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

• 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

• 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

• 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

• 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

• 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

• 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