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Chapter 5: Loops. Outline. Increment and Decrement while loop do-while loop for loop. Slide 5- 2. The Increment and Decrement Operators . ++ is the increment operator. It adds one to a variable. val ++; is the same as val = val + 1;

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slide2

Outline

  • Increment and Decrement
  • while loop
  • do-while loop
  • for loop

Slide 5- 2

slide3

The Increment and Decrement Operators

  • ++ is the increment operator. It adds one to a variable.

val++; is the same as val = val + 1;

  • ++ can be used before (prefix) or after (postfix) a variable:

++val; val++;

Slide 5- 3

slide4

The Increment and Decrement Operators

  • -- is the decrement operator. It subtracts one from a variable.

val--; is the same as val = val - 1;

  • -- can be also used before (prefix) or after (postfix) a variable:

--val; val--;

Slide 5- 4

slide5

Prefix vs. Postfix

  • ++ and -- operators can be used in complex statements and expressions
  • In prefix mode (++val, --val) the operator increments or decrements, then returns the value of the variable
  • In postfix mode (val++, val--) the operator returns the value of the variable, then increments or decrements

Slide 5- 7

slide6

Prefix vs. Postfix - Examples

int num, val = 12;

cout << val++; // displays 12,

// val is now 13;

cout << ++val; // sets val to 14,

// then displays 14

num = --val; // sets val to 13,

// stores 13 in num

num = val--; // stores 13 in num,

// sets val to 12

Slide 5- 8

slide7

Outline

  • Increment and Decrement
  • while loop
  • do-while loop
  • for loop

Slide 5- 9

slide8

Introduction to Loops: The while Loop

The general form of the while statement is:

while (expression)

statement;

OR

while (expression)

{

statement1;

statement2;

}

slide10

The process used by the computer in

  • evaluating a while statement is:
    • 1. test the expression
    • 2. if the expression has a nonzero (true) value
      • a. execute the statement following the parentheses
    • b. go back to step 1
  • else
  • exit the while statement
while is a pretest loop
while is a Pretest Loop
  • expression is evaluated before the loop executes. The following loop will never execute:

int number = 6;while (number <= 5){ cout << "Hello\n"; number++;}

an infinite loop
An Infinite Loop

int number = 1;while (number <= 5){ cout << "Hello\n";}

watch out for infinite loops
Watch Out for Infinite Loops
  • The loop must contain code to make expression become false
  • Otherwise, the loop will have no way of stopping
  • Such a loop is called an infinite loop, because it will repeat an infinite number of times
slide16

Example:

void main ( void )

{

int count = 1;

while(count <= 10)

{

cout << count << " ";

count = count + 1; //count++;

//count += 1;

}

}

Output:

1 2 3 4 5 6 7 8 9 10

slide17

What is the output from the next segment

of code?

void main ( void )

{

int i;

i = 10;

while(i >= 1 )

{

cout << i << " ";

i = i - 1; //count--; count -= 1;

}

}

Output:

10 9 8 7 6 5 4 3 2 1

the power of the while statement
The Power of the while statement
  • To illustrate the power of the while statement, consider the task of printing a table of numbers from 1 to 10 with their squares and cubes. This can be done with a simple while statement:

void main ( void )

{

intnum = 1;

//Display Heading

cout << "Number Square Cube\n"

<< "----------- --------- -------\n";

while (num < 11)

{

cout << num << " "

<< pow(num, 2) << " "

<< pow(num, 3) << " "<< endl;

num = num + 1;

}

}

the power of the while statement1
The Power of the while statement

Note: The expression (num <= 10) could have been used in place of (num < 11).

Number Square Cube

------------ ---------- -- -----------

1 1 1

2 4 8

3 9 27

4 16 64

5 25 125

6 36 216

7 49 343

8 64 512

9 81 729

10 100 1000

If we wanted to use the previous program to produce a table of 1000 numbers, all we have to do is change the expression in the while statement from (num < 11) to (num < 1001).

slide20

cin within a while loop

  • Combining interactive data entry with the repetition capabilities of the
  • while statement produces very adaptable and powerful programs. To
  • understand the concept involved consider the following program which
  • asks the user to enter a user specified amount of numbers and then
  • displays the total of those numbers.
  • Prompt user for how many numbers to be entered.
  • Prompt user for the numbers.
  • Add each number to a variable named total which accumulates the
  • total of all numbers entered.
  • Initialize total to zero before the loop begins.
  • Accumulate the numbers (total = total + number)
slide21

int main ( void )

