Controlling execution iteration a lecture for the c++ Course

}

A summing program

• We wish to write a program that will calculate the sum of the integers between 1 and 20.

// A Summing Program using a while/do Loop

#include

using namespace std;

int main(){

int sum, count ;

sum = 0;

count = 1;

while (count <=20){

sum = sum + count;

count = count + 1;

}

cout << "The sum of the integers from 1 through 20 is ";

cout << sum << endl;

return 0;

}

The variables sum and count are accumulators.

A summing program

• This program produces the same output. Which is better?

// A Summing Program using a for loop

#include

using namespace std;

int main(){

int sum, count ;

sum = 0;

for (count=1; count<=20; count++)

sum = sum + count;

cout << "The sum of the integers from 1 through 20 is ";

cout << sum << endl;

return 0;

}

Just an average program

• We have already seen a program that calculates the average of 3 particular numbers. That’s not very useful because it is not general enough.
• We can do better now that we know how to loop. Here are some runs to show how the output from this program should look.

Just an average program

• This program uses a for loop:

#include

int main(){

int n, count;

float x, sum, avg;

sum = 0;

cout << "How many numbers? ";

cin >> n;

for (count=1; count<=n; count++){

cout << "? ";

cin >> x;

sum = sum + x;

}

avg = sum / n;

cout << "The average is " << avg << endl;

return 0

}

Do we really need to ask the user to count? Counting is something computers do really well!

Using a sentinel value

#include

int main(){

int count;

float x, sum, avg;

const float EOD = -999; //end of data indicator

count = 0;

sum = 0;

cout << “First number?”; cin >> x;

while (x != EOD) {

sum += x;

count++;

cout << "? "; cin >> x;

}

avg = sum / count;

cout << "The average is " << avg << endl;

return 0;

}

Run this program yourself to see how it works.

Choosing a loop structure

• When to use the while structure? When to use for?
• How about do/while?

Branching – emergency use only

• Discuss conditional vs. unconditional branching statements. The goto statement. Branching instructions should be used sparingly, only when absolutely necessary.
• We will never use the unconditional goto statement, so we won’t even cover it here.
• Conditional branching statements:
• break
• continue

break

• The break statement is used to break out of a loop. Control is transferred to the first statement after the end of the loop.
• Example. What is output?

// modified from the Deitel book

// Using the break statement in a for structure

#include

using namespace std;

int main(){

int x;

for (x=1; x<=10; x++) {

if (x==5)

break; //break loop only if x is 5

cout << x << " ";

} //end for

cout << "\nBroke out of loop at x = " << x << endl;

cout<< endl << endl << endl;

return 0;

}

Example using break

Continue

• The continue statement skips the remainder of the body of the loop, and continues the loop. Transfers control to “loop again.”

// This program is modified from the Deitel book,