fundamentals of programming procedural...

Procedural Programming/Lab2 1

Universität Duisburg-Essen

PRACTICAL TRAINING TO THE LECTURE

Session Two: Boolean expressions, Conditional Executions:

Chains and Nested

Prof. Dr.Ing. Axel Hunger

Dipl.-Ing. Joachim Zumbrägel Universität Duisburg-Essen

Faculty of Engineering, Department Electrical Engineering and Information Technology Computer Engineering

Name: Matriculation-Number:

First Name: Group-Number:

Tutor: Date:

Fundamentals of Programming & Procedural Programming


Decision Making Calculations and expressions are merely a small fraction of computer programming. Till now, we concentrated on the linear programming, in which programs were being executed in a straight line, statement after statement, or as it is called officially as “sequential”. But as programs become more and more complex, they need to be equipped with decision making (selection) in order to specify which statements are to be executed in which order.

In this session, we will focus on the control flow of a program with the help of branching statements. Branching statements allow a particular section of the code to be executed or not, depending on the given conditional clauses.

RelationalOperatorsPrograms compare values by using relational operators. In C, supported relational operators are:

Operator Meaning > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != Not equal to

A relational expression consists of operands and a relational operator between them. The result of a relational expression is a Boolean value, represented as either true or false.

Note: The bool data type did not exist in C before C 99 came up. Since then we have the bool type. Before it was 1 seen as true and false seen as a 0. If you want to use the data type bool then you have to include the header file stdbool.h with the statement: “#include <stdbool.h”.

Example: Expression Value 8 == 8 true 8 != 8 false 8 < 8 false 8 > 8 false 8 <= 8 true

The above expressions use operands having literal values, but operands can also be variables, like shown in the following example.


Example: /* example: using relational operators */ #include <stdio.h> int main() { int x = 8, y = 9; printf("%d < %d is %d", x, y, x < y); printf("\n%d > %d is %d", x, y, x > y); printf("\n%d >= %d is %d", x, y, x >= y); printf("\n%d <= %d is %d", x, y, x <= y); printf("\n%d == %d is %d", x, y, x == y); return 0; }

Exercise 2.1: Please trace the result of each of the output statements in the above program and write down the results below. Note: In Boolean expressions, 0 (zero) is the integer value of false, while 1 (one) is the integer value of true Exercise 2.2: Write a program which calculates whether the following expressions are true or false and will print accordingly: 3 > 4; ‘x’ < ‘y’; 6 = = 7;


LogicalOperatorsLogical operators in C enable you to combine comparisons in one if statement or else‐if statement.

The && Operator Also known as the And Operator, the && Operator connects two relational expressions. Both of the expressions should be true for the overall expression to return a true. For example:

if (age >= 13 && gender == 1) femaleTeenager = femaleTeenager + 1;

In the above example, the combined condition becomes true only and if only both conditions are true and the combined condition is false if either or both of the conditions are false.

Expression 1 Expression 2 Expression 1 && Expression 2

False False False

False True False

True False False

True True True

Truth table for the && operator

The || Operator Also known as the Or Operator, the || Operator connects two relational expressions. If either value of the expressions is true, the overall expression returns a true. For example:

if (termAverage >= 80 || finalAverage >= 80) printf("Grade is A");

In the above example, the combined condition becomes true if either or both of the conditions are true and the combined condition is false only if both of the conditions are false.

Expression 1 Expression 2 Expression 1 || Expression 2

False False False

False True True

True False True

True True True

Truth table for the || operator

The ! Operator Also known as the Not Operator, the ! Operator reverses the value of an expression, making the value of a true expression false and the value of a false expression true.


