week selection c

8/3/2019 Week Selection C

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SCK1213 Programming Technique ISEM1 2009/2010

Making Decisions

Week 7

Source from Tony Gaddis, Barret Krupnow, Starting out with C++, 5th editionupdate. 2007. Pearson Addison-Wesley


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Selection structure

Selection will be chosen based onconditions

In C, there are 3 ways for selectionstructure:

1. Selection using if & else

if, expanding if, if/else, if/else if, trailing else, nested if

2. Selection using conditional operators

3. Selection using switch, case & break


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Relational Operators & Logical

Operators

4.1


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Relational Operators

Used to compare numbers to determinerelative order

Operators:


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Logical Operators

Used to create relational expressions fromother relational expressions

Operators, meaning, and explanation:

&& AND New relational expression is true ifboth expressions are true

|| OR New relational expression is true if

either expression is true! NOT Reverses the value of an expression

true expression becomes false, andfalse becomes true


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Logical data

Also called Boolean

In C, there is no True or False Value.

A Zero value is treated as False

A Non-Zero value is treated as True

True and False in arithmetic scale


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Logical operators truth table


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Relational expression T/F examples


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The if Statement4.2


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The if Statement

Allows statements to be conditionallyexecuted or skipped over

Models the way we mentally evaluatesituations:

"If it is raining, take an umbrella."

"If it is cold outside, wear a coat."


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Flowchart for Evaluating a Decision


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Flowchart for Evaluating a Decision


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The if Statement

General Format:

if (expression)

statement;


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if statement what happens

To evaluate:

if (expression)

statement;

If the expression is true, thenstatement is executed.

If the expressionis false, thenstatement is skipped.


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if statement - example


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if statement notes

Do not place ; after (expression)

Place statement; on a separate line

after (expression), indented:if (score > 90)

grade = 'A';

Be careful testing float and doublefor equality

0 is false; any other value is true


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Flowchart if


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Flowchart if with example

Printing a number only if it is smaller than 0


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Exercise Week 7_1

Refer to Lab 7, Exe 2 No. 1 (i) in pg. 60.

Explain and draw the flowchart


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Flags

4.3


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Flags

Variable that signals a condition

Usually implemented as a bool variable

As with other variables in functions, mustbe assigned an initial value before it isused


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Exercise Week 7_2 Trace the following programs if the input is 22 and 68

int main()

{

float mark;

bool pass=false; //this condition does not yet exist

printf ("Enter your mark");

scanf ("%f", &mark);

if (mark >=30)

pass=true;

if (pass)

printf ("You pass the test\n");

if (!pass)printf ("You fail the test\n");

printf ("Program end");

getch();

return 0;

}


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Expanding the if Statement

4.4


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Expanding the if Statement

To execute more than one statement as part ofan if statement, enclose them in { }:

if (score > 90)

{grade = 'A';

cout


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Expanding the if Statement - example


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Exercise Week 7_3 (Solve the problem)

Identify the logic errors, and correct them The program will display Pass message, calculate the carry mark anddisplay the carry mark if the student pass the test.

If the student fail the test the program will display Fail message and

display a message to instruct the student to re-sit the testint main()

{double mark, final20p = 0;

printf ("Enter your mark: ");

scanf ("%lf", &mark);

if (mark >= 30)

printf ("TEST 1 -> Pass\n");

final20p = ((20.0/100.0) * mark);

printf ("Contribution to final mark %ld", final20p);

if (mark < 30)

printf ("TEST 1 -> Fail\n");

printf ("Please re-sit TEST 1\n");

getch();

return 0;}


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The if/else Statement

4.5


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Provides two possible paths of execution

Performs one statement or block if the

expression is true, otherwise performsanother statement or block.


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General Format:

if (expression)

statement1; // or block

else



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if/else what happens

To evaluate:if (expression)

statement1;

else

statement2;

If the expression is true, then statement1is executed and statement2 is skipped.

If the expression is false, thenstatement1 is skipped and statement2 isexecuted.


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if statement exampleif/else statement example

IF score is higher than 50

THEN grade is PASS

ELSE grade is FAIL

In C, this corresponds to if statement with three parts:


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if statement exampleif statement condition

Part 1: the condition an expression thatis evaluated to TRUE or FALSE


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if statement TRUE

Part 2: the TRUE-part a block ofstatements that are executed if thecondition evaluates to TRUE


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else statement FALSE

Part 3: the FALSE-part a block ofstatements that are executed if thecondition evaluates to FALSE

If the conditionevaluates to FALSE,the TRUE-part isskipped


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Flowchart if/else


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if/else statement notes

If the TRUE-part (or FALSE-part) consists ofonly one statement, then the curly braces maybe omitted

E.g. these two statements are equivalent:


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Flowchart 2 - if/else

Example: if two numbers (p and q) are equivalent, resetthem to zero, otherwise, exchange or swap their valueand then print the new values.


