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Repetition:

1.4 Logical structure “Repetition”

A part of the program is repeated many times and the result of a test condition will determine when to stop the repetition.

 There are 3 types of repetition :-

  1. Fixed repetition (FOR … TO … DO)
    The iteration occurs a fixed number of times
  2. Non-zero-based conditional (REPEAT … UNTIL)
    The iteration continues until a condition is satisfied. Since the condition test is at the end of the iteration, the iterated statements are executed at least once.
  3. Zero-based conditional (WHILE … DO)
    The iteration continues while a condition is true. If the condition is false to begin with, the iteration will occur zero times, i.e. not at all
WHILE … DO REPEAT … UNTIL
1. The condition is evaluated before each execution of the iterated statements. The condition is evaluated after each execution of the iterated statements.
2. The iterated statements may not be executed at all. The iterated statements are executed at least once.
3. The iteration is terminated when the condition is false. The iteration is terminated when the condition is true.

1.4.1     Zero-based conditional Iteration (while …. do…)

Example WDRep1

Problem :- Design an algorithm to process any number of orders as in example Seq2. End of data will be indicated by a product number of 999999

Solution :-

Data Required                :-       (i) product code

(ii) quantity ordered

(iii) unit price

Processing involved       :-       cost = quantity ordered * unit price

Output Required            :-       total cost

 

Algorithm :- (only pseudocode shown; as a supplementary exercise do the flowchart)

         Read product code

While product code <> ‘999999’ Do

Read quantity ordered

Read unit price

cost = quantity ordered * unit price

Print the cost

Read another product code

Endwhile


 Dry run

Perform a dry run on the algorithm and fill in the trace table below. Use the following product numbers, quantities and prices as test data.

 

000001,15,100,000008,10,5,000235,42,8,000045,12,75,000482,10,25,

142583,20,15,047256,315,10,012540,15,200,502100,210,120,999999

 

Product number Is product number = ‘999999’? Quantity Unit price Cost
000001 no 15 100 1500
000008 no 10 5 50
000235

Pascal Program

program wdrep1_process_orders;

var

qty         : integer;

price, cost : real;

product     : string[6];

begin

write(‘Please key in the product number > ‘);

readln(product);

while product <> ‘999999’ do

begin

write(‘Please key in a price > ‘);

readln(price);

write(‘Please key in quantity required > ‘);

readln(qty);

cost := qty * price;

writeln;

writeln (‘Product    Quantity  Price    Cost’);

writeln (product:5, qty:10, price:11:2, cost:13:2);

write(‘Please key in product number > ‘);

readln(product)

end

(* endwhile *)

end.

 


Example WDRep2

Problem :- To modify the example NSeq1 so that the results of any number of student can be calculated. In addition, the students’ index number must be input. End of data will be indicated by an index number of ‘00000’

Solution :-

Algorithm :- (only pseudocode shown; as a supplementary exercise do the flowchart)

Read student’s index number

While index number <> ‘00000’ Do

Read marks for Project

Read marks for Theory exam

If mark for project = 0 Then

output result as “absent”

Else

total marks = project + theory

output the total marks

If total marks < 40 Then

output result as “failed”

Else

output result as “passed”

Endif

Endif

          Read another index number

Endwhile


Dry run

Perform a dry run on the algorithm and fill in the trace table below. Use the following index numbers, project marks and theory marks as test data.

25436,23,68,12365,0,65,75698,15,58,12354,12,71,12542,13,23,

24535,21,19,10240,15,24,10245,0,35,96521,20,50,00000

Index number Is index number = ‘00000’? Project marks Theory marks Total marks Results
25436 no 23 68 91 Passed
12365 no 0 65

Pascal Program

program wdrep2_project;

var

    index_no              :integer;

project,theory,sum    :real;

begin

write(‘Please enter student index number > ‘);

readln(index_no);

while index_no <> ‘00000’ do

begin

read(project);

if project = 0 then

writeln(‘Absent’)

else

          begin

write(‘Please enter marks for Theory exam ‘);

readln (theory);

sum:=project +  theory;

writeln (‘The sum is  ‘,sum);

   if sum < 40 then

writeln(‘Failed’)

   else

writeln(‘Passed’)

end

write(‘Please enter another index number > ‘);

readln(index_no)

end

(* endwhile *)

end.


1.4.2     Non Zero-based conditional Iteration (Repeat… Until…)

Example RURep1

Problem :- Modify example WDRep1  by using Repeat … Until…. Instead of  While … Do… program construct.

Solution :-

Algorithm :- (only pseudocode shown; as a supplementary exercise do the flowchart)

Repeat

Read product code

Read quantity ordered

Read unit price

cost = quantity ordered * unit price

Print the cost

Until product code = ‘999999’


Pascal Program

program rurep1_order_for;

var

times, qty  : integer;

price, cost : real;

product     : string[6];

begin

repeat

write(‘Please key in product number > ‘);

readln(product);

write(‘Please key in a price > ‘);

readln(price);

write(‘Please key in quantity required > ‘);

readln(qty);

cost := qty * price;

writeln;

writeln (‘Product    Quantity  Price    Cost’);

writeln (product:5, qty:10, price:11:2, cost:13:2)

until product = ‘999999’

end.


Dry run

Perform a dry run on the algorithm and fill in the trace table below. Use the following product numbers, quantities and prices as test data.

000001,15,100,000008,10,5,000235,42,8,000045,12,75,000482,10,25,

142583,20,15,047256,315,10,012540,15,200,502100,210,120,999999

Product number Quantity Unit price Is product number = ‘999999’? Cost
000001 15 100 no 1500
000008 10 5 no 50
000235

 

What is the problem with this algorithm?

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