Switch statement/choice coding March 2, 20061The The switchswitchStatement and Statement and Programming Choice StructuresProgramming Choice StructuresLarry CarettoComputer Science 106Computing in Engineering and ScienceMarch 2, 20062Outline• Review choice programming– Statements (if, if-else, if-else-if)– Conditions; relational and logical operators– Type bool variables– Nested if statements• End-of-file tests• Programming exercises3Review Choice (if statements)• Three structures if, if-else, and if-else-if• Based on statement if (<condition>)• Condition used relational operators (<, >, <=, >=, ==, !=) and logical operators not(!) and(&&) or(||)• Condition evaluates to true or false• In if-else and if-else-if only one block of code is executed4Review Type bool Variables• Type bool variables have two possible values: true and false• Can be used to hold result of expressions that give these values• leapYear = year % 4 == 0 && ( year % 100 != 0 || year % 400 == 0 )• Test bool variables in if statements and use with logical operatorsif ( leapYear && month == 2 ) days = 29;5Review DeMorgan’s Laws• Have two bool expressions, a and b, that can have values of true or false• Combinations of conditions for a and b satisfy both of the following• !(a && b) = !a || !b• !(a || b) = !a && !b• Proved these using truth table• Application to data validation follows6Review Data Validation• Apply DeMorgan’s Law to ValidationbadData = x < Min || x > Max;goodData = x >= Min && x <= Max;goodData = !badData;goodData = !(x < Min || x > Max);!(a || b ) = !a && !bgoodData = !(x < Min) && !(x>Max)goodData = x >= Min && x <= MaxSwitch statement/choice coding March 2, 200627Exercise• An example of an iteration problem, shown below, computes x = A)()()1(22nnnxAxx +=+• Iterations continue until converged, defined as |x(n+1)–x(n)| < ε1+ ε2 |x(n+1)|• Coding this problem uses variables xNew and xOld for x(n+1)and x(n)and e1 and e2 for error tolerances ε1and ε28Exercise Continued)1(21)1()1()()()1(22+++++<−+=nnnnnnxxxuntilxAxxεε• Define a bool variable, converged, that is true when |x(n+1)–x(n)| < ε1+ ε2|x(n+1)| using fabs(x) for |x|• bool converged = fabs(xNew -xOld) < e1 + e2 * fabs(xNew);• What condition is true the solution is converged or iterations > maximum• converged || iterations > maximum9Nested If Statements• Can have one if block inside another• Example: Find days in month– If the number of the month is 4, 6, 9, or 11 the answer is 30– If the number of the month is 2• If it is a leap year, the answer is 29• Otherwise the answer is 28– For all other month numbers (1, 3, 5, 7, 8, 10, and 12) the answer is 3110Days in Monthif ( month == 4 || month == 6 || month == 9 || month == 11 ){days = 30;}else if ( month == 2 ){if (leapYear) // bool var{ days = 29;} // continue on next chart11Days in Month Continuedelse{ days = 28; }} // ends else if (month==2) else{days = 31;}12The switch Statement• An alternative to the if-else-if– Tests for equality only• Dangerous difference – once a case is selected code for that case and all subsequent cases is executed• Start with switch (<expression>), where <expression> can be of type char or int• Followed by “cases” with particular values of the expression• Following example from Visual C++ HelpSwitch statement/choice coding March 2, 2006313Example of switchchar c;switch ( c ){case 'A':capa = capa + 1;break;case 'a':lettera = lettera + 1;break;default:nota = nota + 1;}Use break to skip subsequent code sectionsBraces not requiredUse colon (:) after each casedefault case is optional14Operation of switch• Starts with switch( <expression> )• Have a series of cases, srarting with the statement case <constant> :• Statements in case whose <constant>matches the value of the <expression> and all subsequent cases are executed– Use a break statement to do only one case• Braces not needed for each case, but overall switch statement has braces15Exercise with switch Statement• Printing an error code– For code = 0 print “no error”– For code = 1 print “value too low”– For code = 2 print “value too high”– For other codes print “incorrect code”• Write the program to print the correct message two ways– Use an if-else-if–Use a switch statement16if-else-if Solutionif ( code == 0 )cout << “No error”;else if (code == 1 )cout << “Value too low”;else if (code == 2 )cout << “Value too high”;elsecout << “Incorrect code”;17switch Statement Solutionswitch ( code ) {case 0:cout << “No error”;break;case 1:cout << “Value too low”;break;// continued on next chart18switch Statement Solution IIcase 2:cout << “Value too high”;break;default:cout << “Incorrect code”;}Switch statement/choice coding March 2, 2006419Alternative Structures• Recall previous example of finding days in a month depending on month of year and condition of a leap year or not• We used a nested if statement to code this • An alternative is to use a combined condition• Look at previous code first then consider alternative20Days in Monthif ( month == 4 || month == 6 || month == 9 || month == 11 ){days = 30;}else if ( month == 2 ){if (leapYear) // bool var{ days = 29;} // continue on next chart21Days in Month Continuedelse{ days = 28; }} // ends else if (month==2) else{days = 31;}22Alternative Days in Monthif ( month == 4 || month == 6 ||month == 9 || month == 11 )days = 30;else if ( month == 2 && leapYear )days = 29;else if ( month == 2 )days = 28; elsedays = 31;At this else-if we know that if month equals two it is not a leap year. Why?23The Dean’s List• Under a proposed change a student makes the dean’s list if the student is a undergraduate and– The student completes 12 units in a with a grade point average of 3.5 or– The student completes at least 6 units with a grade point average of 3.7• Select variables and write code that sets the bool variable deansList to true if the student makes the list (false if not) 24The Dean’s List Code• Use the following variables– status – a string variable equal to “grad” for graduate students and “undgrd” for undergraduaute students– units – a double variable equal to the units taken– gpa – a double variable equal to the grade point average– deansList – a bool variable that is true or false is a student is or is not on the listSwitch statement/choice coding