comp 132 notes 3 program control

8/2/2019 Comp 132 Notes 3 Program Control

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Program Cont rol:Select ion and Repet it ion


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Control Structures

Control structures control the flow of execution in

a program.

Statements are organized into three kinds ofcontrol structures to control execution flow:

sequence

selection

repetition


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Sequential Execution

Statements are executed one after the other in the

order they are written in the curly braces:

{

statement_1;

statement_2;

..

.

statement_n;

}


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Control Structures

Transfer of control Direction of the program can be changed using selection and

also repetition structure. C has three selection structures:

if

if/else

switch

C has three repetition structures:

while

do/whilefor


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The i f Selection Statement Selection structure:

Used to choose among alternative courses of action

Syntax:

if(condition)

statement;

If condition t r u e

statement executed and program goes on to next statement

Iff a l s e , statement is ignored and the program goes onto

the next statement

Indenting makes programs easier to read

C ignores whitespace characters


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The i f Selection Statement i f statement is a single-entry/single-exit structure

true

false

grade >= 60 print Passed

if ( grade >= 60 )

printf( "Passed \n" );


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Conditions

Condition is an expression whose evaluation

produces eitherFALSE orTRUE result.

FALSE is represented by zero

TRUE is represented by non-zero

Most conditions for comparisons have the form:Variable equality-operatorvariable

Variable equality-operatorconstant

Variable relational-operatorvariableVariable relational-operatorconstant


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Equality and Relational Operators

Standard algebraic equalityoperator or relational operator

C equality orrelational operator

Example of Ccondition

Meaning of C condition

Equality Operators

= == x == y x is equal to y ! = x ! = y x is not equal to y

Relational Operators

> > x > y x is greater than y < < x < y x is less than y >= >= x >= y x is greater than or equal to y


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Logical Operators

A logical operator is used to combine conditions

Operator Meaning

&& AND

|| OR ! NOT


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Operator Precedence

Operator Precedence

Function calls highest

! + - & (unary operators)

* / %

+ -< = >

== !=

&&||

= lowest


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The i f e l s e Selection Statement Syntax:

if(condition)

statement_T;

elsestatement_F;

C code:i f ( g r a d e > = 6 0 )

p r i n t f ( " P a s s e d \ n " ) ;e l s e

p r i n t f ( " F a i l e d \ n " ) ;

Ternary conditional operator (? : ) Takes three arguments (condition, value ift r u e , value iff a l s e )

Our pseudocode could be written:p r i n t f ( " %s \ n " , g r a d e > = 6 0 ? " P a s s e d " : " F a i l e d " ) ;

Or it could have been written:g r a d e > = 6 0 ? p r i n t f ( P a s s e d \ n ) : p r i n t f (

F a i l e d \ n ) ;


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The i f e l s e Selection Statement Flow chart of the i f e l s e selection statement

Nested i f e l s e statements

Test for multiple cases by placing i f e l s e selectionstatements inside i f e l s e selection statement

Once condition is met, rest of statements skipped

Deep indentation usually not used in practice

truefalse

print Failed print Passed

grade >= 60


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The i f e l s e Selection Statement Compound statement:

Set of statements within a pair of braces

Example:

i f ( g r a d e > = 6 0 )

p r i n t f ( " P a s s e d . \ n " ) ;

e l s e {

p r i n t f ( " F a i l e d . \ n " ) ;

p r i n t f ( " Y o u mu s t t a k e t h i s c o u r s ea g a i n . \ n " ) ;

}

Without the braces, the statement

p r i n t f ( " Y o u mu s t t a k e t h i s c o u r s e

a g a i n . \ n " ) ;

would be executed automatically


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Nested if/else Statements Test for multiple cases by placing if/else selection structures

inside if/else selection structures

Once condition is met, rest of statements skipped

statement_1;if (condition_1)

{

statement_2;

statement_3;

}else if(condition_2)

{

statement_4;

statement_5;

}.

.

else

statement_n;

statement_m;


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Nested if/else Statements

if (road_status == S)

if (temperature > 0)

{

printf(Wet roads ahead\n);

printf(Stopping time doubled\n);

}

else

{

printf(Icy roads ahead\n);

printf(Stopping time quadrupled\n);

}

else printf(Drive carefully!\n);


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switch Statement

To select one of several alternatives.

