Chapter 10
Control Structures
Chapter 10: Control Structures
1
Flow of Control
Sequence
Selection
Repetition
No
action-1
condition
Yes
action-2
action
Yes
condition
No
Selection structure
Chapter 10
Flow of Control
Repetition structure
2
Selection: ‘if’ construct
Syntax: if (expression)
statement ;
expression is the condition for the ‘if’ construct.
If expression is evaluated to non-zero (true),
statement is executed.
If expression is evaluated to zero (false),
statement is skipped.
Chapter 10
Selection: ‘if’ construct
3
‘if’ construct
Syntax: if (expression)
statement ;
fine = 20;
if (speed > 50)
fine += 10;
printf("Fine is %d\n", fine);
Chapter 10
'if’ construct
4
Compound-statement Action
Syntax: if (expression) {
compound-statement ;
}
if (a > b)
max = a;
if (a > b)
printf("%d is larger than %d\n", a, b);
if (a > b)
{
max = a;
printf("%d is larger than %d\n", a, b);
}
Chapter 10
Compound-statement Action
5
Flags
Flag: an integer variable that simulates a
Boolean variable. Contains 0 (false) or 1 (true).
if (attended == 1)
attendance++;
if (attended == 0)
{
absentees++;
printf("One more absentee.\n");
}
Chapter 10
or
or
Flags
if (attended)
attendance++;
if !(attended)
{
absentees++;
printf("One more absentee.\n");
}
6
Naming a Flag
Appropriate naming is desirable.
Example: a flag ‘attended’ implies
 ‘attended’ if it contains 1 (true)
 ‘did not attend’ if it contains 0 (false)
Chapter 10
Naming a Flag
7
Nested ‘if’ statement
Example:
if (exam >= 80)
if (project >= 90)
grade = 'A';
Above could be rewritten as:
if (exam >= 80 && project >= 90)
grade = 'A';
Chapter 10
Nested ‘if’ statement
8
Short-circuit evaluation
Evaluation stops as soon as value of expression
is known. Evaluation is from left to right.
For logical AND (&&), if the partial expression is
false, the whole expression is false.
For logical OR (||), if the partial expression is
true, the whole expression is true.
Chapter 10
Short-circuit evaluation
9
Short-circuit evaluation
Example:
if (bank_bal < 0.0 || expenses > 100.0)
printf("Red alert!\n");
Chapter 10
Short-circuit evaluation
10
Complementing a Condition
Comlementing or negating a logical expression
means changing the polarity.
Examples:
!(a == 30) is equivalent to (a != 30)
!(a > b) is equivalent to (a <= b)
Chapter 10
Complementing a Condition
11
DeMorgan’s Theorem
NOT(a AND b) same as NOT(a) OR NOT (b)
NOT(a OR b) same as NOT(a) AND NOT(b)
In C:
!(expr1 && expr2) same as !(expr1) || !(expr2)
!(expr1 || expr2) same as !(expr1) && !(expr2)
Chapter 10
DeMorgan’s Theorem
12
DeMorgan’s Theorem
Example:
!(age > 25 && (status == 'S' || status == 'D'))
(age <= 25) || (status != 'S' && status != 'D'))
Chapter 10
DeMorgan’s Theorem
13
Common Mistakes
Do not use == and != on floating-point numbers.
Mixing up == with =.
Wrong placament of semi-colon, resulting in
empty statement:

if (a > b);
printf("a is larger than b\n");
printf() is outside ‘if’ construct.
Chapter 10
Common Mistakes
14
Common Mistakes
Translating condition in English to C:

if (lower <= x <= upper)
...
Let lower be 10, and upper be 30.
If x is 20, (lower <= x) is true, so it is evaluated
to 1. Since (1 <= upper) is also true, condition is
true!
Chapter 10
Common Mistakes
15
Common Mistakes
Wrong condition:

if (lower <= x <= upper)
...
Correct method:

if (lower <= x && x <= upper)
which is equivalent to this (since <= has a higher
precedence than &&):
if ((lower <= x) && (x <= upper))
Chapter 10
Common Mistakes
16
Common Mistakes
Wrong condition:

if (x && y > z)
Correct method:

Chapter 10
if (x > z && y > z)
Common Mistakes
17
Common Mistakes
Forgetting the braces for compound statements:

if (a > b)
max = a;
printf("%d is larger than %d\n", a, b);
Correct method:

