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Structured-Programming Summary. All programs broken down intoSequenceSelectionif, if/else, or switch Any selection can be rewritten as an if statementRepetitionwhile, do/while, or forAny repetition structure can be rewritten as a while statement. . while Repetition Structure. Repetition contr
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1. Looping Lecture 4
2. Structured-Programming Summary All programs broken down into
Sequence
Selection
if, if/else, or switch
Any selection can be rewritten as an if statement
Repetition
while, do/while, or for
Any repetition structure can be rewritten as a while statement
3. while Repetition Structure Repetition control structure
Action repeated while some condition remains true
Psuedocode
while there are more items on my shopping list
Purchase next item and cross it off my list
while loop repeated until condition becomes false
Example
int product = 2;
while ( product <= 1000 )
product = 2 * product;
Two varieties: event-controlled and count-controlled
4. A While Statement SYNTAX
while ( Expression )
{ .
. // loop body
.
}
NOTE: Loop body can be a single statement, a null statement, or a block.
5. Repetition Structure Elements Repetition structure has three required elements:
Statement to be repeated
Condition to be evaluated
Determines loop termination
Initial value for the condition
Repetition statements include
while
for
do while
6. Basic Loop Structures The condition can be tested
At the beginning: pretest or entrance-controlled loop
The condition can be tested
At the end: posttest or exit-controlled loop
Something in the loop body must cause the condition to change in order to avoid an infinite loop (one that never terminates)
7. Pre-test Symbology Pretest loop: condition is tested first; if false, statements in the loop body are never executed
while and for loops are pretest loops
8. Post-test Symbology
9. When the expression is tested and found to be false, the loop exits and control passes to the statement which follows the loop body.
WHILE LOOP
10. Number of Loop Iterations There are essentially two kinds of loop iterations:
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
11. Two Types of Loops count controlled loops
repeat a specified number of times
event-controlled loops
some condition within the loop body changes and this causes the repeating to stop
12. an initialization of the loop control variable
an expression to test for continuing the loop
an update of the loop control variable to be executed with each iteration of the body
13. int count ;
count = 4; // initialize loop variable
while (count > 0) // test expression
{
cout << count << endl ; // repeated action
count -- ; // update loop variable
}
cout << “Done” << endl ; Count-controlled Loop
14. 14 Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
15. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
16. Count-controlled Loop int count ;
count = 4;
while (count > 0) TRUE
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
17. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
18. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
19. Count-controlled Loop int count ;
count = 4;
while (count > 0) TRUE
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
20. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
21. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
22. Count-controlled Loop int count ;
count = 4;
while (count > 0) TRUE
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
23. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
24. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
25. Count-controlled Loop int count ;
count = 4;
while (count > 0) TRUE
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
26. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
27. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
28. Count-controlled Loop int count ;
count = 4;
while (count > 0) FALSE
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
29. Count-controlled Loop int count ;
count = 4;
while (count > 0)
{
cout << count << endl ;
count -- ;
}
cout << “Done” << endl ;
30. Event-controlled Loops 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
31. Examples of Count-controlled and EOF Loops
32. Examples of Sentinel and Flag Loops
33. A Sentinel-controlled Loop requires a “priming read”
“priming read” means you read one set of data before the while
34. // Sentinel controlled loop
total = 0;
cout << “Enter a blood pressure (-1 to stop ) ”;
cin >> thisBP; // The “priming read”
while (thisBP != -1) // while not sentinel
{
total = total + thisBP;
cout << “Enter a blood pressure (-1 to stop ) ”;
cin >> thisBP;
}
cout << total;
35. End-of-File Controlled Loop depends on the fact that a file goes into fail state when you try to read a data value beyond the end of the file
36. // End-of-file controlled loop
37. //End-of-file at keyboard total = 0;
cout << “Enter blood pressure (Ctrl-Z to stop)”;
cin >> thisBP; // priming read
while (cin) // while last read successful; or while (!cin.eof())
{
total = total + thisBP;
cout << “Enter blood pressure”;
cin >> thisBP; // read another
}
cout << total;
38. Flag-controlled Loops you initialize a flag (to true or false)
use a meaningful name for the flag
a condition in the loop body changes the value of the flag
test for the flag in the loop test expression
39. countGoodReadings = 0;
bool isSafe = true; // initialize Boolean flag
while (isSafe)
{
cin >> thisBP;
if ( thisBP >= 200 )
isSafe = false; // change flag value
else
countGoodReadings++;
}
cout << countGoodReadings << endl;
40. Loops are often used to: count all data values
count special data values
sum data values
keep track of previous and current values, etc.
41. initialize outer loop
while ( outer loop condition )
{ . . .
initialize inner loop
while ( inner loop condition )
{
inner loop processing and update
} // end inner loop
. . .
} // end outer loop Pattern of a Nested Loop
45. Trace of Program Variables count price kind total
0.0
1 3.98 ‘P’ 3.98
2 7.41 ‘H’ 11.39
3 8.79 ‘P’ 20.18
4 etc.
20
21 so loop terminates
46. for Loops
47. for Loops (continued) Altering list: provides the increment value that is added or subtracted from the counter in each iteration of the loop
If altering list is missing, the counter must be altered in the loop body
If initializing list is missing, the counter initial value must be provided prior to entering the for loop
Omitting the expression will result in an infinite loop
48. for Loop Example
49. for Loop Symbology
50. In-Class Exercise (follows) Based on following two programs…
51. In-Class Exercise: count.cpp // Program Count prompts for, reads, echo prints, and sums a
// fixed number of integer values. The sum is printed.
#include <iostream>
using namespace std;
const int LIMIT = 10;
int main () {
int counter; // loop-control variable
int sum; // summing variable
int dataValue; // input value
counter = 1;
sum = 0;
// Input and sum integer data values.
while (counter <= LIMIT) {
cout << "Enter an integer value. Press return." << endl;
cin >> dataValue;
sum = sum + dataValue;
counter++;
}
cout << "Sum is " << sum << endl;
return 0;
}
52. In-Class Exercise: count2.cpp // Program Count2 prompts for, reads, and sums integer
// values until a negative number is read. The input
// values and the sum are printed.
#include <iostream>
using namespace std;
int main () {
int sum; // summing variable
int dataValue; // input value
sum = 0;
cout << "To stop processing, enter a negative value." << endl;
cout << " Enter an integer value; press return." << endl;
cin >> dataValue; // Read first data value to prepare for loop.
// Input and sum integer data values
while (dataValue > 0) {
sum = sum + dataValue;
cout << "Enter an integer value; press return." << endl;
cin >> dataValue;
}
cout << "Sum is " << sum << endl;
return 0;
}
53. Exercises: What is printed if the following data values are entered as prompted for count.cpp? 8 5 3 -2 0 9 1 7 3 10
Is the loop in program count.cpp a count-controlled loop or an event-controlled loop? Explain.
What is printed if the following data values are entered one per line for count2.cpp? 8 5 3 -2 0 9 1 7 3 10
Is the loop in program count2.cpp a count-controlled loop or an event-controlled loop? Explain.
Change count.cpp so that the number of values to be read (LIMIT) is input from the keyboard rather than being set as a named constant. Please don’t forget to prompt for limit.