Selection is based on the value of an expression.

Expression could be a single value.

The type of expression can be eitherint orchar

type, but not double.


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switch Statement

Syntax:switch (expression){

case const-expr:statements;

break;

case const-expr:statements;

break;

case const-expr:

statements;break;

default:statements;

break;

}

...


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switch Statement

true

false

.

.

.

case a case a action(s) break

case b case b action(s) break

false

false

case z case z action(s) break

true

true

default action(s)


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switch Statement

switch (class) {

case 'B':

case 'b':

printf("Battleship\n");

break;

case 'C':

case 'c':

printf("Cruiser\n");

break;case 'D':

case 'd':

printf("Destroyer\n");

break;

case 'F':

case 'f':

printf("Frigate\n");

break;

default:

printf("Unknown ship class %c\n", class);

}


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Example for Logical Assignment

int age;

char gender;

int senior_citizen;

senior_citizen = 1; /* Set senior citizen */

scanf(%d, &age);

senior_citizen = (age >= 65) /* Is it right? */

YES


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Example for Logical Assignment

int in_range;

int is_letter, is_even;

int n;

char ch;

in_range = (n> -10 && n < 10);

is_even = (n % 2 == 0)

is_letter = (A


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Repetition in Programs

Most programs contain repetition or looping.

A loop is a group of instructions (or statements) that

the program executes repeatedly while loop-

repetition condition remains true.


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Types of Loops

In a counting loop, we know in advance the

number of loop repetition needed to solve theproblem. An example is calculating N!.

In a conditional loop, it is not known in advancehow many times the loop will be executed.


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while Statement

Syntax:

while (loop repetition condition)statement;

OR

while (loop repetition condition) {

statement1;

statement2;

statementn;

}

...


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Implementation With the while Statement

How it workscounter=0

counter


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Example: Calculating Factorial

Nfactorial=1;

counter=1;

while (counter


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Outline27

Outline

Copyright 19922004 by Deitel & Associates, Inc. and Pearson Education Inc. All Rights Reserved.

fig03_06.c (Part 1 of2)

1 / * F i g . 3 . 6 : f i g 0 3 _ 0 6 . c2 Cl a s s a v e r a g e p r o g r a m w i t h c o u n t e r - c o n t r o l l e d r e p e t i t i o n * /3 # i n c l u d e < s t d i o . h >4

5 / * f u n c t i o n ma i n b e g i n s p r o g r a m e x e c u t i o n * /6 i n t ma i n ( )7 {8 i n t c o u n t e r ; / * n u mb e r o f g r a d e t o b e e n t e r e d n e x t * /9 i n t g r a d e ; / * g r a d e v a l u e * /10 i n t t o t a l ; / * s u m o f g r a d e s i n p u t b y u s e r * /11 i n t a v e r a g e ; / * a v e r a g e o f g r a d e s * /12

13 / * i n i t i a l i z a t i o n p h a s e * /14 t o t a l = 0 ; / * i n i t i a l i z e t o t a l * /15 c o u n t e r = 1 ; / * i n i t i a l i z e l o o p c o u n t e r * /16

17 / * p r o c e s s i n g p h a s e * /18 wh i l e ( c o u n t e r < = 1 0 ) { / * l o o p 1 0 t i me s * /19 p r i n t f ( " E n t e r g r a d e : " ) ; / * p r o mp t f o r i n p u t * /20 s c a n f ( " %d " , & g r a d e ) ; / * r e a d g r a d e f r o m u s e r * /21 t o t a l = t o t a l + g r a d e ; / * a d d g r a d e t o t o t a l * /22 c o u n t e r = c o u n t e r + 1 ; / * i n c r e me n t c o u n t e r * /23 } / * e n d wh i l e * /24

2825 / * t e r mi n a t i o n p h a s e * /


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Outline28

Outline



Program Outputn t e r g r a d e : 9 8E n t e r g r a d e : 7 6E n t e r g r a d e : 7 1E n t e r g r a d e : 8 7E n t e r g r a d e : 8 3E n t e r g r a d e : 9 0E n t e r g r a d e : 5 7E n t e r g r a d e : 7 9

