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ECE 103 Engineering Programming Chapter 22 Selection. Herbert G. Mayer, PSU CS Status 6/22/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip Wong @ PSU ECE. Syllabus. If if-else if-else if Switch Triadic Conditional Expression

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Ece 103 engineering programming chapter 22 selection

ECE 103 Engineering ProgrammingChapter 22Selection

Herbert G. Mayer, PSU CS

Status 6/22/2014

Initial content copied verbatim from

ECE 103 material developed by

Professor Phillip Wong @ PSU ECE


Syllabus
Syllabus

  • If

  • if-else

  • if-else if

  • Switch

  • Triadic Conditional Expression

  • Nested Selection Statements


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


Decisions can be documented by a flowchart or by pseudocode:

expression

branch A

branch B

IF expression is true THEN

Execute branch A

ELSE

Execute branch B

END IF

true

false

Example of 2-way true or false decision


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


if

pre-code;

if (expression)

Statement;

post-code;

Statement

post-code

pre-code

T

expression

F


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.


Example:

#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

if( x < 0 ) {

printf("x is negative.\n”);

y = abs(x);

} //end if

Note: Indentation improves clarity.

7


if–else

pre-code;

if (expression)

Statement_T;

else

Statement_F;

post-code;

post-code

pre-code

T

F

expression

Statement_T

Statement_F


Example:

#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


Example:

/* This is wrong! */

if( y )

m = 1;

printf( "Yes!” );

else

m = -1;

is actually seen by the compiler as

if( y )

m = 1;

printf( "Yes!” );

else

m = -1;

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

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if–else if

pre-code;

if (expression_1)

Statement_1;

else if (expression_2)

Statement_2;

else

Statement_3;

post-code;

post-code

pre-code

expression_1

Statement_1

Statement_2

Statement_3

expression_2

T

F

T

F

Note: It is else if, not elseif.


Example:

#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


switch

switch (expression) if (expression==const_1)

{ {

caseconst_1: Statements_1;

Statements_1; }

break; else if (expression==const_2)

caseconst_2:{

Statements_2; Statements_2;

break; }

… …

default: else

Statements_d; {

break; Statements_d;

} //end switch } //end if


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


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


Example:

#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();

break;

case PANIC:

printf("HELP ME!!!!!\n");

freak_out(10);

break;

default:

printf("All is fine.\n");

}

return 0;

}

switch(experience)

{

case 'n':

printf("Newbie\n");

N++;

break;

case 'a':

disp('Amateur');

A++;

break;

case 'p':

case 'P':

printf("Professional\n");

P++;

break;

}

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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

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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.

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Example:

/*

If voltage > 2,

then set state to high (1)

else set state to low (0)

*/

state = (voltage > 2.0) ? 1 : 0;

Example:

/* Sinc calculation handles x == 0 case */

y = (fabs(x) < 1e-8) ? 1.0 : sin(x)/x;

19


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.

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Example:

Convert the if-else if to a nested if.

/* Nested 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");

}

}

/* 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");

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Example:

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;

else

pc += 10;

}

else

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;

else

pc += 10;

}

else

done = 1;

Use indenting to make it clear what you really mean!

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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");

}

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