lecture 4 introduction to programming in c prof. dr. arne kutzner hanyang university / seoul korea

Lecture 4

Introduction to Programming in C

Prof. Dr. Arne Kutzner

Hanyang University / Seoul Korea

Control Structures in C

copyright © 2009 Arne Kutzner. All right reserved. L4.3

Control Structures (cont.)

• C provides the following control structures– compound structures (sequential Execution)– selection structures:

• if• if/else• switch

– repetition structures:• while• do/while• for

• Control structures are not thought to be statements in C. (Important regarding the placement of semicolons)

Sequential Execution of Statements in C


Sequential Execution (called Compound Statement)

• Statements are executed one by one in the order they are written in the curly braces:

{statement_1;statement_2;…statement_n;

}

begin in Pseudocode

end in Pseudocode

every statement in C ends with a semicolon !

Selection Structures:

if,if/else structures


if statement in C

if condition then statement

if (condition)statement;

Pseudocode

C Code

Parentheses necessary !


if-else statement in C

if condition then statement_1else statement_2

if (condition)statement_1;

elsestatement_2;

Pseudocode

C Code



if statement with one compound alternative

if condition then begin statement_1 statement_2 … end

if (condition) {statement_1;statement_2;…

}

Pseudocode

C Code


if Statement with Two Alternatives that are Compounds

if condition then begin statement_11 statement_12

… statement_1n endelse begin statement_21 statement_22

… statement_2m end

if (condition) { statement_11; statement_12; … statement_1n; } else { statement_21; statement_22; … statement_2m; }

Pseudocode C Code


Sequential if/else Structures• Test for multiple cases by placing if/else selection

structures inside if/else structures• Once condition is met, rest of statements is skipped

statement_1;if (condition_1){ statement_2; statement_3;}else if (condition_2){ statement_4; statement_5; }…else

statement_n;statement_m;


Example Program in Pseudocode

begin write "Enter midterm grade:" read midterm write "Enter final grade:" read final grade ← (0.6 * final) + (0.4 * midterm) write "Grade is " write grade

if grade > 60 then write " and passed" else write " and failed" end


Pseudocode in C#include <stdio.h>int main(){ double midterm, final, grade;

printf("Enter midterm grade:"); scanf ("%lf", &midterm); printf("\n Enter final grade:"); scanf ("%lf", &final); grade = 0.6 * final + 0.4 * midterm; printf("Grade is %lf and", grade);

if (grade > 60) printf(" passed!!\n");

else printf(" failed\n");

}

Variable Declaration necessary !


An ambiguous cases the last else-alternative belongs to the most recent if-structure in the same block. Example: int i = 1; int j = 2; int k = 3; if (i > j) if (i > k) printf("A"); else printf("B");

is equivalent to int i = 1; int j = 2; int k = 3; if (i > j) { if (i > k) printf("A"); else printf("B"); }

Nested if/else


To force the else clause to match the first if clause, you must add a pair of braces: int i = 1; int j = 2;int k = 3; if (i > j) { if (i > k) printf("A"); } else printf("B");

This code prints B.

Nested if/else (cont.)


Caution (weird C)

• Adding a semicolon at the end of the condition of a if structure can be a serious mistake.if (grade > 60); printf("Passed\n");

• This mistake is hard to find, because the above code is correct. It results in a logic error !!

misplaced semicolon

if grade > 60 thenbeginend

write "Passed"

Problem shown Pseudocode:

Repetition Structures

while - loops


while statement in C

while condition do statement

while (condition)statement;

Pseudocode

C Code



while statement combined with compound structure

while condition do begin statement_1 statement_2 … end

while (condition) {statement_1;statement_2;…

}

Pseudocode

C Code


Factorial Computation in Pseudo Code

begin write "Enter n:" read n product ← 1 i ← 1 while i <= n do begin product ← product * i i ← i + 1 end write product end


Factorial Computation in C

#include <stdio.h>void main() {

int n, i, product; printf("Enter n:"); scanf("%d", &n); product = 1; i = 1; while (i <= n) {

product = product * i; i = i + 1; } printf("%d", product);}

while loop in C


Caution (weird C)

• Adding a semicolon at the end of the condition of a while structure can be a serious mistake.i = 10;while (i > 0); i = i – 1;

• The above statements results in an infinite loop

Misplaced semicoloni ← 10while i > 0 do

beginend

i ← i - 1

Problem shownPseudocode:


Nested Loops

• Loops (like all other control structures as e.g. the if-structure) may be nested

• A nested loop consists of an outer loop with one or more inner loops.


Example of nested while loop

compute

N

i

M

j

ji0 0

*

int sum, i, j;sum = 0; i=0;

while (i <= N) { j = 0;

while (j <= M) { sum = sum + (i * j); j = j + 1; } i = i + 1;}

printf("The total sum is %d", sum);

outer loop

inner loop


Exercise

• Develop some program in Pseudocode that contains if/else, while structures and translate this program into C

Boolean Values and Conditional Expressions in

C


Representation of Boolean Values in C

• VERY IMPORTANT:In the C the boolean values true and false are represented by integer values.

