Conditionals

Conditional Execution A conditional statement allows the computer to choose an execution path depending on the value of a variable or expression – if the withdrawal is more than the bank balance, then print an error –if today is my birthday, then add one to my age –if my grade is greater than 3.5, then give ‘A’ as final grade

Conditional statements in C C supports several forms for conditionals: –if –if-else –if-elsif-elsif-…-else –And more… You as a programmer must decide which form to use. General semantics: the conditional statement depends on a boolean condition, the guard; if this condition is true the statement that depends on it will get executed;

Conditional Statement in C if (condition) statement; The statement is executed if and only if the condition is true. if (x < 100) x = x + 1; if (withdrawalAmount > balance) printf( “NSF check.\n”); if (temperature > 98.6) printf(“You have a fever.\n”);

Conditional statements in C if (condition) statement1; else statement2; Semantics: if condition is true, statement1 gets executed, otherwise statement2 gets executed. if (value >= 70) passed = passed + 1; else notPassed = notPassed + 1;

Conditional statements in C if (condition1) statement1; else if (condition2) statement2; else if (condition3) statement3; : : else if (conditionK) statementK; else statement; The first condition that is true (from top to bottom) will make its statement execute. If all conditions are false last statement executes.

Conditional Example if (grade >= 90) countAs = countsAs + 1; else if (grade >= 80) countBs = countBs + 1; else if (grade >= 70) countCs = countCs + 1; else if (grade >= 60) countDs = countDs + 1; else countFs = countFs + 1;

Flow of execution x = y + 10; (s1) if ( x < 100) x = x + 1; (s2) y = y – 10; (s3) S2 depend on the condition (X < 10) S1 and S3 do not depend on the condition (x < 10) x<100 Executes s1 true false executes s2 Does not execute s2 Executes s3

Conditional expressions Also called "logical" or "Boolean" expressions Made up of variables, constants, arithmetic expressions, and the "relational operators": Math symbols:,,=, in C:, >=, ==, != Conditional expression examples: airTemperature > <= body_temperature maritalStatus == ‘M’ divisor != 0

Value of conditional expressions In C conditional expressions are evaluated to yield an integer. –If the resulting integer value is 1, it is equivalent to TRUE. –If the resulting integer value is 0 it is equivalent to FALSE. An arithmetic relational operator (==, >=, !=) will yield 1 when the relation is true and 0 when false. Any integer expression can be used as a conditional expression: –If its value is not zero, it is true –Otherwise it its false. Thus we can write: if (value) equivalent to: if (value !=0)statement;

Formation of complex conditional expressions Boolean operators && stands for the and || stands for the or ! Stands for the negation operator Not Examples (x >= 0) && (x <=10) (y 10) !(x == 0) ! ( (x>=0) && (x <=10))

Boolean Tables for &&, ||, ! Be1 && be2 be1 || be !0 yields 1 !1 yields 0

DeMorgan’s Laws ! ( be1 && be2) == (!be1) || (!2be) ! ( be1 || be2) == (!be1) && (!2be)

Precedence rules of expressions with boolean operators Boolean ! Has higher precedence than && && has higher precedence than || &&, and || are left associative. !x && y equivalent to (!x) && y y && z || u equivalent to (y && z) || u z1 || z2 || z3 equivalent to (z1|| z2) || z3 z1 && z2 && z3 equivalent to (z1&& z2) && z3 ! Has higher precedence than, = &&, || have lower precedence than, = !x < y equivalent to (!x) < y X && y < z equivalent to x && (y < z)

Warning Notice that when we write: if (be) statement1; statement2; the boolean expression is only affecting statement1. Statement2 will always execute after the if is executed. What if we need to perform more than one statement based on a condition?

Blocks Use a block to form a group of statements, which will groups together statements so that they are treated as a single statement: { statement1; statement2; …. } With this notion we can write more complex conditional statements.

Example If (withdrawal <= balance){ balance = balance - withdrawal; monthyCharge = monthlyCharge + 1.0; } Else { monthlyPenalty = monthlyPenalty ; } To note: A single statement ends with semicolon A block does not have semicolon after ‘}’ A block statement can have zero or more statements Its best to use block statements in conditionals, it helps you reduces the number of errors

Pitfalls with if’s Using ‘=‘ instead of ‘==‘ It will not yield a compilation error And accompaning statement might or might not execute. In any case, it is a BUG hard to find!!!!!!!!!!! if ( value = average) statement ;

Pitfalls with if’s Althought the following statement will not give a syntactic error it is incorrect if ( low <= value <= high) value = value * 2; You should write it as: if (low <= value && value <= high) value = value * 2;

Pitfalls with if’s the operator & is different from && the operator | is different from || & and | are not boolean operators!!!!! if used by mistake, you will get no syntax errors, but program will most likely produce incorrect results.

Pitfalls with if’s When using doubles do not compare them for equality (==) or inequality (!=) When using either of these two operators with doubles you might get incorrect results due to accuracy of doubles! 30.0* (1.0/3.0) == 10.0 May yield false! 1 + ½. + 1/3. + ¼. +…..+ 1/n. Will most likely not be equal to 1/n. + 1/(n.-1) + … + ½. + 1 for a large value of n.