E n t e r g r a d e : 8 2E n t e r g r a d e : 9 4Cl a s s a v e r a g e i s 8 1

26 a v e r a g e = t o t a l / 1 0 ; / * i n t e g e r d i v i s i o n * /27

28 / * d i s p l a y r e s u l t * /29 p r i n t f ( " Cl a s s a v e r a g e i s %d \ n " , a v e r a g e ) ;30

31 r e t u r n 0 ; / * i n d i c a t e p r o g r a m e n d e d s u c c e s s f u l l y * /32

33 } / * e n d f u n c t i o n ma i n * /

29


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for Statement

Syntax:

for (initialization; loop repetition condition; update)

statement;

OR

for (initialization; loop repetition condition; update) {statement1;

statement2;

statementn;

}

...

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Implementation With the for Statement

Counting loops have three loop control statements:

Initialization of the loop control variable. Test of the loop repetition condition.

Update of the loop variable.

The for statement designates a place for each ofthese components, resulting in a more compactcode for counting loops.

for (count=0; count


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Implementation With the for Statement Remark: The for statement is a special case of the while

statement. Any for loop can be rewritten as a while loop.

Most programmers prefer the for statement to implement a

counting loop.

for(expr1; expr2; expr3)

statement

expr1;

while (expr2) {

statement;expr3;

}

321 / * F i g . 4 . 6 : f i g 0 4 _ 0 6 . c


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OutlineOutline


fig04_06.c (Part 1 of 2)

2 Ca l c u l a t i n g c o mp o u n d i n t e r e s t * /3 # i n c l u d e < s t d i o . h >4 # i n c l u d e < ma t h . h >5

6 / * f u n c t i o n ma i n b e g i n s p r o g r a m e x e c u t i o n * /7 i n t ma i n ( )8 {9 d o u b l e a mo u n t ; / * a mo u n t o n d e p o s i t * /10 d o u b l e p r i n c i p a l = 1 0 0 0 . 0 ; / * s t a r t i n g p r i n c i p a l * /11 d o u b l e r a t e = . 0 5 ; / * i n t e r e s t r a t e * /12 i n t y e a r ; / * y e a r c o u n t e r * /13

14 / * o u t p u t t a b l e c o l u mn h e a d * /15 p r i n t f ( " %4 s %2 1 s \ n " , " Y e a r " , " A mo u n t o n d e p o s i t " ) ;16

17 / * c a l c u l a t e a mo u n t o n d e p o s i t f o r e a c h o f t e n y e a r s * /18 f o r ( y e a r = 1 ; y e a r < = 1 0 ; y e a r + + ) {19

20 / * c a l c u l a t e n e w a mo u n t f o r s p e c i f i e d y e a r * /21 a mo u n t = p r i n c i p a l * p o w( 1 . 0 + r a t e , y e a r ) ;22

23 / * o u t p u t o n e t a b l e r o w * /24 p r i n t f ( " %4 d %2 1 . 2 f \ n " , y e a r , a mo u n t ) ;25 } / * e n d f o r * /26

33

27 r e t u r n 0 ; / * i n d i c a t e p r o g r a m e nd e d s u c c e s s f u l l y * /


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OutlineOutline



Program Output

Y e a r A mo u n t o n d e p o s i t1 1 0 5 0 . 0 02 1 1 0 2 . 5 03 1 1 5 7 . 6 34 1 2 1 5 . 5 15 1 2 7 6 . 2 86 1 3 4 0 . 1 07 1 4 0 7 . 1 08 1 4 7 7 . 4 69 1 5 5 1 . 3 31 0 1 6 2 8 . 8 9

28

29 } / * e n d f u n c t i o n ma i n * /

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General Conditional Loop:

The loop continues to execute as long as a repetition

condition remains true.

Example:

/* Multiply data while product remains less than 1000. */

product = 1; /* initialization */while (product < 1000) {

printf(%d\n, product); /* Display product so far */

printf(Enter next item > );

scanf(%d, &item);product = product * item; /* update product */

}

Running productRunning product

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Input Validation Loop:

The loop continues to read input data until the data

variable is in the accepted range.

