1. Iterations Very Useful: Ability to repeat a block of code Example:
summing numbers entered at the keyboard
Example:
Could do
int sum;
int x1,x2,x3,x4;
printf(“Enter 4 numbers: ”);
scanf(“%d%d%d%d%d”,&x1,&x2,&x3,&x4);
sum = x1 + x2 + x3 + x4;
OK, but what if we had 100 numbers!
Prefer:
sum =0 No
get x
Use
while
statement
sum = sum + x
are we
done ?
Yes
142 J -1

2. While Statement int sum=0,x,count=1; printf(“Enter 4 numbers: ”);
while(count<=4) {
scanf(“%d”,&x);
sum = sum + x;
count = count + 1; }
test expression
body of the loop
incrementing the
counter variable
Or more generally:
int sum=0,x,count=1;
int number_of_entries;
printf(“How many numbers? ”);
scanf(“%d”,&number_of_entries);
while (count<=number_of_entries) {
printf(“Enter entry %d: ”,count);
scanf(“%d”,&x);
sum = sum + x;
count = count + 1; }
142 J -2

3. While Syntax while (condition) { statement1; statement2; ...}
the loop body is
executed as long as the
condition is true
Flow Chart
True
condition ?
statement1;
statement2;
False
Note:
if the condition is FALSE, the
body of the loop is NOT executed
(not even once)
142 J -3

4. Examples Computing factorial n: n! = n(n-1)(n-2)(n-3)...1
e.g. 9! = 9*8*7*6*5*4*3*2*1=362880
How?
i>1
get n
fact_n = 1
Yes No
i=n
fact_n = fact_n*i
i = i - 1
142 J -4

5. n! in C int n; int i; int fact_n;
printf(“Enter a positive integer: ”);
scanf(“%d”,&n);
i=n;
fact_n=1;
while(i>1) {
fact_n = fact_n * i;
i = i - 1; }
142 J - 5

6. Another example Goal: compute the time needed to double the
amount of money on a 5% interest
account
With a while loop
get the amount
goal = 2*amount
year = 0
Yes
5% increase
year = year + 1
money<goal No
142 J -6

7. In C double money; double goal; int year=0;
printf(“Enter the amount of money: ”);
scanf(“%lf”,&money);
goal = 2.*money;
while(money<goal) {
money = money *money;
year = year + 1; }
printf(“My money will double in %d years”,
year);
142 J -7

8. Sentinel controlled loop
Motivation:
e.g, a program inputs grades from a class
with a loop (e.g.a scanf statement within
a while loop).
How to end the loop when we don’t know the
number of students in the class?
input a value that can’t be a grade (e.g. -1)
and have the condition tests for it
scanf(“%lf”,&grade);
while(grade != -1) {
... }
sentinel
value
Get data
Process data
Get new data No
sentinel
value?
Yes
142 J -8

9. sentinel controlled loop
Example of a
sentinel controlled loop
Find the average of a list of positive
numbers:
printf
(“Enter a positive number (or -1 to end): ”);
scanf(“%d”,&n);
sum = 0;
count = 0;
while(n!=-1) {
sum = sum + n;
count = count + 1;
(Enter a positive number (or -1 to end): ”); }
if (count!=0)
printf(“average=%f”,
(double)sum/(double)count);
sentinel
142 I -9

10. A command interpreter Read in commands and execute them
input command (single character)
if a, execute command a
if b, execute command b
if q, exit (quit)
How?
have a variable done set to FALSE
while not done
read the user’s input
select the proper execution
if quit, set done to TRUE
142 J -10

11. Command interpreter in C
#define FALSE 0
#define TRUE 1
int main(void){
char command;
int done = FALSE;
while(!done){
printf(“Input command: ”);
scanf(“%c”,&command);
switch(command){
case ‘a’:
case ‘A’:
command_a();
break;
case ‘b’:
case ‘B’:
command_b();
case ‘q’:
case ‘Q’:
done = TRUE;
default:
printf(“What?\n”);
} /*end of switch */
}/*end of while */
return 0; }
142 J -11