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11. The Power of Variable Names
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11.1 Considerations in Choosing Good Names
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You can t give a variable a name the way you give a dog a name because it s cute or it has a good sound. Unlike the dog and its name, which are different entities, a variable and a variable s name are essentially the same thing. Consequently, the goodness or badness of a variable is largely determined by its name. Choose variable names with care. Here s an example of code that uses bad variable names:
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Java Example of Poor Variable Names
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x = x - xx; xxx = aretha + SalesTax( aretha ); x = x + LateFee( x1, x ) + xxx; x = x + Interest( x1, x );
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What s happening in this piece of code What do x1, xx, and xxx mean What does aretha mean Suppose someone told you that the code computed a total customer bill based on an outstanding balance and a new set of purchases. Which variable would you use to print the customer s bill for just the new set of purchases Here s a different version of the same code that makes these questions easier to answer:
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Java Example of Good Variable Names
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balance = balance - lastPayment; monthlyTotal = NewPurchases + SalesTax( newPurchases ); balance = balance + LateFee( customerID, balance ) + monthlyTotal; balance = balance + Interest( customerID, balance );
In view of the contrast between these two pieces of code, a good variable name is readable, memorable, and appropriate. You can use several general rules of thumb to achieve these goals.
The Most Important Naming Consideration
The most important consideration in naming a variable is that the name fully and accurately describe the entity the variable represents. An effective technique for coming up with a good name is to state in words what the variable represents. Often that statement itself is the best variable name. It s easy to read because it doesn t contain cryptic abbreviations, and it s unambiguous. Because it s a full
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11. The Power of Variable Names
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description of the entity, it won t be confused with something else. And it s easy to remember because the name is similar to the concept. For a variable that represents the number of people on the U.S. Olympic team, you would create the name numberOfPeopleOnTheUsOlympicTeam. A variable that represents the number of seats in a stadium would be numberOfSeatsInTheStadium. A variable that represents the maximum number of points scored by a country s team in any modern Olympics would be maximumNumberOfPointsInModernOlympics. A variable that contains the current interest rate is better named rate or interestRate than r or x. You get the idea. Note two characteristics of these names. First, they re easy to decipher. In fact, they don t need to be deciphered at all because you can simply read them. But second, some of the names are long too long to be practical. I ll get to the question of variable-name length shortly. Here are several examples of variable names, good and bad:
4 CROSS-REFERENCE
Table 11-1. Examples of Good and Bad Variable Names Purpose of Variable Good Names, Good Descriptors Bad Names, Poor Descriptors written, ct, checks, CHKTTL, x, x1, x2 velt, v, tv, x, x1, x2, train cd, current, c, x, x1, x2, date lpp, lines, l, x, x1, x2
name nChecks uses the Standardized Prefix naming convention described later in Section 11.5 of this chapter.
Running total of runningTotal, checkTotal, checks written to date nChecks Velocity of a bullet train Current date Lines per page velocity, trainVelocity, velocityInMph currentDate, todaysDate linesPerPage
The names currentDate and todaysDate are good names because they fully and accurately describe the idea of current date. In fact, they use the obvious words. Programmers sometimes overlook using the ordinary words, which is often the easiest solution. cd and c are poor names because they re too short and not at all descriptive. current is poor because it doesn t tell you what is current. date is almost a good name, but it s a poor name in the final analysis because the date involved isn t just any date, but the current date. date by itself gives no such indication. x, x1, and x2 are poor names because they re always poor names x traditionally represents an unknown quantity; if you don t want your variables to be unknown quantities, think of better names. Names should be as specific as possible. Names like x, temp, and i that are general enough to be used for more than one purpose are not as informative as they could be and are usually bad names.
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