Repetition, Basic loop structures, Loop programming techniques

Introduction to Computer Programming
Control Structures – Repetition
C++ Programming: From Problem Analysis to Program Design
1

• Basic loop structures
• while loops
• Interactive while loops
• for loops
• Loop programming techniques
• Nested loops
• do whileloops
• Common programming errors
Chapter Topics
2

• Suppose you want to add five numbers to find their
average. From what you have learned so far, you
could proceed as follows:
• Assume that all variables are properly declared
Why Is Repetition Needed?
3

• Suppose you want to add and average 100, 1000, or
more numbers
– You would have to declare that many variables
– List them again in cin statements
• This takes an exorbitant amount of space and time
• Also, if you want to run this program again with
different values or with a different number of
values, you have to rewrite the program
4

• Suppose you want to add the following numbers:
5 3 7 9 4
• Consider the following statements, in which sum
and num are variables of type int:
1. sum = 0;
2. cin >> num;
3. sum = sum + num;
• Each time you need to add a new number, you
repeat statements 2 and 3
5

• Repetition structure has four required elements:
– Repetition statement
– Condition to be evaluated
– Initial value for the condition
– Loop termination
• Repetition statements include:
– while
– for
– do while
Basic Loop Structures
6
f o r ( i = 0 ; i < 1 0 ; i + + )
c o u t < < i ;

• The condition can be tested
– At the beginning: Pretest or entrance‐controlled loop
(while, for)
– At the end: Posttest or exit‐controlled loop (do‐while)
• Something in the loop body must cause the
condition to change, to avoid an infinite loop, which
never terminates
Basic Loop Structures (continued)
7

• Pretest loop: Condition
is tested first; if false,
statements in the loop
body are never
executed
• while and for loops
are pretest loops
Pretest and Posttest Loops
A pretest loop
8

Pretest and Posttest Loops (continued)
• Posttest loop: Condition is
tested after the loop body
statements are executed;
loop body always executes
at least once
• do while is a posttest
loop
A posttest loop
9

• Fixed‐count loop: Loop is processed for a fixed
number of repetitions
• Variable‐condition loop: Number of repetitions
depends on the value of a variable
Fixed‐Count Versus Variable‐Condition
Loops
10
f o r ( i = 0 ; i < 1 0 ; i + + )
c o u t < < i ;
f o r ( i = 0 ; i < a ; i + + )
c o u t < < i ;

while Loops
• while statement is used to create a while loop
– Syntax:
while (expression)
statement;
• Statements following the expressions are executed
as long as the expression condition remains true
(evaluates to a non‐zero value)
11

while Loops (continued)
12
Loop repeated 10 times
Start value 1
End value 10
Increment 1
Last value of count is 11

while Loops (Examples)
13
Start value 0
End value 20
Increment 5
Last value of i is 25

while Loops (Examples)
14
Start value 20
End value N/A
Increment 5

Counter‐Controlled while Loops
• Suppose that a set of statements needs to be executed N
times
• You can set up a counter (initialized to 0 before the while
statement) to track how many items have passed
15

16
Loop repeated limit times
Start value 0
End value limit ‐1
Increment 1
Last value of counter is limit

17

18

while Loops with Sentinels
• Sentinel: A data value used to signal either the start
or end of a data series
• Use a sentinel when you don’t know how many values
need to be entered (looping is controlled by the user)
19

Flag‐Controlled while Loops
• A flag‐controlled while loop uses a bool variable to control
the loop.
• Suppose found is a bool variable. The flag‐controlled while
loop takes the following form:
23

• break statement
– Forces an immediate break, or exit, from
switch, while, for, and do-while
statements
– Violates pure structured programming, but is
useful for breaking out of loops when an unusual
condition is detected
break and continue Statements
24

• Example of a break statement:
(cont’d)
25

• continue statement
– Applies to while, do-while, and for statements;
causes the next iteration of the loop to begin
immediately
– Useful for skipping over data that should not be
processed in this iteration, while staying within the
loop
(cont’d)
26

• A continue statement where invalid grades are
ignored, and only valid grades are added to the total:
(cont’d)
27

• Null statement
– Semicolon with nothing preceding it
• ;
– Do‐nothing statement required for syntax purposes only
The Null Statement
28

for Loops
• for statement: A loop with a fixed count condition
that handles alteration of the condition
– Syntax:
for(initializing list; expression; altering list)
statement;
• Initializing list: Sets the starting value of a counter
• Expression: Contains the maximum or minimum value
the counter can have; determines when the loop is
finished
29

for Loops
30
Start value 0
End value 9
Increment 1
Note: Only this statement is executed with
the loop since there is no curled brackets.

• Altering list: Provides the increment value that is
added or subtracted from the counter in each
iteration of the loop
• If initializing list is missing, the counter initial value
must be provided prior to entering the for loop
• If altering list is missing, the counter must be altered
in the loop body
• Omitting the expression will result in an infinite loop
for Loops (continued)
31

for loop
flowchart.
for Loops (cont’d)
33

• These techniques are suitable for pretest loops ( for
and while):
– Interactive input within a loop
• Includes a cin statement within a while or for loop
– Selection within a loop
• Using a for or while loop to cycle through a set of
values to select those values that meet some criteria
A Closer Look: Loop Programming
Techniques
37

Techniques (continued)
38

39

40

• Nested loop: A loop contained within another loop
– All statements of the inner loop must be completely
contained within the outer loop; no overlap allowed
– Different variables must be used to control each loop
– For each single iteration of the outer loop, the inner loop
runs through all of its iterations
Nested Loops
41

Nested Loops (continued)
For each i, j loops.
42

• do while loop is a posttest loop
– Loop continues while the condition is true
– Condition is tested at the end of the loop
– Syntax:
do
statement;
while (expression);
• All statements are executed at least once in a
posttest loop
do while Loops
44

The do while
loop structure.
do while Loops
45

The do
statement’s
flow of control.
do while Loops
46

do while Loops (Examples)
47
Start value 0
End value 25
Increment 5
Last value of counter is 25

• Useful in filtering user‐entered input and providing
data validation checks
• Can enhance with if-else statement
Validity Checks
48

• Using the assignment operator (=) instead of the equality
comparison operator (==) in the condition expression
• Placing a semicolon at the end of the for clause, which
produces a null loop body
• Using commas instead of semicolons to separate items in
the for statement
• Changing the value of the control variable
• Omitting the final semicolon in a do statement
Common Programming Errors
49

Summary
• Loop: A section of repeating code, whose
repetitions are controlled by testing a condition
• Three types of loops:
– while
– for
– do while
• Pretest loop: Condition is tested at beginning of
loop; loop body may not ever execute; ex., while,
for loops
50

Summary (continued)
• Posttest loop: Condition is tested at end of loop;
loop body executes at least once; ex., do while
• Fixed‐count loop: Number of repetitions is set in
the loop condition
• Variable‐condition loop: Number of repetitions is
controlled by the value of a variable
51

Repetition, Basic loop structures, Loop programming techniques

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