1. ECE 103 Engineering Programming Chapter 22 Selection
Herbert G. Mayer, PSU CS
Status 6/2
6/19/2015
Initial content copied verbatim from
ECE 103 material developed by
Professor Phillip PSU ECE

2. Syllabus If if-else if-else if Switch Triadic Conditional Expression
Nested Selection Statements

3. Making Decisions in Programs
A selection statement uses a conditional test to control the flow of execution in a program
“conditional test” is AKA “conditional expression” or “boolean expression”
A conditional test contains relational and/or logical expressions
Conditional test evaluates to “true” or “false”:
If test is “true” (non-zero), one group of statements is executed, known as the “Then Clause”
If test is “false” (zero), a different group of statements is executed

4. Example of 2-way true or false decision
Decisions can be documented by a flowchart or by pseudocode:
IF expression is true THEN
Execute branch A
ELSE
Execute branch B
END IF
expression
branch A
branch B
true
false
Example of 2-way true or false decision

5. Selection Statements in C
In C, logic states have these definitions:
“false” is the zero value (e.g., 0, 0.0, '\0')
“true” is any non-zero value (e.g., -1, 5, 12.4, 'A’)
C has three types of selection statements based on the if keyword: if
if–else
if–else if any number of times
C has an additional selection called switch

6. if pre-code; if (expression) Statement; post-code; if expression is
Execution Sequence
true ( != 0 )
Statement is executed
Execution then continues at post-code
false ( == 0 )
Statement is skipped
Execution continues at post-code

7. expression must be inside parentheses ( ).
Statement can be a single statement or a block:
if (expression)
Statement; /* Single statement */
if (expression) {
Statement_1; /* Statement block */ 
Statement_k; }
All statements in the block are executed if the expression is true.

8. Example: Note: Indentation improves clarity.
#include <stdio.h>
#define TRUE 1
#define FALSE 0
#define LO_LIMIT 20
#define HI_LIMIT 100
int main( void )
{ // main
int N; /* Input value */
int low = FALSE, high = FALSE;
printf( "Enter N: ” );
scanf( "%d", &N );
/* Check input range */
if( N < LO_LIMIT )
low = TRUE;
if( N > HI_LIMIT )
high = TRUE;
if( low || high )
printf("Out of range.\n");
if( !low && !high )
printf("Within range.\n");
return 0; }
if( temperature <= 65 )
turn_heat_on = 1;
if( temperature > 65 )
turn_heat_on = 0;
if( x < 0 ) {
printf("x is negative.\n” );
} //end if
y = abs(x);
Note: Indentation improves clarity. 7

9. if–else pre-code; if (expression) Statement_T; else Statement_F;
post-code;
pre-code
expression
post-code
Statement_T T F
Statement_F
if expression is
Execution Sequence
true ( != 0 )
Statement_T is executed
Execution continues at post-code
false ( == 0 )
Statement_F is executed

10. Example: if( temperature <= 65 ) turn_heat_on = 1; else
#include <stdio.h>
#define TRUE 1
#define FALSE 0
#define LO_LIMIT 20
#define HI_LIMIT 100
int main( void )
{ // main
int N; /* Input value */
printf("Enter N: ");
scanf("%d", &N);
/* Check input range */
if( N < LO_LIMIT || N > HI_LIMIT )
printf("Out of range.\n");
else
printf("Within range.\n");
// end if
return 0; }
if( temperature <= 65 )
turn_heat_on = 1;
else
turn_heat_on = 0;
if( age >= 18 ) {
printf( "Adult\n” );
allow_vote = 1;
}else{
printf( "Pre-Adult\n” );
allow_vote = 0;
} //end if 9

11. Example: /* This is wrong! */ Write code that does this: if( y )
printf( "Yes!” );
else
m = -1;
is actually seen by the compiler as
Write code that does this:
if y is true, then set m to 1 and display "Yes!". Otherwise, set m to -1 and display nothing.
/* This is correct */
if( y ) {
m = 1;
printf( "Yes!” );
}else{
m = -1;
} //end if 10

12. if–else if pre-code; if (expression_1) Statement_1;
else if (expression_2)
Statement_2;
else
Statement_3;
post-code;
pre-code
expression_1
post-code
Statement_1 T F
expression_2
Statement_2
Statement_3
Note: It is else if, not elseif.

