Sequential Programming Limitation
Sequential Programming
„
Conditional Statements in C
Computer executes instructions in the order in which
they are stated
Limitations of Sequential Programming
„
„
Cannot choose whether or not to perform a
command/instruction
Cannot perform the same command more than once
Based on slides © McGraw-Hill
Additional material © 2004/2005 Lewis/Martin
Modified by Diana Palsetia
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Control Structures
“if” statement
Conditional
„
„
„
if (condition){
statement(s)
}
Making decision about which code to execute, based
on evaluated expression
if
if-else
If the condition is true (expression is evaluated to nonzero), then the statement(s) (i.e. instructions) will be
executed. Otherwise, it/they won’t.
Iteration
„
„
„
„
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Executing code multiple times, ending based on
evaluated expression
while
for
do-while
//Assume x is an integer
if(x > 10) {
x = x * 2;
}
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“if” statement (contd..)
Example if Statements
if (x <= 10)
y = x * x + 5;
{ } indicates the block of code that will get executed given
the condition is true
You can avoid the curly brace after condition if only one statement is to
be performed
//Assume x is an integer
if(x < 0)
x = x * -1;
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if (x <= 10)
y = x * x + 5;
z = (2 * y) / 3;
only first statement is conditional;
second statement is
always executed
if (x <= 10) {
y = x * x + 5;
z = (2 * y) / 3;
}
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compound statement;
both executed if x <= 10
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“if-else” statement
Assignment vs. Equality
if (condition){
statement(s)
}
else{
statement(s)
}
Don't confuse equality (==) with assignment (=)
int x = 9;
i t y = 10
int
10;
if (x == y) {
printf(“not executed\n”);
}
if (x = y) {
printf(“x = %d y = %d”, x, y);
}
Result: “x = 10 y = 10” is printed. Why?
Compiler will not stop you! (What happens in Java?)
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Style: avoid singleton
if statements
(Often cause of errors)
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//Assume x is an integer
if(x > 0) {
x = x * 2;
}
else {
x = x * -1;
}
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If-else Flow chart
Style Rule: Indentation and Spacing
Style
true
condition?
Statement(s)
„
Recommended indentation is from 2 to 4 spaces, but must be
consistent throughout the program
„
Single
g space
p
around every
y binary
y operator,
p
, including
g comparisons
p
and
assignment (=)
false
Statement(s)
Indentation
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< 10) {
= x + 1;
{
= x - 1;
Spacing
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Cascading/Chaining If’s and Else’s Example 2
Cascading “if-else” Example
if(month == 4 || month == 6 || month == 9 || month
== 11) {
printf(“Month has 30 days.\n”);
}
Example
//Assume variable score is entered by user
if (score > 90)
printf(“Grade A\n”);
else if (score > 80)
printf(“Grade B\n”);
else if (score > 65)
printf(“Grade C\n”);
.
.
else
printf(“F\n”);
else if (month == 1 || month == 3 ||
month == 5 || month == 7 ||
month == 8 || month == 10 ||
month == 12) {
printf(“Month has 31 days.\n”);
}
else if (month == 2) {
printf(“Month
printf(
Month has 28 or 29 days
days.\n
\n”);
);
}
//Note: You can avoid the curly brace after condition if
only one statement is to be performed
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if (x
___x
}
else
l
___x
}
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else {
printf(“Don’t know that month.\n”);
}
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Difference between Chaining if-else vs. just using if
int x = -5;
if(x < 0){
x = -x;
}
else if(x >= 1 && x <= 10){
x = x * 2;
}
else if(x > 10){
x = x * 2 - 1;;
}
Nested if-statements
int x = -5;
if(x < 0){
x = -x;
}
if(x >= 1 && x <= 10){
x = x * 2;
}
if(x > 10){
x = x * 2 - 1;;
}
An if within an if
if (condition1){
if (condition2){
( ) A
statement(s)
}
else{
statement(s) B
}
}
else {
statements(s) C
}
Truth Table
What values must the
conditions have in order for
block A to run? B? C?
A
condition1
B
C
T
condition2
If a series of standard "if" statements were used, it would be possible for all
of the statements to be executed, which is not what was intended.
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Matching Else with If
Switch
Else is always associated with closest unassociated if
if (x != 10)
if (y > 3)
z = z / 2;
else
z = z * 2;
is the same as...
if (x != 10) {
if (y > 3){
z = z / 2;
}
else{
z = z * 2;
}
}
is NOT the same as...
if (x != 10) {
if (y > 3)
z = z / 2;
}
Solution: always use braces
else
(avoids
the problem entirely)
z = z * 2;
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switch (integer_expression){
case const1:
statement(s);
break;
case const2:
statement(s);
break;
default:
statement(s);
break;
}
evaluate
expression
= const1?
t1?
T
action1
ti 1
F
= const2?
T
action2
F
Alternative to long if-else chain.
If break is not used, then
case "falls through" to the next.
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action3
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Switch Example
/* same as month example from cascaded if-else */
switch (month) {
case 4: case 6: case 9:case 11:
printf(“Month has 30 days.\n”);
p
y
break;
case 1:case 3:
/* some cases omitted for brevity...*/
printf(“Month has 31 days.\n”);
break;
case 2:
printf(“Month
printf(
Month has 28 or 29 days
days.\n
\n”);
);
break;
default:
printf(“Don’t know that month.\n”);
}
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