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Conditional ExpressionsPowerPoint Presentation

Conditional Expressions

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Control Statements

A control statement will or will not be executed based on the value

of the conditional expression

scanf("%d", &pressureValue);

while(pressureValue <= MAX_PRESSURE)

{

if(pressureValue == NOMINAL_VALUE)

printf("Pressure is nominal\n");

else

printf("Pressure is %d pounds\n", pressureValue);

for (i = 5; i <= pressureValue; i = i + 10)

printf("+");

printf("\n");

scanf("%d", &pressureValue);

} // End while

Boolean Type

- A conditional expression evaluates to a value of true or false
- These are the two values for the Boolean type named after the mathematician George Boole
- When a conditional expression is evaluated, it results in a value of type Boolean
- In the C programming language there is no explicit Boolean type. Instead, false is 0 and true is any non-zero value. Many C programs use a #define macro to set a Boolean variable to true or false
#define FALSE 0

#define TRUE 1

- Conditional expressions are also called Boolean expressions
- A conditional expression is used whenever a true or false decision needs to be made about the value of certain variables or constants in a program

Expression Contents

- A conditional expression in C may contain any valid mathematical expression
- It may also contain relational operators and logical operators
- In addition, it may contain one or more function calls that return a value

Relational Operators

==,!= Equivalent, Not equivalent

<, <= Less than, Less than or equals

>, >= Greater than, Greater than or equals

if (A == B) C = 5;

if (A != B) C = 10;

for (i = 0; i < MAX_INDEX; i++) printf("*");

if (A <= B) D = 20;

while (A > B) B++;

if (A >= B) D = 40;

A = B <= C;

D = A == B;

Logical Operators

&& Logical AND

|| Logical OR

! Logical NOT

if ( (A == B) && (C <= D) ) E = 5;

if ( (A != B) || (C >= D) ) F = 10;

while ( !(A == B) ) B++;

if ( !((A >= B) && (C <= D) || (E != F)))

G = 100;

A = B || C;

D = A && B;

Boolean Algebra

- NOT false = true
- NOT true = false
- true AND true = true
- true AND false = false
- true OR true = true
- true OR false = true
- false AND false = false
- false OR false = false

if (0 || 0) printf("Does false OR false = true?");

if (0 && 0) printf("Does false AND false = true?");

More Examples

ch1 = 'a';

ch2 = 'a';

printf("ch1 OR ch2 = %d\n", ch1 || ch2); // 1

printf("ch1 AND ch2 = %d\n", ch1 && ch2); // 1

ch1 = 'd';

ch2 = 'f';

printf("ch1 OR ch2 = %d\n", ch1 || ch2); // 1

printf("ch1 AND ch2 = %d\n", ch1 && ch2); // 1

ch1 = 'a';

ch2 = '\0';

printf("ch1 OR ch2 = %d\n", ch1 || ch2); // 1

printf("ch1 AND ch2 = %d\n", ch1 && ch2); // 0

Example with Logical and Relational Operators

while ( (w <= x) && ( y >= z) )

{

if ( (w < MAX_HEIGHT) || (y > MAX_DISTANCE) )

{

w = w + newHeight;

y = y - newDistance;

newValuesUsed = TRUE;

}

else

{

w = w + oldHeight;

y = y - oldDistance;

newValuesUsed = FALSE;

} // End if

if ( !silentMode ) // Use of negative logic

printf("Height: %d Distance: &d\n", w, y);

} // End while

Operator Precedence

- Precedence refers to the order in which operations are evaluated in an expressiona = w * x + !y || -u – v && b % c / d++;
- There is a hierarchy of rules that tells which operators are evaluated before other operators (i.e., unary before binary operators, multiplication before addition, relational operators before logical operators)
- The highest precedence goes to parentheses

Rule of thumb: Avoid complicated

expressions and use parentheses

Special Notes

- Avoid complicated conditional expressions, especially
- When they are not a part of the original procedural design
- When they are created just to get the program to finally run

- Use meaningful names for Boolean variables and declare them of type int
- Ex. valueFound, errorStatus, moreData

- Do not compare Boolean variables to TRUE or FALSE
if (valueFound == TRUE) X = 5; // Bad practice

if (valueFound) X = 5; // Good practice

if (valueFound == FALSE) X = 5; // Bad practice

if (!valueFound) X = 5; // Good practice

Special Notes (continued)

- Beware of the bitwise operators; they are not the same as the logical operators
& bitwise AND

| bitwise OR

if (4 & 2) printf("Is this true?"); // Bitwise

if (4 && 2) printf("Or is this true?"); // Logical

- Do not user == or != operator with values of type float
double v = 0.999999999999999999999999999;

. . .

if (v == ZERO) X = 10; // Bad

if ((v >= -0.001) && (v <= 0.001)) x = 10; // Good

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