13. Example: if( x > 0 ) { printf("Positive\n"); p_count++;
#include <stdio.h>
int main( void )
{ // main
int score; /* Numeric grade */
char grade; /* Letter grade */
printf("Enter score: ");
scanf("%d", &score);
/* Check grade brackets */
if (score >= 90)
grade = 'A';
else if (score >= 80)
grade = 'B';
else if (score >= 70)
grade = 'C';
else if (score >= 60)
grade = 'D';
else
grade = 'F';
printf("Grade = %c\n", grade);
return 0;
} //end main
if( x > 0 ) {
printf("Positive\n");
p_count++;
}else if( x < 0 ) {
printf( "Negative\n” );
n_count++;
}else{
printf( "Zero\n” );
z_count++;
} //end ifs 12

14. switch switch (expression) if (expression==const_1) { {
{ {
case const_1: Statements_1;
Statements_1; }
break; else if (expression==const_2)
case const_2: {
Statements_2; Statements_2;
break; }
… …
default: else
Statements_d; {
break; Statements_d;
} //end switch } //end if

15. A case value cannot be a string or floating-point number
expression must be inside parentheses and evaluate to a single char or integer value
A case value must evaluate to a single char or integer constant that is known at compile-time
A case value cannot be a string or floating-point number
If expression matches a case value, then its associated statements are executed until a break is encountered 14

16. When break is encountered, the switch exits and execution continues at post-code
If no break exists in a case block, execution “falls through” to the following case
default handles the “none of the above” case. It can be omitted if not needed 15

17. Example: switch(experience) { case 'n': printf("Newbie\n"); N++;
#include <stdio.h>
#include "myfunctions.h"
#define WARNING 1
#define DANGER 2
#define PANIC 3
int main (void) {
int status, num_warnings = 0;
scanf("%d", &status);
switch (status)
case WARNING:
printf("What?\n");
num_warnings++;
break;
case DANGER:
printf("Leave now!\n");
notify_friends();
case PANIC:
printf("HELP ME!!!!!\n");
freak_out(10);
default:
printf("All is fine.\n"); }
return 0;
switch(experience) {
case 'n':
printf("Newbie\n");
N++;
break;
case 'a':
disp('Amateur');
A++;
case 'p':
case 'P':
printf("Professional\n");
P++; } 16

18. Tip: Selection Ordering
To improve the performance of a multi-conditional selection, arrange it in order of most likely occurrence.
Example:
Use an if-else if selection to test a character variable.
The character value can be 'a', 'b', 'c', 'd', or 'e'. They are not all equally likely.
Sorted from most likely to occur to least likely to occur:
'c', 'd'
'b'
'a'
Assume c and d are equally likely.
Inefficient ordering:
if (ch == 'a')
{…}
else if (ch == 'b')
else if (ch == 'c')
else if (ch == 'd‘)
Better arrangement:
if (ch=='c' || ch=='d')
{…}
else if (ch == 'b')
else if (ch == 'a')
MOST
LEAST 17

19. Triadic Conditional Expression
var = (condition) ? expr1 : expr2;
is equivalent to
if (condition)
var = expr1;
else
var = expr2;
Both expressions should evaluate to the same data type as var. 18

20. Example: /* If voltage > 2, then set state to high (1)
else set state to low (0) */
state = (voltage > 2.0) ? 1 : 0;
/* Sinc calculation handles x == 0 case */
y = (fabs(x) < 1e-8) ? 1.0 : sin(x)/x; 19

21. Nested Selection Statements
The body of a selection statement can contain another selection statement within it.
Each else is matched to the nearest if or else if. 20

22. Example: Convert the if-else if to a nested if.
/* if-else if version */
if (x == 0)
t++;
else if (x==1 || x==2)
printf("zen state\n");
else if (x < 0) {
m = sin(Q);
f = 3 * m * sqrt(m); }
else
printf("Moo\n");
/* Nested if version */
if (x == 0)
t++;
else {
if (x==1 || x==2)
printf("zen state\n");
if (x < 0)
m = sin(Q);
f = 3 * m * sqrt(m); }
printf("Moo\n"); 21

23. Example: Use indenting to make it clear what you really mean!
if (t >= 0 && t < 100)
if (F < 1500e3)
flim = 143.5;
else {
flim = 500.0;
overload = 1;
if (cps == 'x')
cnt++; }
else if (t2 > 750)
printf("D22 max\n");
if (!mox)
mox = 1;
pc += 10;
done = 1;
if (t >= 0 && t < 100)
if (F < 1500e3)
flim = 143.5;
else {
flim = 500.0;
overload = 1;
if (cps == 'x')
cnt++; }
else if (t2 > 750)
printf("D22 max\n");
if (!mox)
mox = 1;
pc += 10;
done = 1;
Use indenting to make it clear what you really mean! 22

24. Nesting can lead to ambiguous situations
Nesting can lead to ambiguous situations. If necessary, use braces to clarify your intentions.
Example:
if (x >= 0)
if (y > 5)
printf("Hi!\n");
else
printf("Goodbye!\n");
Is the "else" associated with the first or second "if"?
if (x >= 0)
if (y > 5)
printf("Hi!\n");
else
printf("Goodbye!\n");
if (x >= 0)
{ /* Braces clarify */
if (y > 5)
printf("Hi!\n");
else
printf("Goodbye!\n"); } 23