Chapter 10
if (a > b)
{
max = a;
printf("%d is larger than %d\n", a, b);
}
Common Mistakes
18
‘if-else’ construct
Syntax:
if (expression)
statement1 ;
else
statement2 ;
Chapter 10
if (expression)
{
compound-statement1 ;
}
else
{
compound- statement2 ;
}
‘if-else’ construct
19
‘if-else’ construct
May be used to avoid redundant code:

if (count == 0)
ave = 0.0;
if (count != 0)
ave = (float) total/count;
Use ‘if-else’ construct:

Chapter 10
if (count == 0)
ave = 0.0;
else
ave = (float) total/count;
‘if-else’ construct
20
‘if-else’ construct
Another example:
if (score1 < score2)
{
better_score = score2;
printf("score2 is better\n");
}
else
{
better_score = score1;
printf("score1 is better\n");
}
Chapter 10
‘if-else’ construct
21
Style
Two common styles:
if (expression)
{
compound-statement1 ;
}
else
{
compound- statement2 ;
}
Chapter 10
Style
if (expression) {
compound-statement1 ;
}
else {
compound- statement2 ;
}
22
Removing common statements
Common statements in the ‘then’ and ‘else’
parts should be moved out of the ‘if’ construct,
if appropriate:

Chapter 10
if (a < 0) {
count++;
neg++;
printf("Enter an integer: ");
scanf("%d", &k);
}
else {
count++;
pos++;
printf("Enter an integer: ");
scanf("%d", &k);
}
Removing common statements
23
Removing common statements
After moving common statements out of ‘if’
construct:

Chapter 10
count++;
if (a < 0)
neg++;
else
pos++;
printf("Enter an integer: ");
scanf("%d", &k);
Removing common statements
24
Logical assignment for Flags
Example:
if (age >= 65)
snr_citizen = 1;
else
snr_citizen = 0;
‘if-else’ statement may be replaced by an
assignment statement.
snr_citizen = (age >= 65);
Chapter 10
Logical assignment for Flags
25
Logical assignment for Flags
Another example:
if (n % 2 == 0)
even = 1;
else
even = 0;
even = (n % 2 == 0);
even = !(n % 2);
Chapter 10
Logical assignment for Flags
26
Nested ‘if-else’ statements
Example:
if (marks < 50)
grade = 'F';
else
if (marks < 70)
grade = 'B';
else
grade = 'A’;
Chapter 10
Nested ‘if-else’ statements
27
Nested ‘if-else’ statements
Which ‘if’ is the ‘else’ associated with?
if (a == 1)
if (b == 2)
printf("***\n");
else
printf("###\n");
‘else’ is associated with nearest ‘if’.
if (a == 1)
if (b == 2)
printf("***\n");
else
printf("###\n");
Chapter 10
Nested ‘if-else’ statements
28
Nested ‘if-else’ statements
To override default association, use braces to
mark out block.
if (a == 1) {
if (b == 2)
printf("***\n");
}
else
printf("###\n");
Chapter 10
Nested ‘if-else’ statements
29
Nested ‘if-else’ statements
Example:


Chapter 10
if (a == 0)
if (b == 0)
printf("Both a and b are zeros.\n");
else
c = b/a;
if (a == 0) {
if (b == 0)
printf("Both a and b are zeros.\n");
}
else
c = b/a;
Nested ‘if-else’ statements
30
Nested ‘if-else’ statements
Example:
if (marks < 50)
grade = 'F';
else
if (marks < 60)
grade = 'D';
else
if (marks < 70)
grade = 'C’;
else
if (marks < 80)
grade = 'B';
else
grade = 'A';
Chapter 10
if (marks < 50)
grade = 'F';
else if (marks < 60)
grade = 'D';
else if (marks < 70)
grade = 'C’;
else if (marks < 80)
grade = 'B';
else
grade = 'A';
Nested ‘if-else’ statements
31
Common Mistakes
Wrong matching of ‘else’ with ‘if’.
Wrong placement of semi-colon.

