1. Programming
Loops (continued)

2. The for Statement Syntax for (ForInit; ForExpression; PostExpression)
Action
How it works:
Execute ForInit statement
While ForExpression is true
Execute Action
Execute PostExpression
Example
int i;
for(i=1; i<=20; i++)
cout << "i is " << i << endl;

3. Iteration Using the for Statement
ForInit
ForExpression
true
false
Action
PostExpression

4. N! (for) int number, factorial, n; cout << "Enter number: ";
cin >> number;
factorial = 1;
for(n=1; n<=number; n++)
factorial *= n;
cout << "The factorial of " << number
<< " is " << factorial << endl;

5. 2N (for) int number, result, n; cout << "Enter number: ";
cin >> number;
result = 1;
for(n=1; n<=number; n++)
result *= 2;
cout << "Two raised to the " << number
<< " power is " << result << endl;

6. Iteration
Key Points
[stopping criterion] Make sure there is a statement that will eventually stop the loop
[initialization]Make sure to initialize loop counters correctly
[side effect] Loop counters should not be modified in the loop

7. The Lazy Student Problem
Four students working together on HW1
Problem will take 200 hours of work that can be done over the next few weeks
The laziest student convinces the other three students to draw straws
Each straw is marked with an amount
The amount represents both the number of days and the numbers of hours per day that the student would work
Example
If the straw was marked three then the student who drew it would work for three hours per day for three days
What are the best markings of the straws for a clever, lazy student?

8. Observations Need to find sets of numbers a, b, c, and d such that
a2 + b2 + c2 + d2 = 200
Maximal legal number is 14 as 152 equals 225
which is greater than 200
Minimal legal number is 1
No advantage to listing the combinations more than once
Implication
Generate the solutions systematically
We will make sure that a <= b <= c <= d

9. Method Generate all possibilities for a where for each a possibility
Generate all possibilities of b where for each b possibility
Generate all possibilities for c where for each c possibility
Generate all possibilities for d where for each d possibility
Determine whether the current combination is a solution

10. Nested For Loop Solution
int a, b, c, d;
for(a=1; a<=14; a++) {
for(b=a; b<=14; b++) {
for(c=b; c<=14; c++) {
for(d=c; d<=14; d++) {
if(a*a + b*b + c*c + d*d == 200)
cout << a << " " << b << " " << c
<< " " << d << endl; }

11. The Do-While Statement
Syntax
do Action
while (Expression)
How it works:
Execute Action
if Expression is true then execute Action again
Repeat this process until Expression evaluates to false
Action is either a single statement or a group of statements within braces
Action
true
Expression
false

12. N! (do-while) int number, factorial, n;
cout << "Enter number: ";
cin >> number;
factorial = 1;
n = 1;
do{
factorial *= n;
n++;
}while(n <= number);
cout << "The factorial of " << number
<< " is " << factorial << endl;

13. 2N (do-while) int number, result, n; cout << "Enter number: ";
cin >> number;
result = 1;
n = 1;
do{
result *= 2;
n++;
}while (n <= number);
cout << "Two raised to the " << number
<< " power is " << result << endl;

14. Maximum (do-while) int value; //input value int max=0; //maximum value
cout << "Enter a value (-1 to stop): ";
cin >> value;
if(value > max)
max = value;
}while(value!=-1);
cout << "The maximum value found is "
<< " is " << max << endl;

15. Waiting for a Reply char reply; do{ cout << "Continue(y/n): ";
cin >> reply;
//do something
}while(reply!='n');

16. Which Loop to Use? For loop While loop Do-while loop
Best for calculations that are repeated a fixed number of times using a value that is changed by an equal amount (usually 1) each time through the loop.
While loop
You want to repeat a segment of code without knowing exactly how many times it will be repeated.
You are working with user input
There are situations when the code segment should not be executed at all.
Do-while loop
The code segment should always be executed at least once.
Otherwise, the situations when do-while loops are used are very similar to those when while loops are used.

17. A simpler lazy student problem
three straws: a, b, c
minimum value = 1
maximum value = 3
a <= b <= c
for (a=1; a<=3; a++)
for (b=a; b<=3; b++)
for (c=b; b<=3; c++)
cout << a << b << c << endl;

18. How to Stop a Loop
Known number of iterations before the loop stops (for)
Test for a user-controlled condition before or after each iteration (while, do-while)
Use the break command.

19. break
The break command is the same as the one used previously in switch.
break leaves the current loop immediately. It is recommended that break be used for situations where the loop needs to be terminated immediately (e.g., due to user intervention or if a fatal error occurs).

20. Maximum (while with break)
int value=0; //input value
int max=0; //maximum value
while(true){
cout << "Enter a value (-1 to stop): ";
cin >> value;
if(value > max)
max = value;
if(value==-1)
break; }
cout << "The maximum value found is "
<< " is " << max << endl;
MAX

21. Common Loop Errors while(balance != 0.0); {
balance = balance - amount; }
This will lead to an infinite loop!
for(n=1; n<=count; n++);
cout << "hello" << endl;
"hello" only printed once!

22. Common Loop Errors (with correct indentation and curly braces)
while(balance != 0.0) {
; // do nothing infinitely !! }
balance = balance - amount;
This will lead to an infinite loop!
for(n=1; n<=count; n++) {
; // do nothing n times!!
cout << "hello" << endl;
"hello" only printed once!

23. Common Loop Errors while(balance != 0.0){ balance = balance - amount;}
balance may not become equal zero due to numerical inaccuracies
int power;
while(power <= 1000){
cout << "Next power of N is " << power << endl;
power *= n;
Be sure to initialize to 0 a variable used for sums
Be sure to initialize to 1 a variable used for products

24. Nested Loops
Nested loops are loops within loops. They are similar in principle to nested if and if-else statements.
Many applications require nested loops.

25. Nested Loops // Find the average score on 8 lab assignments
int n, lastlab=8;
double avg, score, tscore;
char resp;
do{
tscore = 0;
for(n=1; n<=lastlab; n++){
cout << "Enter student’s score for lab " << n << ": ";
cin >> score;
tscore += score; }
avg = tscore/double(lastlab);
cout << "The average score is " << avg << endl;
cout << "Enter another student (y/n)? ";
cin >> resp;
}while(resp=='y' || resp=='Y');

26. Diamond Pattern Print out the following diamond pattern * * * *
* * * * *
* * * * * * *
* * * * * * * * *

27. Diamond Pattern Subproblem: Print out upper half:
print out the upper half
print out the lower half
Print out upper half:
row 1: print 4 spaces, 1 star;
row 2: print 3 spaces, 3 stars;
row 3: print 2 spaces, 5 stars;
row 4: print 1 space, 7 stars;
row 5: print 0 spaces, 9 stars;
Algorithm Refinement:
row 1: print (5-row) spaces, (2*row - 1) stars;
row 2: print (5-row) spaces, (2*row - 1) stars;
row 3: print (5-row) spaces, (2*row - 1) stars;
row 4: print (5-row) spaces, (2*row - 1) stars;
row 5: print (5-row) spaces, (2*row - 1) stars;

28. Diamond Pattern int row, space, star; // loop counters
for(row=1; row<=5; row++){ //top half
for(space=1; space<=5-row; space++)
cout << " ";
for(star=1; star<=2*row-1; star++)
cout << "*";
cout << endl ; }
for(row=4; row>=1; row--){ //bottom half

29. Multiplication Table // Program to output the // multiplication table
int i; //Outer loop counter
int j; //Inner loop counter
for(i=1; i<=10; i++){
for(j=1; j<=10; j++)
cout << i*j << " ";
cout << endl; }