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Logical Expressions

  1. Because the compiler recognizes 10 < value already as an expression, it expects a comma after it to accept it as a legal argument to writeln. Using parentheses around the whole expression would not work either, because this time a closing parenthesis would be expected after the same expression.
  2. Grouping the expression as (10 < value) < 20 removes the compilation error, because in this case first 10 < value is evaluated and then its result is used with < 20.

    We know that the value of a logical expression like 10 < value is either false or true. false and true take part in integer expressions as 0 and 1, respectively. (We will see automatic type conversions in a later chapter.) As a result, the whole expression is the equivalent of either 0 < 20 or 1 < 20, both of which evaluate to true.

  3. The expression "greater than the lower value and less than the upper value" can be coded like the following:
        writeln("Is between: ", (value > 10) && (value < 20));
  4. "There is a bicycle for everyone" can be coded as personCount <= bicycleCount or bicycleCount >= personCount. The rest of the logical expression can directly be translated to D from the exercise:
        writeln("We are going to the beach: ",
                ((distance < 10) && (bicycleCount >= personCount))
                ((personCount <= 5) && existsCar && existsLicense)

    Note the placement of the || operator to help with readability by separating the two main conditions.