Chapter 10
if (a == b) {
a++;
b--;
};
else
a = b;
Common Mistakes
32
Conditional Operator (?:)
Ternary operator:
condition ? expr1 : expr2
First operand is condition.
If condition is true, take value of expr1;
otherwise, take value of expr2.
Chapter 10
Conditional Operator (?:)
33
Conditional Operator (?:)
Example:
max = (a > b ? a : b);
equivalent to:
if (a > b)
max = a;
else
max = b;
Chapter 10
Conditional Operator (?:)
34
Conditional Operator (?:)
Example:
printf("%s\n", grade < 50 ? "Failed" : "Passed");
equivalent to:
if (marks < 50)
printf("Failed\n");
else
printf("Passed\n");
Chapter 10
Conditional Operator (?:)
35
‘switch’ construct
Multi-way selection statement:
switch (expression) {
case v1: s1 ;
break;
case v2: s2 ;
break;
...
default: sn ;
break; /* optional break */
}
Chapter 10
‘switch’ construct
36
‘switch’ construct
May only test constant integral expressions, i.e.,
expressions that evaluate to integers or
characters.
The vi’s are integral values; the si’s are
compound statements.
After expression is evaluated, control jumps to
appropriate ‘case’ label.
‘break’ statements are inserted to avoid falling
through.
Chapter 10
‘switch’ construct
37
‘switch’ construct
Example:
switch (class) {
case 'B':
case 'b': printf
break;
case 'C':
case 'c': printf
break;
case 'D':
case 'd': printf
break;
case 'F':
case 'f': printf
break;
default : printf
("Battleship\n");
("Cruiser\n");
("Destroyer\n");
("Frigate\n");
("Unknown ship class %c\n",
class);
}
Chapter 10
‘switch’ construct
38
Repetition Structure
Counter-controlled repetiton: number of
iterations known.
Sentinel-controlled repetiton: iterate until a
sentinel value is entered, or terminating
condition is true.
Chapter 10
Repetition Structure
39
‘while’ construct
Loop structure with pre-test condition.
while (expression)
statement ;
expression is loop condition.
If expression is true, statement in loop body is
executed, and expression tested again.
If expression is false, loop terminates.
Chapter 10
‘while’ construct
40
‘while’ construct
Example: Print n asterisks.
count_star = 0;
while (count_star < n) {
printf("*");
count_star++;
}
count_star = 0;
while (count_star++ < n)
printf("*");
count_star is the loop control variable.
Chapter 10
‘while’ construct
41
‘while’ construct
Example: Compute sum of first 100 positive
integers.
int num = 1;
int total = 0;
/* declaration and */
/* initialisation */
while (num <= 100) {
total += num;
num++;
}
Chapter 10
‘while’ construct
42
‘while’ construct
Which of these is/are same as previous code?
int num = 1;
int total = 0;
while (num <= 100) {
total += num;
++num;
}
Chapter 10
int num = 1;
int total = 0;
while (num <= 100)
total += num++;
int num = 1;
int total = 0;
while (num <= 100)
total += ++num;
‘while’ construct
43
‘while’ construct
Loop control variable.
 Initialisation: before the loop is entered, the variable
must be initialised.
 Testing: condition involving the loop control variable
is tested before the start of each loop iteration; if
condition is true, loop body is executed.
 Updating: loop control variable is updated during
each iteration (usually at the beginning or the end of
the loop body).
Chapter 10
‘while’ construct
44
Counter-control repetition
A counter is used to keep track of number of
#define N 10
iterations.
. . .
total = 0;
count = 1;
while (count++ <= N) {
printf("Enter score: ");
scanf("%d", &score);
total += score;
}
avg = (float) total / N;
printf("Average is %.2f\n", avg);
Chapter 10
Counter-control repetition
45
Sentinel-control repetition
A sentinel is
used to denote
end of data.
#define SENTINEL –1
. . .
total = 0;
count = 0;
printf("Enter score, –1 to end: ");
scanf("%d", &score);
while (score != SENTINEL) {
total += score;
count++;
printf("Enter score, –1 to end: ");
scanf("%d", &score);
}
if (count) {
avg = (float) total/count;
printf("Average is %.2f\n", avg);
}
else
printf("No scores were entered\n");
Chapter 10
Sentinel-control repetition
46
‘do-while’ construct
Loop structure with post-test condition.
do
statement ;
while (expression);
Loop body is executed at least once.
Chapter 10
‘do-while’ construct
47
‘do-while’ construct
Examples:
count = 1;
do {
printf ("%d ", count);
} while (++count <= 10);
do {
printf("Enter a letter A thru E: ");
scanf("%c", &letter);
} while (letter < 'A' || letter > 'E');
Chapter 10
‘do-while’ construct
48
Flag-controlled loops
When loop condition is complex, flags may be
used.
valid = 1;
while (valid) {
printf("Enter a letter A thru E: ");
scanf("%c", &letter);
valid = (letter >= 'A' && letter <= 'E');
}
Chapter 10
Flag-controlled loops
49
‘for’ construct
Another pre-test loop structure.
Provides more compact form for countercontrolled loops.
for ( initialisation-expression;
loop-condition;
update-expression )
statement;
Chapter 10
‘for’ construct
50
‘for’ construct
The ‘for’ construct is similar to this ‘while’
construct.
initialisation-expression;
while ( loop-condition ) {
statement;
update-expression;
}
Chapter 10
‘for’ construct
51
‘for’ construct
Example:
for (count_star = 0;
count_star < N;
count_star++)
printf ("*");
Chapter 10
‘for’ construct
/* init
*/
/* condition */
/* update
*/
52
‘for’ construct
The initialisation-expression and updateexpression are often comma-separated lists of
expressions.
The comma operator evaluates the list from left
to right.
for (x = 1, total = 0; x <= 100; x++)
total += x;
Chapter 10
‘for’ construct
53
‘for’ construct
Any of the three expressions in the ‘for’ header
may be omitted, but the semi-colons must stay.
If initialisation-expression is omitted, you must
perform necessary initialisation before loop.
x = 1;
total = 0;
for (; x <= 100; x++)
total += x;
Chapter 10
‘for’ construct
54
‘for’ construct
If update-expression is omitted, you must
ensure that necessary update operations are
done in loop body.
for (x = 1, total = 0; x <= 100;) {
total += x;
x++;
}
Chapter 10
‘for’ construct
55
‘for’ construct
If loop-condition is omitted, then the test is
always true.
This loop is infinite:
for (x = 1, total = 0; ; x++)
total += x;
Chapter 10
‘for’ construct
56
Common Mistakes
Wrong placement of semi-colon.