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The three forms of if statements

The conditionisalways inparentheses

All TRUE-partsand all FALSE-parts are a singlestatement of a

block of code /compoundstatements


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Exercise Week 7_4

Refer back to Lab 7, Exe 2, No. 1, pg. 60.

Solve the problem in (ii) and (iii) pg.61


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The if/else if Statement

4.6


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The if/else if Statement

Chain of if statements that test in order

until one is found to be true

Also models thought processes:

If it is raining, take an umbrella,

else, if it is windy, take a hat,else, take sunglasses


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if/else if format

if (expression)


else if (expression)


.

. // other else ifs

.

else if (expression)

statementn; // or block


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(ProgramContinues

)

if/else if format example program 7-12

//This program uses an if/else if statement to assign a//letter grade (A, B, C, D, or F) to a numeric test score.#include #include

int main(){

int testScore; //To hold a numeric test scorechar grade; //To hold a letter grade

//Get the numeric test scoreprintf ("Enter your numeric test score and I will\n");printf ("tell you the letter grade you earned: ");scanf ("%d", &testScore);

//Determine the letter gradeif (testScore < 60)

grade = 'F';else if (testScore < 70)

grade = 'D';else if (testScore < 80)

grade = 'C';else if (testScore < 90)

grade = 'B';


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if/else if format - example

else if (testScore


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Exercise Week 7_5

Refer to Lab 7, Exe 3, No. 1 in pg. 62.

Draw a flowchart for Program 7.3

Refer to Lab 7, Exe 3, No. 2 in pg. 62-63.

Draw a flowchart for Program 7.4

Discuss the differences.


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Using a Trailing else

4.7


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Using a Trailing else

Used with if/else if statement whennone of the expressions are true

Provides default statement/action Used to catch invalid values, other

exceptional situations


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From Program 7-12

if (testScore < 60)

grade = 'F';

else if (testScore < 70)

grade = 'D';


grade = 'C';


grade = 'B';

else if (testScore


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Flowchart with trailing else


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Flowchart with trailing else

Example: identifying the grade of a score


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Menus

4.8


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Menus

Menu-driven program: program executioncontrolled by user selecting from a list of

actions Menu: list of choices on the screen

Menus can be implemented using

if/else if statements


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Menu-driven program organization

Display list of numbered or lettered choicesfor actions

Prompt user to make selection Test user selection in expression

if a match, then execute code for action

if not, then go on to next expression


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Exercise Week 7_6

Refer to Lab 7, Exe. 4, No. 1, Program 7.5. inpg. 63.

Use if / else.if to select the menu

Use trailing else to print We dont have

any

Jump to switch slide #93


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Nested if Statements

4.9


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Nested if Statements

An if statement that is part of the if orelse part of another if statement

Can be used to evaluate more than one

condition:if (score < 100) or,

{

if (score > 90)

grade = 'A';

}

condition1

condition2

condition3TRUE-part,

Nested if Statements Problem-solving


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Nested if Statements Problem-solvingexample

Write a program to calculate and displayan area of a rectangle. Size of each side isgiven by a user from a keyboard. Your

program must ensure that the size of eachside is not zero or negative. If side is zero,display a warning message to the user

and turn the area into zero. If the sidegiven is negative, only take the magnitude.



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i) Problem analysisInput

media: keyboard

data: width and length

Output:media: screen

data: area of a rectangle

Process:

1. To find magnitude of a number:

if number is negative, magnitude = -(number)

if number is positive, magnitude = number

2. Area of a rectangle = width x length



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ii) Pseudo-code1. Read width & length from user

2. if width = 0 or length = 0, then

2.1 begin

2.1.1 print warning message

2.1.2 area = 02.2 end_if

3. else

3.1 begin

3.1.1 if negative length then

3.1.1.1 length = magnitudelength

3.1.2 end_if

3.1.3 if negative width, then

3.1.3.1 width = magnitude width

3.1.4 end_if

3.1.5 area = length * width

3.2 end_else

4. print area



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iii) C program

2-tier

nested if

#include main (){

float area, length, width;

printf (Enterlength and width\n);scanf (%f %f, &length, &width);

if (length == 0 || width == 0){

printf (WARNING- INPUT CANNOT BE ZERO);area = 0;

}else{

if (length < 0)length = -length; //find magnitude length

if (width < 0)width =-width; //find width magnitude

area = length * width}

printf (Area of a rectangle is %f\n);, area);}


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Nested if Statements multiple tiers

Defense Ministry is to list names of malestaffs whose age is between 20-26 andnot yet married

if (gender == M)

if (marital_status == S)

if (age = 20)

printf (%s\n, name);


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Notes on coding nested ifs

An else matches the nearest if thatdoes not have an else:

if (score < 100)

if (score > 90)

grade = 'A';

else ...// goes with second if,

// not first one

Proper indentation helps greatly


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Exercise Week 7_7

Write nested if statements that perform

the following test:

If amount1 is greater than 10 andamount2 is less than 100, display the

greater of the two.


