performed at least once, because the test is peformed after the sequence is executed. At the conclusion of each iteration, the condition is evaluated, and the loop repeats if the condition is false. The loop terminates when the condition becomes true.

FOR

This loop is a specialized construct for iterating a specific number of times, often called a "counting" loop.  Two keywords, FOR and ENDFOR are used. The general form is:

    FOR iteration bounds
        sequence
    ENDFOR

In cases where the loop constraints can be obviously inferred it is best to describe the loop using problem domain vocabulary.

Example
    FOR each month of the year (good)
    FOR month = 1 to 12 (ok)
    FOR each employee in the list (good)
    FOR empno = 1 to listsize (ok)

NESTED CONSTRUCTS

The constructs can be embedded within each other, and this is made clear by use of indenting. Nested constructs should be clearly indented from their surrounding constructs.

Example
    SET total to zero
    REPEAT
        READ Temperature
        IF Temperature > Freezing THEN
            INCREMENT total
        END IF
    UNTIL Temperature < zero
    Print total

In the above example, the IF construct is nested within the REPEAT construct, and therefore is indented.

INVOKING SUBPROCEDURES

Use the CALL keyword. For example:
    CALL AvgAge with StudentAges
    CALL Swap with CurrentItem and TargetItem
    CALL Account.debit with CheckAmount
    CALL getBalance RETURNING aBalance
    CALL SquareRoot with orbitHeight RETURNING nominalOrbit

EXCEPTION HANDLING
    BEGIN
        statements
    EXCEPTION
        WHEN exception type
            statements to handle exception
        WHEN another exception type
            statements to handle exception
    END

Sample Pseudocode
"Adequate"

FOR X = 1 to 10
    FOR Y = 1 to 10
        IF gameBoard[X][Y] = 0
            Do nothing
        ELSE
            CALL theCall(X, Y) (recursive method)
            increment counter                 
        END IF
    END FOR
END FOR

"Better"
Set moveCount to 1
FOR each row on the board
    FOR each column on the board
        IF gameBoard position (row, column) is occupied THEN
            CALL findAdjacentTiles with row, column
            INCREMENT moveCount
        END IF
    END FOR
END FOR

(Note: the logic is restructured to omit the "do nothing" clause)

"Not So Good"

FOR all the number at the back of the array
    SET Temp equal the addition of each number
    IF > 9 THEN
        get the remainder of the number divided by 10 to that index
        and carry the "1"
    Decrement one
Do it again for numbers before the decimal

"Good Enough (not perfect)"

SET Carry to 0
FOR each DigitPosition in Number from least significant to most significant

    COMPUTE Total as sum of FirstNum[DigitPosition] and SecondNum[DigitPosition] and Carry  

    IF Total > 10 THEN
        SET Carry to 1
        SUBTRACT 10 from Total
    ELSE
        SET Carry to 0
    END IF
    STORE Total in Result[DigitPosition]
END LOOP  
IF Carry = 1 THEN
    RAISE Overflow exception
END IF

"Pretty Good"  This example shows how pseudocode is written as comments in the source file. Note that the double slashes are indented.

public boolean moveRobot (Robot aRobot)
{
    //IF robot has no obstacle in front THEN
        // Call Move robot
        // Add the move command to the command history
        // RETURN true
    //ELSE
        // RETURN false without moving the robot
    //END IF
}

Example Java Implementation

    source code statements are interleaved with pseudocode.
    comments that correspond exactly to source code are removed during coding.

public boolean moveRobot (Robot aRobot)
{
    //IF robot has no obstacle in front THEN
    if (aRobot.isFrontClear())
    {
        // Call Move robot
        aRobot.move();
        // Add the move command to the command history
        cmdHistory.add(RobotAction.MOVE);
        return true;
    }
    else // don't move the robot
    {
        return false;
    }//END IF
}
 
  Examples of vague pseudocode

Document History
Date     Author     Change
12/2/03
    JD
    Added Exception Handling and more examples
2/21/03     JD     Added "problem domain vocabulary" paragraph.
Modified FOR loop explanation.