{

int numbers_entered = 0, total = 0, number = 0;

int counter = 1;

cout << "How many numbers do you wanted added?" << endl;

cin >> numbers_entered;

while (counter <= numbers_entered)

{

cout << "Please enter a number: ";

cin >> number;

total = total + number; //total += number;

counter = counter + 1; //counter++; counter += 1;

}

cout << "\nThe total of the numbers entered is " << total;

return 0;

}

slide22

Outline

  • Increment and Decrement
  • while loop
  • do-while loop
  • for loop

Slide 5- 28

slide23

The do-while Loop

The do statement allows us to execute some statements before an expression is evaluated. The general form of the do statement is:

do

statement;

while(expression); //don’t forget the ;

OR

do

{

statement1;

statement2;

}while(expression); //don’t forget the ;

do while example
do-while Example

int x = 1;do{cout << x << endl;} while(x < 0);

Although the test expression is false, this loop will execute one time because do-while is a posttest loop.

slide28

Validity Checks (Data Validation)

The do statement is particularly useful in filtering user-entered

input and providing data validity checks. For example, assume

that an operator is required to enter a valid customer identification

number between the numbers 100 and 1999. A number outside this

range is to be rejected and a new request for a valid number made.

The following section of code provides the necessary data filter to

verify the entry of a valid identification number.

do

{

cout << "\nEnter an identification number: ";

cin >> ident_number;

}while(ident_number < 100 || ident_number > 1999);

slide29

Here, a request for an identification number is repeated

until a valid number is entered. This section of code is

"bare bones" in that it neither alerts the operator to the

cause of the new request for data nor allows premature

exit from the loop if a valid identification number cannot be

found. An alternative that removes the first drawback is

do

{

cout << "\nEnter an identification number: ";

cin >> ident_number;

if (ident_number < 100 || ident_number > 1999)

{

cout << "\nAn invalid number was just entered";

cout << "\nPlease check the ID number and re-enter.";

}

}while(ident_number < 100 || ident_number > 1999);

slide30

Outline

  • Increment and Decrement
  • while loop
  • do-while loop
  • for loop

Slide 5- 38

slide31
Loop
  • There are two types of the loop:
    • Conditional loop: executes as long as a particular condition exists (you don’t know how many times of the loop will continue).
    • Count-controlled loop: when you know the exact number of iterations that a loop must perform.
the for loop
The for Loop
  • Useful for counter-controlled loop
  • General Format:for(initialization; test; update)

statement; // or block in { }

  • No semicolon after 3rd expression or after the )
the for loop1
The for Loop
  • General Format:for(initialization; test; update)

statement; // or block in { }

  • for Loop - Mechanics
    • Performinitialization
    • Evaluate test expression
      • If true, execute statement
      • If false, terminate loop execution
    • Execute update, then re-evaluate test expression
examples
Examples
  • While

intcount = 1;

while (count <= 10)

{

cout << count ;

count++; //or count += 1; or count = count + 1;

}

  • Same loop as above but coded using the for statement:

for (int count = 1; count <= 10; count++)

cout << count;

example
Example
  • What is produced from the following segments of programs?

constint MAXNUMS = 10;

int main ( void )

{

intnum;

cout << "Number Square Cube\n"

<< "------- -------- ------\n" << endl;

for (num = 1; num <= MAXNUMS; num++)

cout << num << " "

<< num * num << " "

<< num * num * num << endl;

return 0;

}

example output
Example - Output

Number Square Cube

----------- --------- --------

1 1 1

2 4 8

3 9 27

4 16 64

5 25 125

6 36 216

7 49 343

8 64 512

9 81 729

10 100 1000

for loop pretest loop
for loop – pretest loop
  • When to Use the for Loop?
    • In any situation that clearly requires
      • an initialization
      • a false condition to stop the loop
      • an update to occur at the end of each iteration
  • The for loop tests its test expression before each iteration, so it is a pretest loop.
  • The following loop will never iterate:for (count = 11; count <= 10; count++) cout << "Hello" << endl;
for loop modifications
for Loop - Modifications
  • You can omit the initialization expression if it has already been done:

int sum = 0, num = 1;

for (; num <= 10; num++)

sum += num;

for loop modifications1
for Loop - Modifications
  • You can declare variables in the initialization expression:

int sum = 0;

for (int num = 0; num <= 10; num++)

sum += num;

The scope of the variable num is the for loop.