• The rules for interpreting integer values:– false : the value 0– true : any value other then 0

(typically 1)

• Remark:C does not have a separated boolean datatype as e.g. Java (serious design lack of C)


Conditional Expression in C - Introductionif (condition)

statement;we call the code inside

the parentheses conditional expression

while (condition)statement;

• A conditional expression in C is an expression whose evaluation delivers a numerical value that is interpreted as a boolean value (true / false)


Constants as Conditional Expressions

• The two constants 0 and 1 are the simplest form of conditional expressions– The following statements prints asterisks

“forever”:

while (1)printf("*");


Single Variables as Conditional Expressions

• A single variable of type int represents a valid conditional expressions– The following statements prints asterisks

“forever”:

int i;i = 1;while (i)

printf("*");


Compound Conditional Expressions

• Compound conditional expressions can be created by the use of conditional operators.

• Simple forms of compound expressions:– variable conditional_operator variable

Example: x > y– variable conditional_operator constant

Example: x == 4


Types of conditional operators

1. Relational and equality operators for comparisons

2. Operators for the representation of the boolean operations and/or/not(called logical operators)


Relational and Equality Operators

Operator Meaning

< less than

> greater than

<= less than or equal to

>= greater than or equal to

== equal to

!= not equal to


Examplesx power y item MIN DAY num MAX Sens

-5 1024 7 1.5 -12.0 'M' 12 1024 12

Operator Condition Meaning Value

<= x <= 0 x less than or equal to 0 1 (true)

< power < MAX power less than MAX 0 (false)

>= x >= y x greater than or equal to y 0 (false)

> item > MIN item greater than MIN 1 (true)

== DAY == 'M' DAY equal to 'M' 1 (true)

!= num != Sens num not equal to Sens 0 (false)


Logical Operators in Conditional Expressions

• Logical Operators in C:

Operator Meaning

&& AND

|| OR

! NOT


Logical Operator: AND Operator

• Example: Suppose x=0.5, y=0.4, z=4

(y>z)&&(x<z) delivers value 0(x>y)&& z delivers value 1

x y x && ynonzero (true) nonzero (true) 1(true)

nonzero (true) 0 (false) 0 (false)

0 (false) nonzero (true) 0 (false)

0 (false) 0 (false) 0 (false)


Logical Operator: OR Operator


(y>z)||(x<z) delivers value 1(x<y)||(z!=4) delivers value 0

x y x || ynonzero (true) nonzero (true) 1(true)

nonzero (true) 0 (false) 1(true)

0 (false) nonzero (true) 1(true)

0 (false) 0 (false) 0 (false)


Logical Operator: NOT Operator


!(y > z) delivers value 1!z delivers value 0

x !xnonzero (true) 0 (false)

0 (false) 1 (true)


Binding Rules (Operator Precedence)

• How to evaluate a == b && c ?Two alternatives:1. First equality check and then AND-operation:

(a == b) && c

2. First AND-operation and then equality check: a == (b && c)

• This ambiguity is resolved by operator precedence rules


Operator PrecedenceOperator Precedence

highest (strong binding)

!

* / %

+ -

< <= >= >

== !=

&&

||

= lowest (weak binding)

arithmetic operators

assignment operator


Exercise• Compute the values for all

following expressions:

z y x a

1 1 0 10

Expression Value

a != 10 || a == 0 0 (false)

a > x && x < y 1 (true)

!x == 0 0 (false)

x && y || z 1 (true)

!a 0 (false)

!a + y + !x 2 (true)


Parentheses in Conditional Expressions

• Using parentheses you can always change the precedence of operators in a conditional expression.Example:!(a && b)The AND-operation is here more binding than the NOT-operation


Operator Associativity

When two operators with the same precedence are evaluated, the associativity of the operators determines the order of evaluation. All binary operators except assignment operators are left-associative. a – b + c – d is equivalent to ((a – b) + c) – d Assignment operators are right-associative. Therefore, the expression a = b = c = 5 is equivalent to a = (b = (c = 5))

Logical Assignments in C


Logical Assignments

• Logical assignments are assignments where the right side represents a conditional expression.Examples:senior_citizen = (age >= 65); in_range = (i > -10) && (i < 10);

is_even = ((n % 2) == 0);

conditional expressions


Caution (weird C)

• Logic assignments can trigger big trouble in the context of an intended equality checki = 0;if (i = 1) printf("true");else printf("false");

• The above statements prints true !

please distinguish = and ==

i ← falsei ← trueif i==true then

write "true"else

write "false"

identical logic in Pseudocode:

lecture 4 introduction to programming in c prof. dr. arne kutzner hanyang university / seoul korea

Documents

compound statement statements

c slide

final grade

pseudocode c code slide

failed end slide

ifelse selection structures

pseudocode end

pseudocodec code slide