Example: /* using the not operator */ #include <stdio.h> int main() { int marks; printf("Please enter your marks:"); scanf("%d",&marks); if ( ! (marks <= 30) ) printf("You have passed the exam!"); else printf("Sorry, You failed the exam!"); return 0; }

Expression ! Expression

False True

True False

Truth table for the ! operator

Exercise 2.3: Determine whether the following expressions return a value true or false: Considering the given declarations: int cutoff=0, count=5, I=0, j=1; a. (cutoff ==0) && (count <=5)

b. (cutoff ==0) && (count< 10)

c. (count <20) || (cutoff >5)

d. ( !( ( (cutoff < 100) || (i <j) ) && (count <=10) ) )

e. ((cutoff < 20) || (j< i))


TheifStatementThe if statement executes the subsequent code (either one statement or a block of statements embraced witch {}) when the value of a relational expression is true. The relational expression should be contained within parentheses, and should not be terminated with a semicolon. Syntax: if (relational expression) statement;//executed if the relation expression is true

if (relational expression) { // the statements in this block are executed // if the relational expression is true statement1; statement2; }

The statement respectively the block of statements are only executed if the value of the relational expression is true, otherwise it’s skipped.

Example:

/* example: using the if statement */ #include <stdio.h> int main() { int num; printf("Enter an integer number:"); scanf("%d",&num); if (num > 0) printf("Your given number is positive"); return 0; }


Exercise 2.4: Draw a flowchart for the above example program.


Theif‐elseStatementConsidering the previous example, what if we want to have an output if the entered number is ‘odd’ instead of ‘even’. There’s no consequent statement if the condition is false. For this purpose, we can use the if/else statement.

Syntax:

if (relational expression)

statement;

else

statement;

Example: /* example: using the if-else statement */ #include <stdio.h> int main() { int num; printf("Please enter an integer number:"); scanf("%d",&num); if (num > 0) { printf("Your given number is a positive number"); } else { printf("Your given number is a negative number or zero"); } return 0; }


The following figure shows the flowchart for the above program:

Display the number is negative

True

False

Prompt user to input number

(num)

num > 0

Input num

Display the number is positive

End

Start


Exercise 2.5: Write a program which prompts the user to enter two numbers of type double. The program then should decide which number is smaller of the two, and then displays an appropriate message.


Exercise 2.6: Write a program which asks the user to enter a number of type integer, decides whether the integer number is an even or an odd number, and then displays an appropriate message.

Exercise 2.7: Find the final value of the variable result in the following code:

float x = 52.5, y = 37.6; int result = 0; if ( (x > 40.0) && (y <= 40.0) ) result = result + 1; else result = result - 1;


The Conditional Operator There is another short way to express a form of if‐else statement. C uses a special type of operator called the conditional operator ?: to do this in a conditional expression.

This conditional operator takes three operands, each as an expression.

Syntax:

expression1 ? expression2 : expression3

If expression1 is true, the value of the whole expression equals the value of expression2. Else, it will equals the value of expression3.

For example:

(8 > 4) ? 1 : 2 has a value of 1.

(2 > 6) ? 1 : 2 has the value 2.

(i > j) ? i : j has the value of the larger of i or j.

Theif‐else‐if‐elseStatementScenarios where there are more than two options, you need another expression for each additional alternative. That expression is known as the ‘else if’ statement. Syntax:

if (relational expression) statement; else if statement; else statement;

Example: /* example: using the if-else-if-else statement */ #include <stdio.h> int main() { int a, b, c; printf("Enter the first num: "); scanf("%d",&a); printf("Enter the second num: "); scanf("%d",&b); printf("Enter the third num: "); scanf("%d",&c);


if (a > b && a > c) { printf("%d \t is greater\n", a); } else { printf("%d \t is smaller\n", a); } if(b > a && b > c) { printf("%d \t is greater\n", b); } else { printf("%d \t is smaller\n", b); } if(c > a && c > b) { printf("%d \t is greater\n", c); } else { printf("%d \t is smaller\n", c); } return 0; }

Exercise 2.8: Please trace the result of each of the output statements in the above program and write down the results below.