keeping a running total
Keeping a Running Total
  • running total: accumulated sum of numbers from each repetition of loop
  • accumulator: variable that holds running total

intsum=0, num=1; // sum is the accumulator

while (num <= 10)

{ sum += num;

num++;

}

cout << "Sum of numbers 1 – 10 is"

<< sum << endl;

slide42
int main ( void )

{

inttotal_numbers, number, total = 0 ; // total is the accumulator

cout << "How many numbers would you like to total?" << endl;

cin >> total_numbers;

for(int counter = 0; counter < total_numbers; counter++)

{

cout << "Please enter a number: " << endl;

cin >> number;

total = total + number; //or total += number;

}

cout << "\nThe total of the numbers you entered is: "<< total;

cout <<"\nThe average of the numbers you entered is: " << total/double (total_numbers);

return 0;

}

output
Output

How many numbers would you like to total?

4

Please enter a number:

56

Please enter a number:

45

Please enter a number:

85

Please enter a number:

43

The total of the numbers you entered is: 229

The average of the numbers you entered is: 57.25

sentinels
Sentinels
  • sentinel: value in a list of values that indicates end of data
  • Special value that cannot be confused with a valid value, e.g., -999 for a test score
  • Used to terminate input when user may not know how many values will be entered
slide47

Outline

  • Increment and Decrement
  • while loop
  • do-while loop
  • for loop
    • nested for loop

Slide 5- 63

nested loops
Nested Loops
  • A nested loop is a loop inside the body of another loop
  • Inner (inside), outer (outside) loops:

for (row=1; row<=3; row++) //outer

for (col=1; col<=3; col++)//inner

cout << row * col << endl;

example1
Example

for (inti = 1; i <= 5; i++) // start outer loop

{

cout << "\n i is now " << i << endl;

for (int j = 1; j <= 4; j++) // start inner loop

cout << " j = " << j; // end of inner loop

} // end of outer loop

The first loop, controlled by the value of i, is called the outer loop. The second loop, controlled by the value of j, is called the inner loop. For each single trip through the outer loop, the inner loop runs through its entire sequence. Thus, each time the i counter increases by 1, the inner for loop executes completely.

output1
Output
  • Below is the output from the previous nested loop:

i is now 1

j = 1 j = 2 j = 3 j = 4

i is now 2

j = 1 j = 2 j = 3 j = 4

i is now 3

j = 1 j = 2 j = 3 j = 4

i is now 4

j = 1 j = 2 j = 3 j = 4

i is now 5

j = 1 j = 2 j = 3 j = 4

example2
Example
  • Use a nested loop to compute the average grade for each student in a class of 20 students.
  • Each student has taken four exams during the course of the semester.
  • The final grade is calculated as the average of these examination grades.
example3
Example
  • The outer loop in our program will consist of 20 passes.
  • Each pass through the outer loop is used to compute the average for one student.
  • The inner loop will consist of 4 passes. One examination grade is entered in each inner-loop pass. As each grade is entered, it is added to the total for the student, and at the end of the loop, the average is calculated and displayed.
slide53
const int NUMGRADES = 4;

const int NUMSTUDENTS = 20;

int main ( void )

{ double grade = 0.0, total = 0.0, average = 0.0;

for (inti = 1; i <= NUMSTUDENTS; i++) // start outer loop

{

total = 0;

for (int j = 1; j <= NUMGRADES; j++) // start inner loop

{

cout<< "Enter an exam grade for this student: ";

cin>> grade;

total = total + grade;

} // end inner loop

average = total / NUMGRADES;

cout<< "\nThe average for student " << i << " is " << average << "\n\n";

} // end of outer loop

return 0;

}

slide54
Note: Pay attention to the initialization of total within the outer loop, before the inner loop in entered. Total is initialized 20 times, once for each student. Also note that the average is calculated and displayed immediately after the inner loop is finished. Because the statements that compute and print the average are also contained within the outer loop, 20 averages are calculated and displayed.
  • Output:

Enter an exam grade for this student: 99

Enter an exam grade for this student: 87

Enter an exam grade for this student: 56

Enter an exam grade for this student: 78

The average for student 1 is 80

Enter an exam grade for this student: 100

Enter an exam grade for this student: 99

Enter an exam grade for this student: 98

Enter an exam grade for this student: 97

The average for student 2 is 98.5

Etc…..

breaking out of a loop
Breaking Out of a Loop
  • The continue statement causes the program to skip the rest of the loop in the current iteration as if the end of the statement block had been reached, causing it to jump to the start of the following iteration.
  • For example, we are going to skip the number 5 in our countdown: