1 chapter 3 flow of control. 2 review of class on sep 23

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Chapter 3Flow of Control

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Review of Class on Sep 23

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Outline of Chapter 3

Three types of flow of controlHow to specify conditions?

Relational, Equality and Logical Operators

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Introduction

Sequential flow of control Statements in a program are executed one

after another.

/*The traditional first program in honor of Dennis Ritchie who invented C at Bell Labs in 1972.*/

#include <stdio.h>int main(void){ printf(“Hello, world!\n”); return 0;}

hello.c

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Introduction

Flow of Control Sequential flow of control

Statements in a program are executed one after another.

Selection: select among alternative actionsThe condition to select a specific actionSelect: if, if-else, switch statement

Iteraton: achieve iterative actionsThe condition to end the iterative action.Iteration: while, for, do statement

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Outline of Chapter 3

How to specify conditions? Relational, Equality and Logical Operators

Statements Statements: compound statement and empty

statement Select among alternative actions

The if and if-else statementThe switch statementSelect using operator: The conditional Operator

Achieve iterative actionsThe while statementThe for statementThe do statement

Nested Flow of Control

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Relational, Equality, and Logical Operators

How true and false are implemented in C Representation of true and false

false: represented by any zero valueo int 0o floating 0.0o Null character ‘\0’o Null Pointer ( will be introduced in Chapter 8)

true: represented by any nonzero valueThese operators yield either the intint value

0 (false) or the intint value 1 (true).

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Relational Operators and Expressions

Relational operators: Four types of operators: <,>,<=,>= Operands: takes two expressions as operands Result: int value 0(false) or int value

1(true).Examples

1<1 1<=1 3>9 Variable: a<3 Complex Expression: -1.1>=(2.2*x+3.3)

false 0 true 1 false 0

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Equality Operators and Expressions

Equality Operators: == and != Operands: takes two expressions as operands Result: int value 0(false) or int value 1(true).

Examples: ‘A’ == ‘B’, ‘A’ != ‘B’ Variable: count !=-2 Complex expression: x+y == 2*z -5 int A = 2;

A==1A =3

operator == is different from

operator =

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Logical Operators

Logical negation !a Return 1 if a is zero value Otherwise return 0

a && b Return 1 (true) if both a and b are nonzero Otherwise return 0 (false).

a || b Return 0 (false) if both a and b are zero Otherwise return 1 (true)

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Precedence and associativity

Operators Associativity

++(postfix) –(postfix) Left to right

!,++(prefix),--(prefix),+(unary),-(unary)

Right to left

…… ……

+ - Left to right

Relational: < <= > >= Left to right

Equality: == != Left to right

Logical: && Left to right

Logical: || Left to right

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Summary Introduction

True: non zero valueFalse: zero value

Relational Operators< > <= >=

Equality Operators== !=

Logical Operators! && ||

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End of Review

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Class on Oct 06, Tuesday

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Outline

How to specify conditions? Relational, Equality and Logical Operators

Statements Statements: compound statement and empty

statement Select among alternative actions

The if and if-else statementThe switch statementsThe conditional Operator

Achieve iterative actionsThe while statementThe for statementThe do statementThe break and continue statements


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Statements: compound statement and empty statement

Expression statement Expression:

meaningful combinations of constants, variables and function calls.

Most expressions have both a type (such as int, char, float number) and a value.

Expression statementAn expression followed by a semicolon

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Examples of statement Examples:

Expression: a =bStatement: a=b;

Expression: a + b + cStatement: a+b+c; /* legal, but no useful work done

*/

Expression: printf(“%d\n”,a)Statement: printf(“%d\n”, a);

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What is a compound statement? A series of declarations and statements

surrounded by braces. The chief use is to group statements into an

executable unit. Example:

{

a =1;{

b = 2;c = 3;

}

}

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What is empty statement? A single semicolon. Useful where a statement is required by the

syntax, but no action is required.in some situations of if-else and for

statement Example:

a =b;; /* an empty statement */printf(%d\n”,a);

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What is statement? Expression statement

An expression followed by a semicolon Compound Statement

A series of declarations and statements surrounded by braces.

Empty StatementA single semicolon.

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OutlineHow to specify conditions?

Relational, Equality and Logical OperatorsStatements


Select among alternative actionsThe if and if-else statementThe switch statementsThe conditional Operator

Achieve iterative actionsThe while statementThe for statementThe do statementThe break and continue statements


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Introduction

Weekday

Weekend

Long distancecharges

8am to 5pm5pm to 11pm

11pm to 8am

Sat.