Exercise 2.9: The following flowchart is an example of the if‐else‐if‐else statement. Translate the flowchart into C code:

Start

End

num > 0 True

False

num < 0 True

False

Prompt user to input number

input number

Display the number is positive

Display the number is negative

Display the number is a zero


Place here the C‐Code for Exercise 2.9!

NestedifStatementsA nested if statement is an if statement which appears inside another if statement. They are used to resolve if either one of them of the expressions is true or whether both of the relational expressions hold true.

Example: /* example: using the nested if-else statement */ #include <stdio.h> int main() { int num; printf("Please enter a positive integer number: "); scanf("%d",&num); if (num > 0) { if (num%2 ==0) printf("The entered number is positive and even"); else printf("The entered number is positive and odd"); } else printf("The entered number is negative. Please try again."); return 0; }


Exercise 2.10: Rewrite the above code without using the nested if‐else statement. Modify it so that it shows whether an entered number is negative, and whether it is even or odd in all cases.


MistakestobeAvoided• An ‘else’ keyword always comes with the ‘if’ keyword. You can’t use an ‘else’ without the ‘if’

keyword.

• A Relational Expression is not supposed to be used after the ‘else’ keyword.

• A Semicolon is not used after the ‘else’ keyword

Exercise 2.11: Find and correct the errors in the following program segments: a) if (code == 'y') tax = pay * 0.07; else; tax = pay * 0.045; }

b) if (percentage >= 75); printf("Percentage is greater than or equal to 75"); else printf("Percentage is less than 75");


Exercise 2.12: State whether the following nested‐if‐else statement is correct or not. If not, then rewrite it using proper indentation.

if (a > b) if (j == k) {x = y; t = s;} else u = t; else if (f > e) h=h+1;

Exercise 2.13: Translate the following pseudocode into a valid C program:

Set count to 1 If (balance >= 0) begin Set current_balance to balance Display balance end Else Set count to 0 End if


TheSwitchStatementThe switch statement allows a particular set of statements to be selected from several available choices. The selection is done based upon the current value of an expression, which is included within the switch statement.

Syntax:

The general form of a switch statement is

Switch (expression) statement

where expression results in an integer value.

Note that expression can also be of type char, since individual characters have equivalent integer values.

The switch statement body is made up of a set of cases. Each case is identified with a case label. A case label starts with a case keyword, then a case value, followed with a colon. The case labels identify the different group of the statement (i.e. the different alternatives) and distinguish them from one another.

The case label is followed by the body for that case. If the expression equals the case value that is in the case label, the program then executes the statements in the statement body.

Example: /* example: using the switch statement */ #include <stdio.h> int main() { int a, b; int choice; printf("Please enter first number: "); scanf("%d",&a); printf("Please enter second number: "); scanf("%d",&b); printf("Please enter your choice: "); printf("\n1 - Addition: "); printf("\n2 - Multiplication: "); printf("\n3 - Subtraction: "); printf("\n4 - Division: "); scanf("%d", &choice);


switch(choice) { case (1): printf("The sum is of the two numbers %d", a + b); break; case (2): printf("The multiplication of the two numbers is %d", a * b); break; case (3): printf("The subtraction of the two numbers is %d", a - b); break; case (4): printf("The division is of the two numbers %d", a / b); break; default: printf("Incorrect choice! Please try again."); return 0; }

Note that each of the groups is end with the break statement. The break statement causes control to be transferred out of the switch statement, thus preventing more than one group of statement from being executed. One of the labeled groups of the statement within the switch statement may be labeled default. This is executed if none of the case labels match the value of the expression. The default case label is optional and doesn’t need to be included at the end of the body of the switch statement.


Exercise 2.14: What output would the following piece of code give?

int const choice = 1; switch(choice + 1) { case '1': printf("Tea\n"); break; case '2': printf("Coffee\n"); break; case '3': printf("Milkshake\n"); break; default: printf("Bon appétit!\n"); return 0; }

Note:

bon appétit, originated in French, basically means good appetite or enjoy your meal.

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