indexed pseudo code

8/10/2019 Indexed Pseudo Code

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A Pseudo-CodeStandard

byJ F Pratt


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1A Pseudo-Code Standard

What is Pseudo-Code ?Pseudo-Code is a programming-like

formal form of English, that can be usedto express a programming strategy or

algorithm, namely, a recipe to solve

a problem.

Problem

Find the mean of two numbers.

Pseudo-Code

float a, b, mean;

read (a, b) ;

mean = ( a + b ) / 2 ;

write( when a = , a, and b = , b ) ;write( their mean is , mean);


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Pseudo-Code StandardWhen using pseudo-code it is useful for a

programming community to stick to astandard way of expressing code which

is programming language independent.

Meta-Names

A name in angled brackets eg

means that an instance of the indicated

kind needs to be supplied.

Symbols+ , - , / , * , = , =, . . . and so on

are written literally as shown.

Syntax

Meta-names and symbols are used to definethe correct format (syntax) of pseudo-code

statements. Their meaning (semantics) is

what they do.


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More Assignments

(ii)

x = y + 2 ; means put the value of y + 2

into x.

x 12 y 3

y + 2

|

3 + 2 = 5

x 5 y 3

(iii) x = x + 1 ; means add one to x.

x 5 x 6


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Operators

Some standard operators that can be used

in expressions, are given below. Use BODMAS

- School arithmetic - to evaluate expressions.

( ) brackets highest

* multiplication medium

/ division

+ addition lowest- subtraction

Examples

a 3 b 4

(i) b + a * 4 = 4 + 12 = 16

(ii) ( b + a ) * 4 = 7 * 4 = 28

Operator Usage Priority


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I/O : Input Statement

Assign value(s) from the keyboard to

the variable(s) in the order given. This

is an example of an input statement.

Example

read( a, b, c ) ;

Get three values from the keyboard,

the first goes into a, the second goes into b,and the third one goes into c.

User types : 3 4 5

a 3 b 4 c 5

read();


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I/O : Output Statement

Print out the value(s) of the expression(s)

on the screen in the order given. This is

an example of an output statement.

Example

write( the answer is , a+2) ;

Supposing a were currently 42, then wewould get as output.

the answer is 44

write();


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Constructs

Constructs build simple statements such

as assignment and I/O into higher levelstatements.

They have one entry point and one exit

point - this is important for producing

well-structured (easily maintained) code.

The Three Fundamental Constructs

- Sequence

- Selection

- Iteration

There are several selection constructs

and several iteration constructs available,

the ones considered here are sufficient

for most purposes.


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Sequence Construct

______;______;

______;

Execute the statements in order, namely

top to bottom and left to right.

Example

temp = x ;

x = y ;

y = temp ;

These three assignment statements mean

swap x and y.

NB______ stands for assignment,

I/O or a construct.


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Comparisons

In the constructs that follow often we will need

to compare one value with another, namely, werequire simple conditions called comparisons

of the form :-

value1 value2

We give below a table of the various relational

operators that are available for comparing

values.

Comparisons evaluate to either trueor false.

Symbol Relational Operator

< strictly less than

= greater than or equal to

> strictly greater than

! = not equal to


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Comparison Examples

n 4 m 3

m + 1 = = n is true

m != n is truen != m + 1 is false

mn = = 0 is false

Compound Conditions

More complex conditions can be tested for byusing and, or, not. With andall subconditions

have to be truefor the condition to be true.

With orat least one of the subconditions needs

to be truefor the condition to be true. With

notthe subcondition needs to be false if thecondition overall is to betrue.

eg

not( m = = n and n != m+1) is true


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Selection Constructs

Selection constructs are used to make decisions

in programs, there are three main idioms.

-One-way choice

-Two-way choice-Multi-way choice

The first idiom is distinct from the other

two, in that the option of doing nothingis available.


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One-way Choice

if then

______;

______;

______;

end if

Execute the first arm provided the condition

is true.This means do nothing when the

condition is false.

Example

Perform a conditional swap to order a and b.

if a > b then

temp = a;a = b;

b = temp;

end if

swap a and b.


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Two-way Choice

if then______;

______;

______;

else

______;______;

______;

end if

Execute the first arm (the thenpart) providedthe condition is true, otherwise execute the

second arm (the elsepart).

if height > 1.42 then

write( ok to go on ride);

else

write(sorry youre not tall enough);

end if

Example


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Multi-way Choice

if then______;

______;

______;

else if

______;______;

______;

else

______;

______;______;

end if

Execute the first arm (the thenpart) if

condition 1 is true, otherwise execute the

second arm if condition 2 is true,

otherwise execute the third arm.

NBExtra else if s can

be added, as necessary.

NBno

condition

here.


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Iteration Constructs

While loop

while do

______ ;

______ ;

______ ;

end while

An unpredictable - non-deterministic loop.

For loop

for = todo

______ ;______ ;

______ ;

end for

A predictabledeterministic loop.

Execute the statements repeatedly providing

the condition is trueor ,in the case of for loops,

providing the for loop variable is within range.


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A While Example

Sum a negatively terminated list of integers.

egFor the data set :-

1 2 3 -11

the sum of the positives is 6.

The negative value -11in this case simply

signals the end of the positive integers -

playing the role of a sentinel.

sum=

0;read(x);

while x >= 0 do

sum = sum + x ;

read(x) ;

end whilewrite(total is ,sum);


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A For ExampleProduce a table of squares and cubes, of

the integers from 1 to n inclusively.

read(n);

for i = 1 ton do

write(i); write (i2

); write (i3

);write();

end for

NBalthough some programming languages

provide powers, in others we might have touse i i to get i2 and likewise for i3.

NBWe put downin front of the tokeyword

to get a downward counting for loop.

*

1 1 1

2 4 8

.

n n2 n3


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Index

2

5

6

7

1

3

4

8

What is Pseudo-Code ?

Pseudo-Code Standard

Assignment Statement

More Assignments

Operators

I/O : Input Statement

I/O : Output Statement

Constructs

Sequence Construct

Front Page0

9


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Index

11

12

13

15

16

14

17

18

10

Comparison Examples

Selection Constructs

One-way Choice

Two-way Choice

Multi-way Choice

Iteration Constructs

A While Example

A For Example

Comparisons

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