Flow of Control
True and False in C
Conditional Execution
Iteration
True and False in C
•
False
is represented by any
zero
value
.
– The int expression having the value 0.
– The floating expression having the value 0.0. – The null character ‘\0’.
– The NULL pointer (for pointer see chap. 8).
•
True
is represented by any
nonzero
value
.
A logical expression, such as a<b, is either
true or false.
Examples
int
i=1, j=2, k=3
;
double
x=5.5, y=7.7, z=0.0
;
i
<
j-k
i<(j-k)
0
-i+5*j
>=
k+1 ((-i)+(5*j))>=(k+1)
1
x-y
<=
j-k-1
(x-y)<=((j-k)-1)
1
x+k+7
<
y/k ((x+k)+7)<(y/k)
0
i
!=
j
1
!!
5
!(!5)
1
Selection
•
if
if statement
• Conditional Execution:
if (Boolean expression) statement; else statement;
Where a statement may consist of a single statement, a code block, or empty statement.
if – logic: if (Boolean Expression) statement_1;
If-else-logic: if (Boolean Expression){ compound_1
} else{
compound_2 }
• Conditional execution allows you write
No Semi-colon
Yes-For a
Example
#include <stdio.h>
WHY?
int main ( )
{
double salary, payrate;
int hours, union;
printf (“Enter 1 if a union member, 0 if not”);
scanf (“%d%lf%d”, &hours, &payrate, &union);
if (hours > 40 && union = = 1)
salary = (40 * payrate) + ((1.5 * payrate) * (hours - 40));
else
salary = payrate * hours;
Nested ifs
Nested: One statement coded within another statement.
Nested ifs: An nested if is an if that is the
target of another if or else.
Why Nested ifs?
Needed to handle tasks
where we have 3 or More
options that are mutually exclusive.
The if-else-if ladder
Linear nested If
•
General form:
OLD STYLE
if (expression) statement; else
if (expression) statement; else
if (expression) statement; .
• The conditions are evaluated from the top
downward.
• As soon as a true condition is found, the
statement associated with it is executed and the rest of the ladder is bypassed.
• If none of the conditions are true, the final else
is executed. That is, if all other conditional tests
fail, the last else statement is performed.
E.g.
You are a salesperson for the Widget
Manufacturing Co. You will earn a salary
bonus according to the
following rules
:
Sales > = $50,000
earn $5,000
Sales > = $100,000 earn $15,000
Sales > = $150,000
earn $30,000
double sales, bonus;
printf (“please enter total sales”); scanf (“%lf”, &sales);
if (sales < 50000)
bonus = 0;
else if (sales < 100000)
bonus = 5000;
else if (sales < 150000)
bonus = 15000;
In a nested if, an else always refers to the nearest if
that is within the same block as the else and that is not already associated with an else.
if(i) {
if(j) dosomething1();
if(k) dosomething2(); /* this if */
else dosomething3(); /* goes with this else */ }
Conditional Expression
• The expressions must simply evaluate to
either a
true or false
(zero or nonzero)
value.
x = 10;
y =
x>9 ? 100 : 200;
x = 10;
if
(x>9)
y = 100;
else
y = 200;
The ?: Alternative
#include <stdio.h> int f1(int n);
int f2(void);
int main(void) {
int t;
printf("Enter a number: "); scanf("%d", &t);
t ? f1(t) + f2() : printf("zero entered."); printf("\n");
/* Divide the first number by the second. */ #include <stdio.h>
int main(void) {
int a, b;
printf("Enter two numbers: "); scanf(''%d%d", &a, &b);
if(b)
printf("%d\n", a/b);else printf("Cannot divide by zero.\n");
return 0;
switch statement
•
switch
is a
multiple-branch selection
statement, which successively tests the
value of an expression against a list of
integer or character constants
(floating
point expression, for example, are not
allowed).
• When a match is found, the statements
General Form
switch (expression) { case constant1:
statement sequence break;
case constant2:
statement sequence break;
. .
default
Execution
• The value of the expression is tested against the constants specified in the case statements in a
top-down order..
• When a match is found, the statement sequence associated with that case is executed until the break statement or the end of the switch
statement is reached.
• When break is encountered in a switch, program execution "jumps" to the line of code following the switch statement.
• The switch differs from the if in that switch can only test for equality, whereas if can evaluate any type of relational or logical expression.
• No two case constants in the same switch can
have identical values. Of course, a switch
statement enclosed by an outer switch may have
case constants that are in common.
• If character constants are used in the switch
• The switch statement is often used to process keyboard commands, such as menu selection. The following function will when called: display the options, allow the user to make a selection, and then
evoke the appropriate function to perform the task selected.
void menu(void) {
char ch;
printf("1. Check Spelling\n");
printf(''2. Correct Spelling Errors\n"); printf("3. Display Spelling Errors\n");
printf("Strike Any Other Key to Skip\n"); printf(" Enter your choice: ");
24
switch(ch) {
case '1':
check_spelling (); break;
case '2':
correct_errors (); break;
case '3':
display_errors (); break;
default :
int flag, k; /* Assume k is initialized */ flag = -1;
switch(k) {
case 1: /* These cases have common */ case 2: /* statement sequences. */
case 3:
flag = 0; break; case 4:
flag = 1; case 5:
error(flag); break;
default: •The break inside
the switch is optional.