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Logical Operators

4.10


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Logical operators

Used to create relational expressions fromother relational expressions

Operators, meaning, and explanation:

&& AND New relational expression is true ifboth expressions are true

|| OR New relational expression is true if

either expression is true! NOT Reverses the value of an expressiontrue expression becomes false, andfalse becomes true

Using relational expressions for logical


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Using relational expressions for logicalcomparison

Examples:

l i l i i l i l i


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Relational operators pair in logical expressions

Revisited - Nested if Statements multiple


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Revisited Nested if Statements multipletiers

The statement can also be written as:

if (gender == L) && (marital_status == S) && (age = 20)printf(%s\n, name);

L i l O


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Logical Operators - notes

! has highest precedence, followed by &&,then ||

If the value of an expression can be

determined by evaluating just the sub-expression on left side of a logical operator,then the sub-expression on the right side willnot be evaluated (short circuit evaluation)

!(x > 2)

!x > 2


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Checking Numeric Ranges withLogical Operators

4.11

Checking Numeric Ranges with Logical


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g g gOperators

Used to test to see if a value falls inside arange:

if (grade >= 0 && grade


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Validating User Input

4.12

V lid ti U I t


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Validating User Input

Input validation: inspecting input data todetermine whether it is acceptable

Bad output will be produced from bad

input Can perform various tests:

Range

Reasonableness

Valid menu choice

Divide by zero

F P 7 12


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From Program 7-12

//Get the numeric test score

printf ("Enter your numeric test score and I will\n");printf ("tell you the letter grade you earned: ");scanf ("%d", &testScore);

if (testScore < 0 || testScore > 100) //Input validation{

//An invalid score was enteredprintf ("%d is invalid score.\n", testScore);printf ("Run the program again and enter a value\n");

printf ("in the range of 0 to 100.\n");}else{

//Determine the letter gradeif (testScore < 60)grade = 'F';else if (testScore < 70)

grade = 'D';else if (testScore < 80)

grade = 'C';else if (testScore < 90)

grade = 'B';else if (testScore


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More About Variable Definitionsand Scope

4.13

M Ab t V i bl D fi iti d S


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More About Variable Definitions and Scope

Scope of a variable is the block in which itis defined, from the point of definition tothe end of the block

Usually defined at beginning of function

May be defined close to first use

Still More About Variable Definitions and


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Scope

Variables defined inside { } have local

or block scope

When inside a block within another block,can define variables with the same nameas in the outer block.

When in inner block, outer definition is not

available

Not a good idea


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Exercise Week 7_8

What will the following program display ifuser enter test1 40 and test2 30?

if (sum>60){

int bonus=10;

test1+=bonus; test2+=bonus;

int sum=test1+test2;printf ("Test 1 with bonus: %d \n",

test1);

printf ("Test 2 with bonus: %d \n",

test2);

printf ("Sum with bonus: %d \n",

sum);

}

printf ("Test 1 : %d \n", test1);

printf ("Test 2 : %d \n", test2);

printf ("Sum: %d \n", sum);

getch();

return 0;}

int main ()

{

int test1;

printf ("Enter Test 1 score: ");scanf ("%d", &test1);

int test2;

printf ("Enter Test 2 score: ");

scanf ("%d", &test2);

int sum=test1+test2;printf ("Sum: %d \n", sum);

(cont. next)


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Comparing Strings

4.14

Comparing Strings


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Comparing Strings

You cannot use relational operators withC-strings

Must use the strcmp function to compare

C-strings strcmp compares the ASCII codes of the

characters in the C-strings. Comparison is

character-by-character

Comparing Strings


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Comparing Strings

The expressionstrcmp(str1, str2)

compares thestrings str1 and str2 It returns 0 if the strings are the same

It returns a negative number if str1 < str2

It returns a positive number if str1 > str2

Comparing strings example


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Comparing strings - example

strcmp(Newyork,Newyork)

will return zero because 2 strings are equal.

strcmp(their,there)

will return a 9 which is the numeric difference betweenASCII i and ASCII r.

strcmp(The, the)

will return 32 which is the numeric difference betweenASCII T & ASCII t.

