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LISP:Predicates in lisp

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- 1. Predicates in Lisp<br />
- 2. Predicates<br />A predicate is a function used to test for some condition involving its arguments and returns nil if the condition is false, or some non-nil value if the condition is true.<br />Name of the predicates in CL usually ends with p.<br />Ex: numberp, standard-char-p etc.<br />
- 3. overview<br />Logical values<br />Data Type predicates<br />General Type Predicates<br />Specific data type predicates<br />Equality predicates<br />Logical operators<br />
- 4. Logical values<br />The names nil and t are constants in Lisp<br />This object represents logical false value and also the empty list.<br />It can also be written as ()<br />The values of nil and t are not permitted to be changed.<br />Data type predicates:<br />using data type predicates one can determine<br /> whether or not it belongs to a given type, or one <br /> can compare two type specifies.<br />
- 5. General Type predicates<br />Syntax:<br />typep object type<br />Typep is the predicate that is true if the object is of type type false otherwise.<br />Subtypep type1 type2<br /><ul><li>This predicate returns T if type1 is a subtype of type1 else it returns nil.</li></ul>Ex: (subtypep t t) returns T<br />
- 6. Subtypep are supposed to return a second value of nil unless one or both of its arguments involves satisfies and, or, not or member.<br />Subtypep should signal an error when one or both of its arguments involves values or the list form of the function type specifier<br />Subtypep must always return the two values t and t in the case where its arguments, after expansion of specifies defined by deftype are equal.<br />(subtypep ‘(array A) ‘ (array B)) is true if and only if (upgraded-array-element-type ‘A) is the same type as (upgraded-array-element-type ‘ b)<br />(subtypep ‘ (complex A) ‘ (complex B)) is true if and only if (upgraded-complex-part-type ‘A) and (upgraded-complex-part-type ‘B) is of same type.<br />
- 7. Specific data type predicates<br />The following data types test for specific individual data types.<br />null object returns true if the argument is () or false otherwise.<br />symbolp object returns true if the argument is a symbol else returns false<br />atom object is true if the argument is not cons otherwise its false.<br />consp object is true if the argument is a cons, false otherwise<br />listp objectis true if the argument is a cons or an empty list (), false otherwise<br />Numberp object is T if the argument is any kind of number<br />
- 8. Rationalp object is T if the argument is a rational number false otherwise<br />Similarly we have floatp, charaterp, vectorp, stringp, complexp, simple-bit-vector-p, simple-string-vector-p, arrayp etc.<br />packagep object is T if its argument is a package and otherwise is false<br />functionp object is T if its arguments are suitable for applying to arguments, using for ex the funccall or apply function and otherwise is false<br />Compiled-function-p object is T if its argument is any compiled code object and otherwise is false<br />Commonp object is T if its argument is any standard CL data type, and otherwise is false<br />
- 9. Equality predicates<br />Cl provides these predicates to test for the equality of the two objects: eq, eql, equal, equalp <br />eq x y is true if and only if x and y are the same identical objects.<br />Ex: (eq ‘g ‘h) false<br /> (eq 3 3)true<br /> (eq “hi” “HI”) false<br />Eql x y is true if the arguments are eq, or of there are numbers of the same type with the same value, or if there are character objects that represent the same character<br />
- 10. equal x yis true if the arguments are structurally similar objects. (isomorphic objects)<br />Ex: (equal #B #B) true<br /> (equal “FOO” “FOO”) true<br />equalp x y is true if the two objects are equal, and if and only if they are eq that is, the same identical object.<br /> two hash tables are considered the same by equalp if and only if they satisfy the following four conditions:<br /><ul><li>They must be of the same kind ( equivalent :test arguments must be given to make hash-table when the two hash-tables are created.
- 11. They must have the same number of entries.
- 12. For every entry(key1, value1) in one hash table there must be the corresponding entry(key2, value2) in the other hash table.
- 13. For every entry(key1, value1) in one hash table and its corresponding entry(key2, value2) in the other , such that key1 and key2 are the same, equalp must be true of value1 and value2.</li></li></ul><li>Logical operators<br />CL provides three operators as Boolean value: and, or, and not<br />Ex: not y<br /><ul><li>Returns t if y is nil else returns nil.</li></ul>And returns t if the condition is true else NIL.<br />Ex: (and form1, form2,…)<br /><ul><li>Evaluates each form one at the time from left to right. If at least one form returns a nil, the evaluation is stopped by returning a nil.</li></ul>Ex: (and (< 1 2)<br /> (< 3 4)<br /> (princ “true”))<br />true<br /> “true”<br />
- 14. Or operator returns NIL for false conditions and true conditions return non-nil characters<br />(or form1,form2,…..)<br /><ul><li>Performs or operation the forms one by one from left to right. It is true if at least one of the forms are true </li></ul>Ex; (or (< 3 1)<br /> (> 2 1))<br /> T<br />
- 15. Visit more self help tutorials<br />Pick a tutorial of your choice and browse through it at your own pace.<br />The tutorials section is free, self-guiding and will not involve any additional support.<br />Visit us at www.dataminingtools.net<br />

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