for (x = 1; x <= 10; x++);
printf("%d\n", x);

x = 1;
while (x <= 10);
printf("%d\n", x++);
Chapter 10
Common Mistakes
57
Common Mistakes
Omitting semi-colons in ‘for’ header.
Mixing up semi-colons with commas in ‘for’
header.
Off-by-one error, where the loop executes one
more or one fewer iteration than intended. How
many iterations does this loop execute?
for (count = 0; count <= n; ++count)
sum += count;
Chapter 10
Common Mistakes
58
‘break’ statement
Used in loops, ‘break’ causes execution to break
out of the loop that contains the statement.
#include <stdio.h>
main()
{
int x;
for (x = 1; x <= 10; x++) {
if (x == 5)
break;
/* break loop only if x == 5 */
printf("%d ", x);
}
printf("\nBroke out of loop at x == %d\n", x);
return 0;
}
1 2 3 4
Broke out of loop at x == 5
Chapter 10
‘break’ statement
59
‘continue’ statement
The ‘continue’ statement causes execution to
skip remaining loop body and proceed to next
#include <stdio.h>
iteration.
main()
{
int x;
for (x = 1; x <= 10; x++) {
if (x == 5)
continue; /* skip remaining code in
loop only if x == 5 */
printf("%d ", x);
}
printf("\nUsed 'continue' to ");
printf("skip printing the value 5\n");
return 0;
}
1 2 3 4 6 7 8 9 10
Used 'continue' to skip printing the value 5
Chapter 10
'continue’ statement
60
Nested loops & combined
structures
An example of nested ‘for’ loops.
for (i = 1; i <= 4; ++i) {
for (j = 1; j <= 6; ++j)
printf("*");
printf("\n");
}
******
******
******
******
Chapter 10
Nested loops & combined
structures
61
Nested loops & combined
structures
Example 2:
for (i = 1; i <= 4; ++i) {
for (j = 1; j <= i; ++j)
printf("*");
printf("\n");
}
*
**
***
****
Chapter 10
Nested loops & combined
structures
62
Nested loops & combined
structures
Example 3:
for (i = 1; i <= 6; ++i) {
if (i <= 3)
printf("%d", i);
else
for (j = 1; j <= i; ++j)
printf("*");
printf("\n");
}
Chapter 10
Nested loops & combined
structures
1
2
3
****
*****
******
63
Homework
Try exercises behind chapter 10.
Chapter 10
Homework
64