of Flow Control
True and False in C
Conditional Execution
Iteration
Nested Code(Nested-ifs, Nested-loops)
Jumps
1
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.
This expression yields the int value 1 if it is
true or the int value 0 if it is false.
2
Examples
int i=1, j=2, k=3;
double x=5.5, y=7.7, z=0.0;
i<j-k
i<(j-k)
-i+5*j>=k+1
((-i)+(5*j))>=(k+1)
x-y<=j-k-1
(x-y)<=((j-k)-1)
x+k+7<y/k
((x+k)+7)<(y/k)
i!=j
!!5
!(!5)
!i-j+4
((!i)-j)+4
x||i&&j-2
x||(i&&(j-2))
0
1
1
0
1
1
2
1
3
Selection
if
switch
4
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.
The else clause is optional.
5
if logic:
If-else-logic:
if (Boolean Expression)
statement_1;
YesFor a
Semi-colon
if (Boolean Expression){
compound_1
}
No Semi-colon
else{
compound_2
}
Conditional execution allows you write
code that reacts to tested conditions.
6
Example
#include <stdio.h>
WHY?
int main ( )
{
double salary, payrate;
int
hours, union;
printf (Please enter hours worked, payrate,
and union status);
7
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;
printf (Salary is: $ % 6.2 f, salary);
}
8
Nested ifs
Nested:
Nested ifs:
One statement coded within
another statement.
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.
ANSI C specifies that at least 15 levels of nesting
must be supported by the compiler.
9
The if-else-if ladder
Linear nested If
General form:
OLD STYLE
if (expression) statement;
else
if (expression) statement;
else
if (expression) statement;
.
.
.
else statement;
10
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.
If the final else is not present, no action takes
place if all other conditions are false.
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Example
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
Sales > = $100,000
Sales > = $150,000
earn $5,000
earn $15,000
earn $30,000
12
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;
else
bonus = 30000;
13
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 */
}
else dosomething4(); /* associated with if(i) */
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Conditional Expression
The expressions must simply evaluate to
either a true or false (zero or nonzero)
value.
The expressions are not restricted to
involving the relational and logical
operators.
15
The ?: Alternative
Exp1 ? Exp2: Exp3
x = 10;
y = x>9 ? 100 : 200;
x = 10;
if(x>9)
y = 100;
else
y = 200;
16
#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");
return 0;
}
17
/* 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);
VALID?
else printf("Cannot divide by zero.\n");
return 0;
}
18
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
associated with that constant are executed.
19
General Form
switch (expression) {
case constant1:
statementsequence
break;
case constant2:
statementsequence
break;
.
.
default
statementsequence
} //ANSI C - at least 257 case constants.
20
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 default statement is executed if no matches
are found.
21
The default is optional.
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
statement, they are automatically converted to
integers (as is specified by C's type conversion
rules).
22
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: ");
ch = getchar(); /* read the selection from the keyboard */
23
switch(ch) {
case '1':
check_spelling ();
break;
case '2':
correct_errors ();
break;
case '3':
display_errors ();
break;
default :
printf("No option selected")
;
}
24
int flag, k;
The break inside flag = -1;
the switch is
optional.
/* Assume k is initialized */
switch(k) {
case 1: /* These cases have common */
case 2: /* statement sequences. */
If the break is
case 3:
flag = 0;
omitted,
break;
execution will
case 4:
continue on into
flag = 1;
the next case until case 5:
either a break or
error(flag);
the end of the
break;
switch is reached. default:
process(k);
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}
Nested Switch
You can have a switch as a part of the
statement sequence of an outer Even switch.
if the case constants of the inner and
the outer switch contain common values,
no conflict arise.
26
switch(x) {
case 1:
switch(y) {
case 0:
printf(''Divide by zero error.\n");
break;
case 1:
process(x, y);
break;
}
break;
case 2:
.
27
Iteration
Iteration statements (also called loops)
allow a set of instructions to be
repeatedly executed until a certain
condition is reached.
This condition may be predetermined
(as in the for and while loop) or open
ended (as do-while loops).