Example: Suppose a programmer wishes to ask for a

number and keeps asking until a valid positive number issupplied by the user.

printf(Enter number of items> );

scanf(%d, &num_item); /* initialize number of item */

while (num_item < 0) {printf(Invalid number; enter a positive number>);

scanf(%d, &num_item); /* update */

}

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Sentinel-controlled Loops

Many programs with loops input data items each time the

loop body is repeated. In most cases, the number of data

items is not known in advance. One way to signal the end

data and terminate the loop execution is to use a sentinelvalue.

A sentinel must be a value that could not occur as data.

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Example:

/* computes the sum of a list of exam scores */

#include

#define SENTINEL 99

intmain(void) {

int sum = 0; /* outputsum of scores input */

int score; /* inputcurrent score */

printf(Enter first score (or %d to quit)>,SENTINEL);scanf(%d, &score);

while (score != SENTINEL) {

sum += score;

printf(Enter next score (%d to quit)>,SENTINEL);scanf(%d, &score); /* Get next score */

}

printf(\n Sum of exam scores is %d\n, sum);

return 0;

}

Running sumRunning sum

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Any conditional loop can also be implemented by using the

for statement.

Most programmers prefer the while statement to

implement conditional loops.

The previous example could also be implemented with the

for statement though it seems quite bizarre.

for (scanf(%d,&score); score!=SENTINEL; scanf(%d, &score)) {

sum += score;printf(Enter next score (%d to quit)>,SENTINEL);

}

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Nested Loops

Loops may be nested just like other control

structures (e.g. the if statement). A nested loop

consists of an outer loop with one or more inner

loops.

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Example With the for Statement

sum = 0;

for (i=0; i


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Example With the for Statement

Suppose that we also want to compute the inner sums.

sum = 0;

for(i=0; i


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Example With the while Statement

Likewise the for and while loops

can be nested one inside the other.

Continue = Y;while (Continue == Y || Continue == y)

{

company_total = 0;

printf(At any time, type 99 when all data is entered.);

printf(\n Enter the first salesman\s total>);scanf(%lf,&individual_total);

while (individual_total != -99)

{

company_total += individual_total;

printf(Enter the next salesman\s total>);scanf(%lf,&individual_total);

}

printf(The company made %f in sales today.\n,

company_total);

printf(Type Y to continue, anything else to quit>);

scanf(%c,&Continue);

}

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do while Statement


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do-while Statement

The for and while statements both evaluate the loop

repetition condition before the execution of the loop body.

Sometimes we may need to check the repetition condition atthe end of the loop body.

Such a loop can be created by the do-while statement:

do {

statament1;statement2;

.

.

} while (condition);

Note that there isa semi-colon here.

Note that there isa semi-colon here.

Syntax:

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do-while Statement

With the do-while statement, first the loop is executed andthen the condition is tested. If the condition is true, the loopbody is repeated.

Therefore, a do-while loop is always executed at least once.

Example: Entering a valid letter from the prompt:

Since the user must enter at least one data character, the do-while is an ideal statement to implement this loop.

do {

printf(Enter a letter from A through Z>);

scanf(%c, &letter_choice);

} while (letter_choiceZ);

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Alternative With the while Statement

printf(Enter a letter from A through Z>);scanf(%c, &letter_choice);

while (letter_choiceZ) {

printf(Enter a letter from A through Z>);

scanf(%c, &letter_choice);

}

Which one is better?

Why?

Which one is better?

Why?

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C L E


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Common Loop Errors

The following code should sum up the values from 0 to n.

The following code should ask for numbers and calculate a

running sum until a sentinel value (-99) is encountered.

for (i=0, i


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break and continue Statements

A break statement can be used to exit from:

for

while, and

do/while

like in switch statement.

A break statement causes the innermostenclosing loop to be exited immediately.

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d St t t


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break and continue Statements

continue causes the next iteration of the

enclosing: for while, or

do/while

loop to begin

when continue statement is used in while anddo loops, this means that the condition part is

executed immediately

in the for loop, control passes to loop variableupdate step.

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Example for continue


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Example for continue

#include

int main(void){

int sum=0, n=1;while (n


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Reading

Sections 3.1-3.10 and

Chapter-4 of the textbook

comp 132 notes 3 program control

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