c# ean 13 generator Java Example of an Expression Containing Too Few Parentheses in Visual C#

Creation EAN-13 in Visual C# Java Example of an Expression Containing Too Few Parentheses

Java Example of an Expression Containing Too Few Parentheses
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if ( a < b == c == d ) ...
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19. General Control Issues
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This is a confusing expression to begin with, and it s even more confusing because it s not clear whether the coder means to test ( a < b ) == ( c == d ) or ( ( a < b ) == c ) == d. The following version of the expression is still a little confusing, but the parentheses help:
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Java Example of an Expression Better Parenthesized
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if ( ( a < b ) == ( c == d ) ) ...
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In this case, the parentheses help readability and the program s correctness the compiler wouldn t have interpreted the first code fragment this way. When in doubt, parenthesize.
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Man y programmer-oriented text editors have commands that match parentheses, brackets, and braces. For details on programming editors, see Editing in Section 30.2.
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1 CROSS-REFERENCE
Use a simple counting technique to balance parentheses If you have trouble telling whether parentheses balance, here s a simple counting trick that helps. Start by saying zero. Move along the expression, left to right. When you encounter an opening parenthesis, say one. Each time you encounter another opening parenthesis, increase the number you say. Each time you encounter a closing parenthesis, decrease the number you say. If, at the end of the expression, you re back to 0, your parentheses are balanced.
Java Example of Balanced Parentheses
8 Read this. Say this.
if ( ( ( a < b ) == ( c == d ) ) && !done ) ... | | | 0 1 2 3 | 2 | 3 | | 2 1 | 0
In this example, you ended with a 0, so the parentheses are balanced. In the next example, the parentheses aren t balanced:
Java Example of Unbalanced Parentheses
Read this. Say this.
if ( ( a < b ) == ( c == d ) ) && !done ) ... | | 0 1 2 | 1 | 2 | | 1 0 | -1
The 0 before you get to the last closing parenthesis is a tip-off that a parenthesis is missing before that point. You shouldn t get a 0 until the last parenthesis of the expression.
Fully parenthesize logical expressions Parentheses are cheap, and they aid readability. Fully parenthesizing logical expressions as a matter of habit is good practice.
Knowing How Boolean Expressions Are Evaluated
Many languages have an implied form of control that comes into play in the evaluation of boolean expressions. Compilers for some languages evaluate each term in a boolean expression before combining the terms and evaluating the whole expression. Compilers for other languages have short-circuit or lazy
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19. General Control Issues
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evaluation, evaluating only the pieces necessary. This is particularly significant when, depending on the results of the first test, you might not want the second test to be executed. For example, suppose you re checking the elements of an array and you have the following test:
Pseudocode Example of an Erroneous Test
while ( i < MAX_ELEMENTS and item[ i ] <> 0 ) ...
If this whole expression is evaluated, you ll get an error on the last pass through the loop. The variable i equals maxElements, so the expression item[ i ] is equivalent to item[ maxElements ], which is an array-index error. You might argue that it doesn t matter since you re only looking at the value, not changing it. But it s sloppy programming practice and could confuse someone reading the code. In many environments it will also generate either a run-time error or a protection violation. In pseudocode, you could restructure the test so that the error doesn t occur:
Pseudocode Example of a Correctly Restructured Test
while ( i < MAX_ELEMENTS ) if ( item[ i ] <> 0 ) then ...
This is correct because item[ i ] isn t evaluated unless i is less than maxElements. Many modern languages provide facilities that prevent this kind of error from happening in the first place. For example, C++ uses 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. In other words, in C++ the only part of
if ( SomethingFalse && SomeCondition ) ...
that s evaluated is SomethingFalse. Evaluation stops as soon as SomethingFalse is identified as false. Evaluation is similarly short-circuited with the or operator. In Java and C++, the only part of
if ( SomethingTrue || SomeCondition ) ...
that is evaluated is SomethingTrue. The evaluation stops as soon as SomethingTrue is identified as true. As a result of this method of evaluation, the following statement is a fine, legal statement.
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