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Conditional Expressions

Conditional Expressions. 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)

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Conditional Expressions

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  1. Conditional Expressions

  2. 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

  3. 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

  4. 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

  5. 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;

  6. 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;

  7. 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?");

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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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