Why Mess with Logical Operations in Visual Studio .NET

Draw PDF417 in Visual Studio .NET Why Mess with Logical Operations

C++ programs have to make decisions A program that can t make decisions is of limited use The temperature-conversion program laid out in 1 is about as complex you can get without some type of decision-making Invariably a computer program gets to the point where it has to figure out situations such as Do this if the a variable is less than some value, do that other thing if it s not That s what makes a computer appear to be intelligent that it can make

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decisions (By the same token, that same property makes a computer look really stupid when the program makes the wrong decision) Making deci sions, right or wrong, requires the use of logical operators

Table 4-1

== != >, < >=, <= && || !

Meaning Equality; true if the left-hand argument has the same value as the right Inequality; opposite of equality Greater than, less than; true if the left-hand argument is greater than or less than the right-hand argument Greater than or equal to, less than or equal to; true if either > or == is true, OR either < or == is true AND; true if both the left-and right-hand arguments are true OR; true if either the left-or the right-hand argument is true NOT; true if its argument is false

The first six entries in Table 4-1 are comparison operators The equality opera tor is used to compare two numbers For example, the following is true if the value of n is 0, and is false otherwise:

n == 0;

Looks can be deceiving Don t confuse the equality operator (==) with the assignment operator (=) Not only is this a common mistake, but it s a mis take that the C++ compiler generally cannot catch that makes it more than twice as bad

The greater-than (>) and less-than (<) operators are similarly common in everyday life The following expression logical comparison is true:

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int n1 = 1; int n2 = 2; n1 < n2;

It s easy to forget which operator is greater than and which is less than Just remember that the operator is true if the arrow points to the smaller of the two You may think that n1 is greater than or less than n2; however, this ignores the possibility that n1 and n2 are equal The greater-than-or-equal-to opera tor (<=) and the less-than-or-equal-to operator (>=) include that bit of mathe matical nuance They are similar to the less-than and greater-than operators, with one major exception: They include equality; the other operators don t The && (AND) and || (OR) can combine with the other logic operators, like this:

// true if n2 is greater than n1 but n2 smaller than n3 // (this is the most common way determining that n2 is in // the range of n1 to n3, exclusive) (n1 < n2) && (n2 < n3);

The result of a logical operation can be assigned to a variable of type bool:

int n1 = 1; int n2 = 2; bool b; b = (n1 == n2);

This expression highlights the difference between the assignment operator = and the comparison operator == The expression says, Compare the vari ables n1 and n2 Store the results of this comparison in the variable b The assignment operators are about as low down on the precedence totem pole as you can get The equality operator is executed before the assignment The parentheses are not required so the following is an equally valid form of logical confusion:

b = n1 == n2; // compare n1 with n2; generate a true if n1 // if n1 has the same value as n2, false if not // store the result, true or false, in b