Control Structures

Chapter 14-19

Outline

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Organizing Straight-Line Code Using Conditionals Table-Driven Methods Control Structures and Complexity

Organizing Straight-Line Code

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Putting statements and blocks of statements in sequential order - a trivial task?        fopen( )

             fread( )              // fclose( ) should go here            ProcessData( )            printf( )             fclose( )

But some organizational subtleties influence code quality, correctness, readability and maintainability

Problem: dependencies

Statements in a Specific Order

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Statements in which order counts: obviousdata = readData();results = calculateResultsFromData(data);printResults(results);

The 2nd statement depends on the 1st

The 3rd statement depends on the 2nd

Statements in which order counts: less obvious

revenue.computeMonthly();revenue.computeQuarterly();revenue.computeAnnually();

Hidden Dependencies: Example

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computeMarketingExpense();computeSalesExpense();computeTravelExpense();computePersonnelExpense();displayExpenseSummary();

Suppose computeMarketingExpense() initializes the variables that other routines put their data into

It needs to be called before the other routines!

Guidelines

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Organize code to make dependencies obvious initializeExpenseData()

Name routines to make dependencies obvious computeMarketExpense() is misnamed – it does

more than compute marketing expenses computeMarketingExpenseAndInitializeData

Use parameters to make dependencies obvious initializeExpenseData(expenseData); computeMarketExpense(expenseData); or

expenseData=computeMarketExpense(expenseData);

Making Dependencies Obvious: Example

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initializeExpenseData(expenseData);computeMarketingExpense(expenseData);computeSalesExpense(expenseData);computeTravelExpense(expenseData);computePersonnelExpense(expenseData);displayExpenseSummary(expenseData);

What if initializeExpenseData(expenseData) fails?

Guidelines - cont

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Check for dependencies with assertions or error-handling code, if the code is critical enough Constructor may initialize isExpenseDataInitialized

to false, then initializeExpenseData sets it to true Each function depending on expenseData’s

initialization can check isExpenseDataInitialized. The benefits should be weighted against the

additional complexity. It creates new variables and new checking code – all of

which create additional possibilities for error.

Document unclear dependencies with comments

Making Dependencies Obvious: Example

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initializeExpenseData(expenseData);if (isExpenseDataInitialized) {

computeMarketingExpense(expenseData);computeSalesExpense(expenseData);computeTravelExpense(expenseData);

computePersonnelExpense(expenseData);displayExpenseSummary(expenseData);

}

Statements Whose Order Doesn’t Matter

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MarketingData marketingData;SalesData salesData;TravelData travelData;

travelData.computerQuartly();salesData.computeQuartly();marketingData.computeQuartly

();

travelData.computerAnually();salesData.computeAnnually();marketingData.computeAnnuall

y();

travelData.print();salesData.print();marketingData.print();

Bad code that jumps around!

How marketingData is calculated?

Start at the last line and track all references back to the first line

Although used in a few places, you have to keep in mind how it is used everywhere between the first and last references

Making Code Read from Top to Bottom

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Experts agree that top-down order contributes most to readability

Better organizationMarketingData marketingData;marketingData.computeQuartly();marketingData.computeAnually();marketingData.print();…

References to each object are kept close together The # of LOC in which the objects are “live” is small The code now looks as if it could be broken into

separate routines for marketing, sales, and travel data.

Grouping Related Statements

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Statements are related because they operate on the same data, perform similar tasks, or depending on each other’s being performed in

order

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If the code is well organized into groups, boxes drawn around related sections don't overlap. They might be nested

If the code is organized poorly, boxes drawn around related sections overlap

14

Using Conditionals Put the normal case after the if rather than after the else

if(){normal case;

}else{

unnormal case}

Don't use NULL then clauses       

Can You Improve This Code?

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if ( ( (‘a’<=inputChar) && (inputChar <=‘z’)) || ( (‘A’<=inputChar) && (inputChar <=‘Z’))) {

charType = CharacterType.Letter;}else if ( (inputChar==‘ ‘) ||(inputChar == ‘,’) ||

(inputChar==‘.‘) || (inputChar==‘!‘) || (inputChar==‘(‘) ||(inputChar==‘)‘) || (inputChar==‘:‘) || (inputChar==‘;‘) ||(inputChar==‘?‘) || (inputChar==‘-‘)) {charType = CharacterType.Punctuation;

} else if ((‘0’<=inputChar) && (inputChar <=‘9’)) {charType = CharacterType.Digit; }

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Simplify complicated tests with boolean function calls

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Put the most common cases first

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Make sure that all cases are covered Code a final else clause with an error message or assertion to catch cases you

didn't plan for.