28
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.
The increment defines how the loop control
variable changes each time the loop is repeated.
29
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
following the body of the for loop.
30
#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)
{
z = x*x;
printf(''The square of %d, %d", x, z);
}
31
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.
for (count=0; count <n; count=count+1)
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Pieces of the loop definition need not be
there.
for(x=0; x != 123; ) scanf("%d", &x);
The incrementation of the loop control
variable can occur outside the for
statement.
for( x=1 ; x < 10; )
{
printf("%d", x);
++x;
}
33
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( ; ; )
Terminate
the infinite
loop
printf("This loop will run forever.\n");
for( ; ; ) {
ch = getchar();
/* get a character */
if(ch == 'A') break; /* exit the loop */
}
34
printf("you typed an A");
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?
for(t=0; t < SOME_VALUE; t++ ) ;
35
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;
for( int i = 0; i<10; i++)
j = i * i;
i = 10;
/*** Error ***-- i not known here! */
36
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.
When the condition becomes false, program control
passes to the line of code immediately following the
loop.
37
Example
char wait_for_char(void)
{
char ch;
ch = '\0'; /* initialize ch */
while(ch != 'A') ch = getchar();
return ch;
}
while((ch=getchar()) != 'A') ;
38
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)
working = process3();
}
}
39
For loop Vs While Loop
A-(Assignment), T-(testing), I-(Increment)
for (A; T; I)
{
Body;
}
A;
While (T)
{
Body;
I;
}
40
NESTED LOOPS
Nested
Loop
Why?-
1 loop syntax coded inside another
syntax.
Single -Loop?
Why? - Nested-Loops?
Have a statement or
statements that you
want to repeat. A
repetitive task
that you must solve.
You have a single repetitive task - THAT
YOU MUST REPEAT
i.e., a repetition of an
already repetitive task.41
General Format:
while (boolean expression){
while (boolean expression) {
}
}
You may have any combination of the 3 loop syntax
inside each other. The problem dictates what
combination should be used.
42
/* 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);
}
printf(\n Count: %d\n, cnt);
}
43
How many times will if be executed?
512 times
j range 0
k range 0
m range 0
7
7
7
8 * 8 * 8 = 512
44
What will the values of j, k, and m be in sequence
J
0
0
0
K
0
0
0
M
0
1
2
0
0
7
0
0
0
1
1
1
0
1
2
0
1
7
45
Cont...
0
0
2
2
0
1
0
2
7
.
.
.
46
do-while Loop
General form:
do {
statement;
} while(condition);
Execution:
Always executes at least once.
Iterates until condition becomes false.
do {
scanf(''%d", &num);
} while(num > 100);
47
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");
printf("2. Correct Spelling Errors\n");
printf("3. Display Spelling Errors\n");
printf("
Enter your choice: ");
48
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;
}
} while(ch=='1' || ch=='2' || ch=='3');
}
49
Jump
break
continue
return (will be introduced in Ch. 4,
Functions).
50
break Statement
#include <stdio.h>
Two uses:
You can use it to terminate
a case in the switch
statement.
int main (void)
{
int t;
for(t=0; t < 100; t++) {
printf(''%d ", t);
if(t == 10) break;
}
You can also use it to force
immediate termination of a
return 0;
loop, bypassing the normal
}
loop conditional test.
0 1 2 3 4 5 6 7 8 9 10 51
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;
}
}
123456789123456789
100 times
52
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:
A return will cause a function to be aborted
immediately.
Control returns to the calling function.
53
Continue Statement
General form
continue;
break forces termination of the loop.
continue forces the next iteration of the
loop to take place, skipping any code in
between.
54
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.
For the while and do-while loops, program
control passes to the conditional tests.
55
This function codes a
void code(void)
message by shifting all
{
characters you type
char done, ch;
one letter higher. For
done = 0;
example, an A
while(!done) {
becomes a B. The
ch = getchar();
function will terminate
if(ch == S') {
when you type a S.
done = 1;
continue; } /* test the condition now */
putchar(ch+1);
}
56