Download Controlled Loop - CSE, IIT Bombay

Loop Construct
Control flow constructs
• Control Flow constructs seen so far
– sequential flow
– two-way or multi-way branching flow
• These constructs are not enough!
Sum of Square roots
• Problem: Compute sum of square roots of numbers from 1 to 5
• One solution:
program sum_sqroot
implicit none
real :: sum,sum1,sum2,sum3,sum4
sum1 = sqrt(1.0)
sum2 = sum1 + sqrt(2.0)
sum3 = sum2 + sqrt(3.0)
sum4 = sum3 + sqrt(4.0)
sum = sum4 + sqrt(5.0)
print *, sum
end program
Summing Square roots
• What are the problems with this solution?
• No need for so many sum variables; use instead
sum = sum + sqrt(..)
• what if 10, 20 or 100 numbers to be added?
– code repetition possible but undesirable
• what if an unknown numbers (given as input) to be added?
– code repetition not possible at all
Looping
• Loop constructs are designed to solve the above problems
• Loop constructs enable execution of same piece of code a specified number of time
• Two kinds of loop constructs:
– Controlled loop
– Uncontrolled loop
• The above problem can be solved using controlled loop
Example
Program sum_sqrt
implicit none
real :: x,sum
sum = 0.0
!initialization needed. why? see the loop
!looping construct
do i = 1,5 !loop body executed five times
x = real(i)
sum = sum + sqrt(x) !each time i incremented by 1
end do
print *, sum
end program
Controlled Loop Construct
• General form:
do var = init, fin, inc
loop_body
end do
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var, called control variable, must be integer
init,fin,inc are integer expressions
inc must be non-zero, can be negative and default value is 1
loop_body must not modify value of var
Iteration
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loop_body is executed repeatedly
each execution is called an iteration
initially, var = init
after every iteration inc is added to var
loop is terminated when value of var exceeds fin (or smaller than fin)
Loop body is executed a fixed number of times
number of iterations may depend on values of variables
Controlled Loop
• number of iterations (for +ve inc)
iter-count = (fin-init+inc)/inc
• iter-count is computed when the do statement is first encountered
• if iter-count <= 0 , loop is not executed
• if iter-count > 0, loop_body is executed, iter-count is decremented by 1 and var is
incremented in every iteration
• loop terminates finally when iter-count <= 0, i.e., the control flows to the statement
immediately following the loop statement
Controlled Loop
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iter-count is computed when the do statement is first encountered
Its value depends upon the values at that point
values of init, fin, inc should not be changed inside the loop
var is incremented by the value of inc in every iteration
It is assigned the value init when do is first encountered
Cycle and Exit statements
• loop_body can contain two special statements
• Cycle
– It terminates the current iteration and the next iteration is started, if there is one
• Exit
– This terminates the loop moving the control to the statement following the loop
Example
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Read a line of characters,
Count the number of e occurring in the line
Replace every a by #
Lines of length 80 or ended by $ sign
program e_count
character(len=80) :: line
integer :: counter,index
read *, line
index=1
counter = 0
do index = 1, 80
if (line(index:index) = = "$“) exit !exit the loop if end of line
if (line(index:index) == "e“) then
counter = counter +1
cycle !skip the rest of the loop
endif
if (line(index:index) == "a“) then
line(index:index) = "#"
endif
end do
print *, counter, line
Uncontrolled Loop
• The controlled loop is a simple loop computation
• The loop body executed for a number of times that is fixed a priori or at least prior to
the execution of the do statement
• Many times
– repeat a computation till certain condition holds
– No upper bound on the number of iterations exists
• More general loops are required for describing these
• Uncontrolled loops are meant for this
Another Example
• Similar to the earlier problem
• there is no upper bound on the length of the line
• all lines should be terminated by $
index =0
do !uncontrolled loop
index = index +1
if (line(index:index) == "$“) exit
if (line(index:index) == "e" ) then
counter = counter +1
cycle
endif
if (line(index:index) == "a" ) then
line(index:index) = "#"
endif
end do
Uncontrolled loop
• Controlled loop does many things implicitly
– control variable updation, exit condition checking
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In uncontrolled version, these have to be done explicitly
Termination is guaranteed in the controlled one
Termination need to be ensured in the uncontrolled one
explicit inclusion of exit statements is required
In e_count1, for termination of the loop, the end of line condition is required
In e_count, the loop terminates automatically after reading at most 80 characters
Controlled loop is safe but less general
General Form
do
loop_body
end do
• Any number of exit statements can occur in the loop_body
• At least one required
• Possibility of non termination, in any case
Loops
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Loop is an important construct
Most programs will involve loops
Loops encode potentially unbounded computations
Mastering loops is important
Loops cause non termination, a most common programming error
Correct programs should be terminating
Every loop is associated with a condition called loop invariants
Loop invariant holds at the beginning of every iteration
Nested Loops
• Like many other constructs loops can also be nested
do
statement1
do
statement2
end do
statement3
end do
• Note that `inner do' ends before the `outer do‘
• Exit and cycle in statement2 refers to the inner loop
• Exit and cycle in statement1 and statement3 refer to the outer loop
Labeling loops
• For ease of understanding loops can be labeled:
outer: do
statement1
inner: do
statement2
end do inner
statement3
end do outer
• exit and cycle statements can name the loop
exit outer, cycle outer
• unnamed statements refer to the innermost enclosing loops
Labeling control constructs
• Like loops all other constructs can be labeled
• Examples:
cond1: if (test1) then
statement
elseif (test2) then cond1
statement
else cond1
statement
endif cond1
• Similarly for select case construct