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Replace if-then-else chains with other constructs if your language

supports them

Can You Improve This Code?

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if (month==1) days =31;

else if (month==2)days = 28;

else if (month==3)days = 31;

…else if (month == 12)

days = 31;

Leap year: more complicated

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Controlling loop

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23

Guidelines

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25

26

27

28

29

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Calculate charge movies Three types: regular, new released, kid Regular

$2 for 2 days, $1.5 for each extra day

New released $3 per day

Kid $1.5 for 3 days $1.5 for each extra day

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Table-Driven Methods

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Can You Improve This Code?

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If your filing status is single If your filing status is marriedTax Bracket Percentage Tax Bracket Percentage

$0 … $21,450 15% $0 … $35,800 15%

Amount over $21,451, up to $51,900

28% Amount over $35,800, up to $86,500

28%

Amount over $51,900

31% Amount over $86,500

31%

TaxReturn.java: 1992 tax return Filing status: single, married (jointly/separately) Cutoffs/rates

Table-Driven Methods

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What is a table-driven method? A scheme that allows you to look up info in table Rather than using logic statements (if and case) to

figure it out Why?

In simple cases, logic statements are easier and more direct

As the logic chain becomes more complex, tables become increasingly attractive

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The CharacterType Example

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Use a lookup table Store the type of each character in an array that’s

accessed by character code charType = charTypeTable[inputChar]; Assume charTypeTable has been set up earlier

Put your program’s knowledge into its data rather than into its logic (if tests)

The Days-in-Month Example

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int daysPerMonth[] = {31,28, 31, 30,…, 31};

days = daysPerMonth[month-1];

Leap year days = daysPerMonth[month-1] + leapYearIndex(); // 0/1

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Stair-Step Access

Stair-Step Access Tables

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Entries in a table are valid for ranges of data rather than for distinct data points

Example<=100.0 A<90.0% B<75.0% C<65.0% D<50.0% F

Table structure?

Example

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double rangeLimit[] = {50.0, 65.0, 75.0, 90.0, 100.0};String grade[] = {“F”, “D”, “C”, “B”, “A”};int maxGradeLevel = grade.length -1;

int gradeLevel = 0;String studentGrade = “A”;while (studentGrade==“A” &&

gradeLevel<maxGradeLevel) {if (studentScore>rangeLimit[gradeLevel]

studentGrade = grade[gradeLevel];gradeLevel = gradeLevel+1;

}

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Try it out!

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Boolean Expression

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Boolean Expression – cont’ I ain't not no undummy

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Boolean Expression – cont’ Apply DeMorgan's Theorems to simplify boolean tests

with negatives

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Boolean Expression – cont’ Using Parentheses to Clarify Boolean

Expressions

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Boolean Expression – cont’ Short-circuit evaluation

if the first operand of the and is false, the second isn't evaluated because the whole expression would be false anyway.

if ( SomethingFalse && SomeCondition ) ... Only SomethingFalse will be evaluated Evaluation stops as soon as SomethingFalse is

identified as false

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Boolean Expression – cont’ using number-line ordering for boolean tests

Subtleties

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Watch the endpoints Consider using a binary search rather than a

sequential search for a larger list Consider using indexed access

Control Structures and Complexity

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Three Components of Structured Programming Sequence, Selection, Iteration Any control flow can be created Use of any other structure should be viewed with a

critical eye: break, continue, return, throw-catch Control structures are a big contributor to

overall program complexity Poor use of control structures increases

complexity. Programming complexity

Number of mental objects you have to keep in mind simultaneously in order to understand a program

How to Measure Complexity?

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McCabe’s Cyclomatic Number C = E-N+2 E (number of edges), N (number of nodes) C = R+1, R is the number of legitimate regions C = D+1, D is the number of primitive decisions

How to Measure Complexity

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McCabe’s Decision Points Start with 1 for the straight path through the

routine Add 1 for each of the following keywords, or their

equivalence: if, while, repeat, for, and, or Add 1 for each case in a case statement

Exampleif ((status==Success) && done) || (!done && (numLines >=maxLines)))…

5 decision points

Decision Points and Complexity

53

0-5: probably fine 6-10: start to think about ways to simplify 10+: brake part of the routine

It just moves the decision points around; doesn’t reduce the overall complexity

Reduces the amount of complexity you have to deal with at any ONE time

10 isn’t an absolute limit – use it as a warning flag Don’t use it as an inflexible rule

It might be foolish to break up a case statement with many cases

Reducing Complexity

54

Improve your own mental juggling abilities by doing mental exercises Programming itself is usually enough exercise People seem to have trouble juggling more than

about 5 to 9 mental entities Decrease the complexity of your programs

and the amount of concentration required to understand them