Comparing Strings example


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Comparing Strings - example

//This program correctly tests two C-strings for equality

//with the strcmp fuction.#include #include #include

int main (){

const int SIZE = 40;char firstString[SIZE], secondString[SIZE];

//Get two stringsprintf ("Enter a string: ");gets(firstString);printf ("Enter another string: ");gets(secondString);

//Compare them with strcmpif (strcmp(firstString, secondString) == 0)

printf ("You entered the same string twice.\n");else

printf ("The strings are not the same.\n");getch();

return 0;}

Comparing Strings example


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Comparing Strings - example


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Exercise Week 7_9

Refer back to Lab 7, Exe. 4, Program 7.5 inpg. 63.

Change the program that you wrote in

Exercise Week 7_6 : Change variable choice to variableiceCream[20]

Instead of using menu, use gets so the usercan enter the flavor and use strcmp to if /elseif statement.


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The Conditional Operator

4.15



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Can use to create short if/else

statements

Format: expr ? expr : expr;x


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The value of a conditional expression is The value of the second expression if the first

expression is true

The value of the third expression if the firstexpression is false

Parentheses () may be needed in an

expression due to precedence ofconditional operator



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Condition operator vs if/elsestatements

(x100)

a=0;

else

a=1;

printf(Your grade is );

printf((score


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The Conditional Operator - example

#include

#include

int main(){

const double PAY_RATE = 50.0;double hours, charges;

printf ("How many hours were worked? ");scanf ("%lf", &hours);

hours = hours < 5 ? 5 : hours; //conditional operatorcharges = PAY_RATE * hours;

printf ("The charges are $%.2lf", charges);getch();

return 0;}


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Exercise Week 7_10

Rewrite the followingif/else statements asconditional expressions

if (x>y)

z = 1;

elsez = 20;

if (hours> 40)

wages *= 1.5;

else

wages *= 1;

if (result >= 0)

printf("The result is +ve);

else

printf("The result is -ve);

Rewrite the followingconditional expressions asif/else statements

j = k > 90 ? 57 : 12;

total += count == 1 ? sales :count * sales;

printf (((num % 2) == 0) ?

"Even\n" : "Odd\n");


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The switch Statement

4.16



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Used to select among statements fromseveral alternatives

In some cases, can be used instead of

if/else if statements

switch statement format


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switch statement format

switch (expression) //integer or char

{

case exp1: statement1;

case exp2: statement2;...

case expn: statementn;

default: statementn+1;}

Conversion from if/else if to switch


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Conversion from if/else if to switch

switch statement requirements


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switch statement requirements

1) expression must be an integer

variable or an expression that evaluates

to an integer value2) exp1 through expn must be constant

integer expressions or literals, and must

be unique in the switch statement3) default is optional but recommended

switch statement how it works


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switch statement how it works

1) expression is evaluated

2) The value of expression is comparedagainst exp1 through expn.

3) If expression matches value expi, the

program branches to the statement followingexpi and continues to the end of the switch

4) If no matching value is found, the programbranches to the statement after default:

switch statement example I


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switch statement example I

switch statement example II


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s tc state e t e a p e II

switch statement example III


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a e e e a p e III

break statement


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Used to exit a switch statement

If it is left out, the program "falls through"the remaining statements in the switch

statement

break statement - example


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p

Flowchart - switch


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Flowchart switch example


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p

Printing the description of a grade

Using switch with a menu


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g

switch statement is a natural choice formenu-driven program:

display the menu

then, get the user's menu selection use user input as expression in switch

statement

use menu choices as expr in casestatements

E i W k 7 11


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Exercise Week 7_11

Change Program 7.5 in pg. 63 to : Input 1=> Output :Muhammads favorite

Ismaels favorite

Input 3=> Output :Adibahs favorite Munirahsfavorite

Input 2 => Output :Ismaels favorite

Input 4 => Output :Munirahs favorite

Back to slide #54


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Testing for File Open Errors

4.17

Testing for File Open Errors


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g p

To ensure there is no error in opening a file Example: opening and reading a file stor12.dat

#include

main()

{FILE *failptr;

failptr = fopen(stor12.dat, r);

if (failptr == NULL)

{

printf(Error in opening the file.\n);

exit(-1); //Program ends)

}

return 0;

}

E i W k 7 12


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Exercise Week 7_12

Refer to Lab 6, Exe.2. Program 6.3 in pg.58.

Modify the program to detect if the open

files operation failed.


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Thank You

Q & A

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