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Java Example of a Test That Works Because of Short-Circuit Evaluation
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if ( ( denominator != 0 ) && ( ( item / denominator ) > MIN_VALUE ) ) ...
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If this full expression were evaluated when denominator equaled 0, the division in the second operand would produce a divide-by-zero error. But since the
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19. General Control Issues
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second part isn t evaluated unless the first part is true, it is never evaluated when denominator equals 0, so no divide-by-zero error occurs. On the other hand, since the && (and) is evaluated left to right, the following logically equivalent statement doesn t work:
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Java Example of a Test That Short-Circuit Evaluation Doesn t Rescue
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if ( ( ( item / denominator ) > MIN_VALUE ) && ( denominator != 0 ) ) ...
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In this case, item / denominator is evaluated before denominator != 0. Consequently, this code commits the divide-by-zero error. Java further complicates this picture by providing logical operators and conditional operators. Java and C++ s && and <;$LB><;$LB> operators function similarly. Java s logical & and <;$LB> operators do not necessarily short-circuit the evaluation of the right-hand term when the left-hand term determines the truth or falsity of the expression. In other words, in Java, this is safe:
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Java Example of a Test That Works Because of Short-Circuit (Conditional) Evaluation
if ( ( denominator != 0 ) && ( ( item / denominator ) > MIN_VALUE ) ) ...
but this is not:
Java Example of a Test That Doesn t Work Because Short-Circuit Evaluation Isn t Guaranteed
if ( ( denominator != 0 ) & ( ( item / denominator ) > MIN_VALUE ) ) ...
6 KEY POINT
Different languages use different kinds of evaluation, and language implementers tend to take liberties with expression evaluation, so check the manual for the specific version of the language you re using to find out what kind of evaluation your language uses. Better yet, since a reader of your code might not be as sharp as you are, use nested tests to clarify your intentions instead of depending on evaluation order and short-circuit evaluation.
Writing Numeric Expressions in Number-Line Order
Organize numeric tests so that they follow the points on a number line. In general, structure your numeric tests so that you have comparisons like
MIN_ELEMENTS <= i and i <= MAX_ELEMENTS i < MIN_ELEMENTS or MAX_ELEMENTS < i
The idea is to order the elements left to right, from smallest to largest. In the first line, MIN_ELEMENTS and MAX_ELEMENTS are the two endpoints, so they go
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19. General Control Issues
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at the ends. The variable i is supposed to be between them, so it goes in the middle. In the second example, you re testing whether i is outside the range, so i goes on the outside of the test at either end and MIN_ELEMENTS and MAX_ELEMENTS go on the inside. This approach maps easily to a visual image of the comparison:
Valid values for
Valid values for
F19xx01
Figure 19-1 Examples of using number-line ordering for boolean tests.
If you re testing i against MIN_ELEMENTS only, the position of i varies depending on where i is when the test is successful. If i is supposed to be smaller, you ll have a test like
while ( i < MIN_ELEMENTS ) ...
But if i is supposed to be larger, you ll have a test like
while ( MIN_ELEMENTS < i ) ...
This approach is clearer than tests like
( i > MIN_ELEMENTS ) and ( i < MAX_ELEMENTS )
which give the reader no help in visualizing what is being tested.
Guidelines for Comparisons to 0
Programming languages use 0 for several purposes. It s a numeric value. It s a null terminator in a string. It s the lowest address a pointer can have. It s the value of the first item in an enumeration. It s False in logical expressions. Because it s used for so many purposes, you should write code that highlights the specific way 0 is used.
Compare logical variables implicitly As mentioned earlier, it s appropriate to write logical expressions such as
while ( !done ) ...
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This implicit comparison to 0 is appropriate because the comparison is in a logical expression.
Compare numbers to 0 Although it s appropriate to compare logical expressions implicitly, you should compare numeric expressions explicitly. For numbers, write
while ( balance != 0 ) ...
rather than
while ( balance ) ...
Compare characters to the null terminator (<;$QS>\0<;$QS>) explicitly Characters, like numbers, aren t logical expressions. Thus, for characters, write
while ( *charPtr != '\0' ) ...
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