1. do - while  while: Execute the body of the loop at least once
False
statements;
True
condition ?
do - while 
while:
False
statements;
True
condition ?
142 K -1

2. do-while statement do { /*play the game*/ game(); /*again?*/
printf(“Another game (yes=1,no=0)? ”);
scanf(“%i”,&answer);
if (answer==1)
go_on = TRUE;
else
go_on = FALSE; }
while(go_on);
#define constants
TRUE is 1
FALSE is 0
don’t forget
condition
Play the game at least once
142 K -2

3. for loops for(initialization; condition; update expression) {
statement1;
statement2;
... }
Don’t forget
update is written
at the front of the
loop, but is
executed
at the end
Do not
write ","
Flow Chart
initialization
True
condition ?
statement1;
statement2;
update
False
142 K -3

4. Counting in for loops /* print n asterisks */
for(count=1; count<=n; count=count+1)
printf(“*”);
initialization
update
condition
don’t need { and } if only 1 statement
What is printed?
**************
n times
Could also do:
/* print n asterisks */
for(count=0; count<n; count=count+1)
printf(“*”);
start at 0
How would you print?
*****
142 K -4

5. for(row=1; row<=nrows; row = row+1) {
*****
to print
use:
repeat 5 times
printf(“*”);
repeat 3 times
Nested for loops
for(row=1; row<=nrows; row = row+1) { }
printf(“\n”);
for(col=1;col<=ncols;col=col+1)
printf(“*”);
inner loop: print
one row of ncols *
outer loop: once a line
of ncols * is printed,
print a newline
All of this nrows times
*****
Get
with nrows=3 and ncols=5
142 K - 5

6. Another 2D figure How would you print * ** *** ? **** *****
Note that the first row contains 1 star,
the second 2 stars, the third 3 stars...
Need only one input: the number of rows,
nrows
for(row=1; row<=nrows; row = row+1) {
for(col=1; col<=row; col=col+1)
printf(“*”);
printf(“\n”); }
142 K -6

7. row at position row is made of row spaces
And what about
*****
****
*** ** * ?
Take row number 4:
made of 3(=4-1) spaces
2(=5-(4-1)) *
number the rows 0,1,2,...
row at position row is made of
row spaces
nrows - row * (in our example
above, nrows is 5)
for(row=0; row<nrows; row = row+1) {
for(col=1;col<=row;col=col+1)
printf(“ ”);
for(col=1;col<=nrows-row;col=col+1)
printf(“*”);
printf(“\n”); }
142 K -7

8. To simplify use a function
Given an integer n and a character, print the
character n times.
void repeat_char(int n, char symbol) {
int i;
for (i=0; i<n; i=i+1)
printf(“%c”,symbol); }
Then
for (row=0; row<nrows; row=row+1) {
repeat_char(row,‘ ’);
repeat_char(nrows-row,‘*’);
printf(“\n”); }
142 K -8

9. Another Example Printing a multiplication table: Print this 1 1 2 3 2 1 2 3
2*3
How?
use two nested loops
_ outer loop: Select the row to print
for (row=1;row<=nrows;row=row+1) {
printf(“\n”); }
_ inner loop: Print the row
for(col=1;col<=ncols;col=col+1)
printf(“%4i”,row*col);
142 K -9

10. Loop Pitfalls sum = 0; while (sum < 10); sum = sum + 2;
Get a never ending
loop
What is sum?
sum = 0;
for (i=0; i<=10; i=i+1);
sum = sum + i;
i is 11 thus sum is 11
What is sum?
sum = 0;
for(i=1; i!=10; i=i+2)
sum = sum + i;
Get a never
ending loop
i is never 10
What is sum?
142 K -10

11. Loop Pitfalls double x; for (x=0.0; x<10.0; x=x+0.2)
printf(“%.16f\n”,x);
Get (MSVC 6.0):
Expect:
...
...
do not use doubles as counters in loops
142 K -11

12. Better: int i; double x; for (i=0; i<50; i=i+1) {
x = (double)i/5.0;
printf(“%.16f\n”,x); }
...
142 K -12

13. The continue statement
Directly go back
to the update
for(init; condition; update) {
statements;
if(cond) continue;
more statements; }
if cond is true,
the rest of
the loop body
is skipped.
What happens here?
for(i=1; i<=10; i=i+1) {
if (i%2==0) continue;
printf(“The square of %i is %i\n”,
i,i*i); }
print the squares of the odd numbers
between 1 and 10.
However, use continue carefully and only
if it clarifies your program.
(not the case for the example above!).
Can always be avoided.
142 K -13

14. The break statement Use break in a loop to exit the loop
example: print all of the prime numbers
between 3 and 100
for(i=3; i<=100; i=i+1) {
for(j=2; j<i; j=j+1)
if(i%j==0)
break;
if(j==i-1)
printf(“%i is a prime\n”,i); }
if i can be divided by j,
exit the j loop and
check the next integer
in the i loop
use carefully
142 K -14

15. Handy Shorthand In a loop, very often use: i=i+1 or i=i-1
Instead, can use the unary operators:
++ and --
post
increment
i++ means i=i+1
i-- means i=i-1
post
decrement
Do you see the pun in C++?
Example:
for(i=1; i<=10; i++) {
... }
Note:
Also have
/=, *=, +=, -=, %=
e.g.
x += a; means x=x+a;
142 K -15

16. Warning Also have pre-increment and pre-decrement ++i and --i
What is the difference with i++ and i-- ?
None if used in isolation
But NOT the same in an expression
or an assignment.
post:
the variable is updated after being used
pre:
the variable is updated before being used
int i=0, j=0, a, b;
a = i++; /* a is 0 and i is 1 */
b = ++j; /* b is 1 and j is 1 */
USE ++ and -- in isolation
do not write x = (y+1)/(3*(x-1));
as x = ++y/(3*(--x));
y is also updated!
(before the expression is evaluated)
142 K -16

17. while, do-while, or for? Between do-while and while when we want
to execute the
loop at least once
Initialization
Test
Update
for
while
in () after for
prior to the loop
in the loop body
in () after while
Better when know
the number of
iterations
Better when don’t
know number of
iterations
142 K -17