•If the break is omitted,
execution will continue on into the next case until either a break or the end of the
Nested Switch
• You can have a
switch
as a part of the
statement sequence of an outer switch.
• Even if the
case
constants of the inner and
the outer
switch
contain common values,
switch(x) {
case 1:
switch
(y) {
case 0:
printf(''Divide by zero error.\n");
break;
case 1:
process(x, y);
break;
}
Iteration
• Iteration statements (also called
loops
)
allow a set of instructions to be
repeatedly executed until a certain
condition is reached.
for loop
for (initialization; testing; increment)
Loop Body;
• The initialization is an assignment statement
that is used to set the loop control variable.
• The testing is a relational expression that determines when the loop exits.
Execution
• The
for
loop
continues to execute
as long
as the condition is
true.
• Once the condition becomes false, program
execution resumes on the statement
#include <stdio.h>
int main(void) {
int x;
for(x=1; x <= 100; x++)
printf("%d ", x);
return 0; }
for(x=100; x != 65; x -= 5)
{
The Elements of the For-loop
• The initialization, testing and
incrementation can be any valid C
expression.
for (
x=0
; Ajax>Manchester;
Ajax=Q*7/i)
• Common use as a
counting loop
.
•
Pieces of the loop definition need not be
there.
• The
i
ncrementation
of the loop control
variable can occur
outside the
for
statement.
for(x=0; x != 123; ) scanf("%d", &x);
for( x=1 ; x < 10; ) {
The Infinite Loop
• Since none of the three expressions that
form the
for
loop are required, you can
make an
endless loop
by leaving the
conditional expression empty.
for( ; ; ) printf("This loop will run forever.\n");
for( ; ; ) {
ch = getchar(); /* get a character */ if(ch == 'A') break; /* exit the loop */
For Loops With No Bodies
• A loop body may be
empty.
• This fact is used to simplify the coding of
certain algorithms and to create time delay
loops.
•
Does what
?
Declaring Variables within a For
Loop
• A variable so declared has its
scope
limited
to the block of code controlled by
that statement.
/* i is local to for loop; j is known outside loop.*/ int j;
While Loop
• General form:
while(condition)
statement;
• Execution:
– Check the test condition at the top of the loop. – The loop iterates while the condition is true.
Example
char wait_for_char(void)
{
char ch;
ch = '\0'; /* initialize ch */
while(ch !
= 'A') ch = getchar();
Example 2 void func1()
{int process1(void);
int process2(void); int process3(void);int working; working = 1; while (working) {
working = process1(); if (working)
working = process2(); if (working)
For loop Vs While Loop
A
-(Assignment),
T
-(testing),
I
-(Increment)
for
(A; T; I)
{
Body;
}
A;
While (T)
{
NESTED LOOPS
Nested 1 loop syntax coded inside another
Loop syntax.
Why?
- Single -Loop? Have a statement or statements that youwant to repeat. A
repetitive task that you must solve.
Why?
- Nested-Loops? You have a single -General Format:
while (boolean expression){
while (boolean expression) {
}
}
/* Find triples of integers that add up to n. */ #include <stdio.h>
#define N 7
main()
{ int cnt = 0, j , k , m;
for(j = 0; j <= N; ++j)
for( k = 0; k <= N; ++k)
for( m = 0: m <= N; ++m)
if ( j + k + m == N) { ++cnt;
printf(“%d%d%d”, j , k , m); }
How many times will “if” be executed
?
512 timesj range 0
7
k range 0
7
m range 0
7
What will the values of j, k, and m be in sequence
J K M
0 0 0
0 0 1
0 0 2
0 0 7
0 1 0
0 1 1
0 1 2
0
2
0
0
2
1
0
2
7
Cont...
do-while Loop
•
General form:
do {
statement
;
} while(
condition
);
•
Execution:
– Always executes at least once.
– Iterates until condition becomes false.
•The most common use of the
do-while
loop is in a
menu selection
function.
void menu(void)
{
char ch;
printf("1. Check Spelling\n");
do {
ch = getchar(); /* read the selection from the keyboard */
switch(ch) { case '1':
check_spelling(); break;
case '2':
correct_errors(); break;
case '3':
display_errors(); break;
Jump
•
break
•
continue
break Statement
Two uses:
You can use it to
terminate
a case in the switch
statement
.
You can also use it to force immediate termination of a loop, bypassing the normal
#include <stdio.h>
int main (void) {
int t;
for(t=0; t < 100; t++) { printf(''%d ", t);
if(t == 10) break; }
•A
break
causes an exit from only the
innermost loop
.
for
(t=0; t < 100; ++t) {
count = 1;
for(;;)
{
printf(''%d ", count);
count++;
if(count == 10)
break
;
}
}
Breaking out of loops
•
Sometimes loops need to be cut short:
– You can break out of any loop with break ;
statement.
– A break will cause the loop to be aborted immediately.
– The next statement executed is the statement following the loop.
• Compare this with
return
:
Continue Statement
• General form
continue;
•
break
forces termination of the loop.
•
continue
can expedite the exit from a loop
by
forcing the conditional test to be
performed sooner.
– For the for loop, continue causes the increment and then the conditional test portions of the loop to execute.
void code(void)
{
char done, ch;
done = 0;
while(!done)
{
ch = getchar();
if(ch == ‘S') {
done = 1;
continue
;
}
/* test the condition now */
putchar
(ch+1);
This function codes a message by shifting all characters you type
one letter higher. For example, an A
becomes a B. The
