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Basic Problem Solving with
Programming – Iteration
(or loops)
What is a loop?
A loop is a repetition control structure
It causes a single statement or block to
be executed repeatedly while an
expression is true
When use a Loop?
Stated simply, one should use a loop at
any point where you wish to repeat a
process, idea, or function.
Example: Write a small program that
will display the numbers 1 – 100 and
their squares.
Loop Usage
A. Solve the equation 2x2 + x + 5 for all
integer x between 5 and 10
B. Sum inputted integers until the user
enters -1
C. The user enters in the current year and
then his/her birth year. Your program
computes the users age. Perform this
task again if he or she wishes.
Two types of loops
count-controlled loops
repeat a specified number of times
event-controlled loops
something happens inside the loop body
that causes the repetition to stop after
the current iteration completes
• The problem describes how many times the
iteration is to execute
• The problem indicates a starting value,
ending value and increment for the loop
control value.
• Sometime the loop control value is needed
for some calculation within the block of the
loop, sometimes not.
• Sentinel controlled - Keep processing
data until a special value which is not a
possible data value is entered to
indicate that processing should stop
• End-of-file controlled - Keep
processing data as long as there is
more data in the file
• Flag controlled - Keep processing data
until the value of a flag changes in the
loop body
Sentinel-controlled loop
• Requires a “priming read”
• “Priming read” means you read one
data value (or set of data values)
before entering the while loop
• Process data value(s) and then read
next value(s) at end of loop
Flag-controlled loops
Use meaningful name for the flag
Initialize flag (to true or false)
Test the flag in the loop test expression
Change the value of the flag in loop
body when the appropriate condition
Nested loops
• Some problems require iteration structures
to be nested
• Examples:
– Process data in rows and columns of a
table, outer loop is for one row at a time,
inner loop is for each column in the current
– Outer loop is to allow the user to “play
again” or “re-enter data”, inner loop is to do
some iterative processing for each outer
Loop Testing & Debugging
• Test data should test all sections of the program
• Beware of infinite loops - the program doesn’t
• Check loop termination condition, and watch for
an OBOB (off-by-1 bug)
• Use algorithm walk-through to verify that
appropriate conditions occur in the right places
• Trace execution of loop by hand with code walkthrough
• Use a debugger to run program in “slow motion”
or use debug output statements
Scratch iteration control structures
forever if (condition)
repeat until (condition)
repeat (number of times)