copyright © 1998-2015 curt hill the if revisited if part 4 style and testing

Copyright © 1998-2015 Curt Hill

The IF Revisited

If part 4Style and Testing


Now what?• There are several more issues that

need consideration• Nesting• Testing• If problems


Nesting

• Any statement may be the THEN or ELSE statement of an if

• It is most often a compound statement

• The interesting one is an if in an if• The usual problem is how the elses

match the ifs


Which else?

• The else may only match one of these

• Which one is it?• The answer is 3!

if(a>b) // 1 if(b>c) // 2 if(c>d)// 3 x = y;else x = z;


Matching Elses

• The general rule is that an else matches the closest, previous, unmatched if

• The language ignores white space and also the layout on the page– Thus indenting has no relevance to

the compiler– It may make the program more (or

less) readable


A Matched Else

• How is an if matched?• An else is encountered for it• It is enclosed within a compound

statement• An unrelated statement follows


Three examplesif(a>b) // 1 if(b>c){ // 2 if(c>d)// 3 x = y; } else // matches 2 x = z;

if(a>b) // 1 if(b>c) // 2 if(c>d)// 3 x = y; else x = 2*y; else // matches 2 x = z;

if(a>b) // 1 x = y; y = x*z; else // error x = z;


Testing

• Ifs complicate our testing• With only sequential flow we test it

once and it is covered• With multiple paths one test is

generally inadequate• Testing strategies then come into

view


Every Statement• The first strategy is to make sure

that in our testing every statement is tested once

• We generate test data accordingly• We have enough test data if each if has both its then and else exercised once

• This usually means several runs


Every statement

if(a>b) { // 1 b = a++ * 2; c /= 2; }else // 2 c = a * b / 2;

Two tests are needed.One forces a > b and the other a <= b

1 2


A path is a unique way through the code

1 2

3 4

There are four paths

1, 3

2, 3

1, 4

2, 4


Test Data Again• In the above, every statement testing

only needs two runs– The paths {1,3} and {2,4} executes every

statement

• However, there may be interactions between 1 and 4 that are not exercised

• Thus every path testing is always as good or better than every statement– In programs without decisions or loops one

set does both


How ifs increase paths

• An if added at the end of a sequence doubles the number of paths

• An if nested within an if adds one path to the existing number

• The generalities do not always capture the actual practice

• Reconsider the quadratic formula evaluation


Two different approaches

if(discrim < 0) …else if(discrim == 0){ … } else { … }

if(discrim < 0){ …}if(discrim == 0){ …}if(discrim > 0){ …}


Commentary on above

• The nested if version is to be preferred

• If any of the conditions are true then no further ones will be checked– Typically check the most likely one first

rather than the straight order I used

• However, provided that discrim is not changed in the ifs both have three paths


Largest of three variables• Here are two ways to display the largest

of three variables, a, b and c

if(a>b) if(a>c) cout << a; else cout << c;else if(b>c) cout << b; else cout << c;

double big;

big = a;if(big<b) big =b;if(big<c) big = c;

cout << big;

Which do you like better? Why?


Comparison• The first one has two comparisons

and one write• The second one has two

comparisons, one to three assignments, one write and needs one temporary variable

• Both have four paths• Most professional programmers

like the second one• Easier to categorize where you are


Paths and Complexity

• More paths cause more complexity• Complexity promotes errors• As programmers we work hard to

subdivide the problem to manage the complexity

• The magic number is 7 ± 2• TMI and cockpit stories

Error Checking

• If an error is found do one of two things– Do no other processing– Correct and continue

• At least two ways to do the former• The latter cannot be done in

general way– For any particular program there may

be a good correction


Do Nothing Further• Again two ways

– Place rest of method in an else– Use the return statement

• Big else:if(ed->GetValue().length()<1) // complainelse { // rest of method in this


Problems with this• If there are lots of separate checks then

the ifs and else get nested rather deeply– Deep nesting causes problems with

complexity

• Use the return statement:if(ed->GetValue().length()< 1){ ed->SetValue(“Empty field.”); return; }

• No else is needed


Return Statement• Two forms:• In a void method, such as an event

handler:return ;

• In a method that returns a value:return expr;– This form will be considered during

function definition presentations

• Return exits the method and optionally returns a value– No else is needed following


Correction• If there is an obvious correction use

that• Empty fields imply zero:if(edit1->GetValue().length()<1){ edit1->SetValue(“0”); }d = edit1->GetValue().ToInt();

• However, there is usually no easy fix• We will examine this again later


Separating Ranges• Often we want to classify a value

by range• For instance a descriptive word for

the age of person:infant is less than 1 yearpreschool is less than 5 yearsetc

• This is usually done with a series of ifs– There is a right and wrong way


Wrong Way• Not nested:if(age < 1) s = “infant”;if(age>=1 && age < 5) s = “preschool”;if(age>=5 && age < 12) …

• This works but there is lots of redundancy

• If s was set to “preschool” do we want to do another test


Right Way• Nested with simple conditions:if(age < 1) s = “infant”;else if(age < 5) s = “preschool”;else if(age < 12) …

• Also works, but the first one to find it eliminates any further tests


Not so right Way• Nested with ANDed conditions:if(age < 1) s = “infant”;else if(age>=1 && age<5) s = “preschool”;else if(age>=5 && age < 12) …

• Also works, but the age>=1 can never be false because of the nesting


copyright © 1998-2015 curt hill the if revisited if part 4 style and testing

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