4' REPETITIVE EXECUTION

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4. REPETITIVE EXECUTION

• PROGRAM REPETITION AND
• THE DO CONSTRUCT
• COUNT - CONTROLLED DO LOOPS
• MORE FLEXIBLE LOOPS
• GIVING NAMES TO DO CONSTRUCTS
• Ellis Philips, 1998, p135-161

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PROGRAM REPETITION AND THE DO CONSTRUCT

• A repetition structure or loop makes possible the
  repeated execution of one or more statements.
• A program loop
• 1.Repeat the following 10 times
• 1.1 Read Celsius temperature
• 1.2 Calculate Fahrenheit equivalent
• 1.3 Print both temperatures

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Do loop

• The do construct provides the means for
• controlling the repetition of statements within
• a loop.
• do countinitial,final,inc
• block of statements
• end do

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Do loop

• do countinitial,final
• block of statements
• end do

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Do loop

• do
• block of statements
• end do

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COUNT-CONTROLLED DO LOOPSSome examples of do
statements and their effects

• do statement Iteration count do variable values
• do i1,10 10 1,2,3,4,5,6,7,8,9,10
• do j20,50,5 7 20,25,30,35,40,45,50
• do p7,19,4 4 7,11,15,19
• do q4,5,6 1 4
• do r6,5,4 0 (6)
• do x-20,20,6 7 -20,-14,-8,-2,4,10,16
• do n25,0,-5 6 25,20,15,10,5,0
• do m20,-20,-6 7 20,14,8,2,-4,-10,-16
• Maximum iteration count (final-initialinc)/inc

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COUNT-CONTROLLED DO LOOPS

• In a count-controlled do loop
• The value of the iteration count is calculated
  before the first iteration.
• The do variable is incremented on each pass
• through the loop.

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Example 6.1 p 141Write a program which first
reads the number of students taking an exam. It
should then read their marks (or scores) and
print the highest andlowest marks followed by
the average mark for the class.

• program Examination
• integer i,number,mark,maximum,minimum,total
• real average
• total0
• maximum-1000
• minimum1000
• print ,How many marks are there ?
• read ,number
• print ,Please type,number,marks on per line
• do i1,number
• read ,mark
• totaltotalmark
• if (markgtmaximum) then
• maximummark
• end if
• if (markltminimum) then
• minimummark
• end if
• end do

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Example 6.2, p142Write a program to print a set
of multiplication tables from 2 times up to 12
times where each table should take the form x
times 1 is x x times 2 is 2x

• program Multiplication_tables
• integer i,j
• do i2,12
• print ,
• print ,i,times table
• do j1,12
• print ,i,times,j,is,ij
• end do
• end do
• end program mutiplication_tables
• Output
• ...
• 4 times table
• 4 times 1 is 4
• 4 times 2 is 8

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More flexible loops

• An infinite loop
• do
• .
• if (termltsmall) then
• exit
• end if
• .
• end do
• An fail-safe mechanism
• do count1,max
• .
• if(termltepsilon) then
• exit
• endif
• .
• end do

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Exit and Cycle statements

• Execution of an exit statement in a loop causes
  the next statement to be executed to be the one
  immediately after the end do statement of the
  innermost loop surrounding the exit statement,
  unless the exit statement is named, in which case
  it will be the statement immediately after the
  end do statement having the same name.
• The cycle statement is very similar to the exit
  statement except that instead of transferring
  control to the statement after the end do
  statement, it transfers control back to the start
  of the loop in exactly same way as if it, in
  fact, transferred control to the end do
  statement.

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Examples for the exit statement

• An infinite loop
• do
• .
• if (termltsmall) then
• exit
• end if
• .
• end do
• An fail-safe mechanism
• do count1,max
• .
• if(termltepsilon) then
• exit
• endif
• .
• end do

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Example for the cycle statement

• ! End of data
• case (X)
• exit
• ! Invalid code
• case default
• print ,invalid code please re-enter data
• cycle
• end select

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GIVING NAMES TO DO CONSTRUCTS

• block_name do
• .
• .
• end do block_name

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Existing from a nested do loop

• do
• .
• do
• .
• do
• .
• exit
• .
• do
• .
• end do
• ! This one (1) ?
• .
• end do
• ! or this one (2) ?
• .
• end do
• ! or this one (3) ?
• .

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Named do constructs

• outer do
• .
• inner do
• .
• select case (n)
• case (1)
• exit outer
• case (2)
• exit inner
• case (3)
• cycle outer
• case (4)
• cycle inner
• end select
• .
• end do inner
• .
• end do outer

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Dealing with exceptional situations

• The stop statement causes an immediate
  termination of the execution of a program.
• The return statement causes an immediate
  termination of the execution of a procedure.

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IN-CLASS PROBLEM SESSION - 4

• Objective Learning repetitive execution
  Numerical
• Methods (Numerical Integration of Functions)
• Assignment
• Write a program to approximate the integral of a
  given
• function over the interval (a,b) using the
  rectangle method
• with altitudes chosen at the midpoints of the
  subintervals.

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Numerical Integration Method (Nyhoff Leestma,
1992, page 157)
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HOMEWORK - 4

• Objective Learning repetitive execution
  Numerical Methods (Infinite
• Series)
• Assignment
• The infinite series
• converge to the number e. For a positive integer
  k, k! is the product of the
• integers from 1 through k 0! is defined to be 1.
  The nth partial sum of such
• a series is the sum of the first n terms of the
  series for example,
• is the 4th partial sum. Write a program to
  calculate and print the first
• 10 partial sums of this series.