Sun

evening rate

full rate

night rate

8am to 5pm

5pm to 11pm

11pm to 8am

evening rate

night rate

night rate

night rate

Flow of Control: Select among alternative actions

if (isweekday(day) && (time==11pm)) rate = nightrate;

Decision Tree

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The if and if-else Statement

General form of if statementif (expr)

statementnext statement

expr: usually is a relational, equality, or logical expression. (to specify the condition)

If expr is nonzero (true), then statement is executed; otherwise statement is skipped, and control passes to the next statement.

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General form of if-else statementif (expr)

statement1else

statement2 expr: usually is a relational, equality, or logical

expression. (to specify the condition) If expr is nonzero (true), then statement1 is

executed; if expr is zero (false), then statement1 is skipped and statement2 is executed.

Then control passes to the next statement.

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Example:

if (grade >= 90) printf(“Congratulations! \n”);

printf(“Your grade is %d. \n”, grade);

What is the output if grade = 91?Congratulations!Your grade is 91.

What is the output if grade = 89?Your grade is 89.

If (expr) statement1

Next statement

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Example:

#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) min = x; else min = y; printf("x = %d, y = %d, min = %d\n", x, y, min); return 0;}

% gcc if.c% a.outx = 10, y = 11, min = 10

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Notes: expr is enclosed by parentheses


elsestatement2


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#include <stdio.h>int main(void){ if 0==1 printf(" 0 is equal to 1 \n"); return 0;} % gcc if1.c

if1.c: In function `main7':if1.c:4: error: parse error before numeric constant

Example:

Condition expr should be enclosed by parentheses

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Notes: Where appropriate, compound statements

should be used to group a series of statements under the control of a single if expression if (expr)

statement1else statement2

if (expr) statement1

if (expr){ statements}else{ statements}

if (expr) { statements}

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The if and if-else Statement#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) { min = x; printf(“x is min\n”); } else { min = y; printf(“y is min\n”); } printf("x = %d, y = %d, min = %d\n", x, y, min); return 0;}

if (expr) statement1else statement2

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#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) min = x; printf(" x is smalller than y \n"); else min = y; printf(" y is smalller than x \n");

printf("x = %d, y = %d, min = %d\n", x, y, min); return 0;}

% gcc if2.cif2.c: In function `main':if2.c:9: error: parse error before "else"

Example:

if(expr) statement1 statement2else statement3 statement4

if (expr) statement1else statement2

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#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) min = x; printf(" x is smaller than y \n"); if ( y<x ) min = y; printf(" y is smaller than x \n");


% gcc if3.c% a.out x is smaller than y y is smaller than xx = 10, y = 11, min = 10

Example:


statement2if (expr)

statement3statement4

Statement1:Statement2:

Statement3:Statement4:

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#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) min = x; printf(" x is smaller than y \n"); if ( y<x ) min = y; printf(" y is smaller than x \n");


Example:

{

}

{

}

% gcc if3_correct.c% a.out x is smaller than yx = 10, y = 11, min = 10

if (expr) compound statement1if (expr) compound statement2

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The if and if-else StatementExample:

#include <stdio.h>int main(void){ int x = 10, y = 11; int min; if ( x<y ) { min = x; }; else{ min = y; } return 0;}


elsestatement2

% gcc if5.cif5.c: In function `main':if5.c:10: error: parse error before "else


If (expr) compound

statementempty statement

elsecompound

statement

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Notes: Because an if or if-else statement is itself a

statement, it can be used as the statement part of another if or if-else statement.


elsestatement2


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Example

if ( a==2 ) if ( b==2 ) printf(“Hi\n");

if (exp )

if statement 1if (exp) statement2


if(c==2) if ( a==2 ) if ( b==2 ) printf(“Hi\n");

if (c==2 )

if statement 1if (a==2)

if statement 2if (b==2) printf(“Hi\n”);

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Example if ( a==2 ) if ( b==2 ) printf(“1\n");else printf(“2\n");

if (exp )

if-else statementIf (exp) statementelse statement

if (exp )

else statement

if statementIf (exp) statement

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Rule: When an if or if-else statement is used as

the statement part of another if or if-else statement, an else attaches to the nearest if.


elsestatement2


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Example if ( a==2 ) if ( b==2 ) printf(“1\n");else printf(“2\n");

if (exp )

An if-else statementIf (exp) statementelse statement

if (exp )

else statement

An if statementIf (exp) statement

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Summary

exp is enclosed by parentheses Where appropriate, compound statements

should be used to group a series of statements under the control of a single if expression

An if or if-else statement can be used as the statement part of another if or if-else statement.an else attaches to the nearest if.


elsestatement2


1 chapter 3 flow of control. 2 review of class on sep 23

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