Putting Constraints on Patterns
Mathematica provides a general mechanism for specifying constraints on patterns. All you need do is to put
/;condition at the end of a pattern to signify that it applies only when the specified condition is
True. You can read the operator
/; as "slashsemi", "whenever" or "provided that".
pattern/;condition  a pattern that matches only when a condition is satisfied 
lhs:>rhs/;condition  a rule that applies only when a condition is satisfied 
lhs:=rhs/;condition  a definition that applies only when a condition is satisfied 
Putting conditions on patterns and transformation rules.
This gives a definition for fac that applies only when its argument n is positive. 
The definition for fac is used only when the argument is positive.
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This gives the negative elements in the list.
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You can use
/; on whole definitions and transformation rules, as well as on individual patterns. In general, you can put
/;condition at the end of any
:= definition or
:> rule to tell
Mathematica that the definition or rule applies only when the specified condition holds. Note that
/; conditions should not usually be put at the end of
= definitions or
> rules, since they will then be evaluated immediately, as discussed in
"Immediate and Delayed Definitions".
Here is another way to give a definition which applies only when its argument n is positive. 
Once again, the factorial functions evaluate only when their arguments are positive.
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You can use the
/; operator to implement arbitrary mathematical constraints on the applicability of rules. In typical cases, you give patterns which
structurally match a wide range of expressions, but then use
mathematical constraints to reduce the range of expressions to a much smaller set.
This rule applies only to expressions that have the structure v[x_, 1x_]. 
This expression has the appropriate structure, so the rule applies.
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This expression, while mathematically of the correct form, does not have the appropriate structure, so the rule does not apply.
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This rule applies to any expression of the form w[x_, y_], with the added restriction that y1x. 
The new rule does apply to this expression.
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In setting up patterns and transformation rules, there is often a choice of where to put
/; conditions. For example, you can put a
/; condition on the righthand side of a rule in the form
lhs:>rhs/;condition, or you can put it on the lefthand side in the form
lhs/;condition>rhs. You may also be able to insert the condition inside the expression
lhs. The only constraint is that all the names of patterns that you use in a particular condition must appear in the pattern to which the condition is attached. If this is not the case, then some of the names needed to evaluate the condition may not yet have been "bound" in the patternmatching process. If this happens, then
Mathematica uses the global values for the corresponding variables, rather than the values determined by pattern matching.
Thus, for example, the condition in
f[x_, y_]/;(x+y<2) will use values for
x and
y that are found by matching
f[x_, y_], but the condition in
f[x_/;x+y<2, y_] will use the global value for
y, rather than the one found by matching the pattern.
As long as you make sure that the appropriate names are defined, it is usually most efficient to put
/; conditions on the smallest possible parts of patterns. The reason for this is that
Mathematica matches pieces of patterns sequentially, and the sooner it finds a
/; condition which fails, the sooner it can reject a match.
Putting the /; condition around the x_ is slightly more efficient than putting it around the whole pattern.
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You need to put parentheses around the /; piece in a case like this.
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It is common to use
/; to set up patterns and transformation rules that apply only to expressions with certain properties. There is a collection of functions built into
Mathematica for testing the properties of expressions. It is a convention that functions of this kind have names that end with the letter
Q, indicating that they "ask a question".
IntegerQ[expr]  integer 
EvenQ[expr]  even number 
OddQ[expr]  odd number 
PrimeQ[expr]  prime number 
NumberQ[expr]  explicit number of any kind 
NumericQ[expr]  numeric quantity 
PolynomialQ[expr,{x_{1},x_{2},...}] 
 polynomial in x_{1}, x_{2}, ... 
VectorQ[expr]  a list representing a vector 
MatrixQ[expr]  a list of lists representing a matrix 
VectorQ[expr,NumericQ], MatrixQ[expr,NumericQ] 
 vectors and matrices where all elements are numeric 
VectorQ[expr,test], MatrixQ[expr,test] 
 vectors and matrices for which the function test yields True on every element 
ArrayQ[expr,d]  full array with depth matching d 
Some functions for testing mathematical properties of expressions.
The rule applies to all elements of the list that are numbers.
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This definition applies only to vectors of integers. 
The definition is now used only in the first case.
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An important feature of all the
Mathematica propertytesting functions whose names end in
Q is that they always return
False if they cannot determine whether the expression you give has a particular property.
This returns False, since x is not known to be an integer.
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Functions like
IntegerQ[x] test whether
x is explicitly an integer. With assertions like
x Integers you can use
Refine,
Simplify and related functions to make inferences about symbolic variables
x.
SameQ[x,y] or xy  x and y are identical 
UnsameQ[x,y] or xy  x and y are not identical 
OrderedQ[{a,b,...}]  a, b, ... are in standard order 
MemberQ[expr,form]  form matches an element of expr 
FreeQ[expr,form]  form matches nothing in expr 
MatchQ[expr,form]  expr matches the pattern form 
ValueQ[expr]  a value has been defined for expr 
AtomQ[expr]  expr has no subexpressions 
Some functions for testing structural properties of expressions.
With , the equation remains in symbolic form; yields False unless the expressions are manifestly equal.
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The expression n is not a member of the list {x, x^n}.
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However, {x, x^n} is not completely free of n.
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You can use FreeQ to define a "linearity" rule for h. 
Terms free of x are pulled out of each h.
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pattern?test  a pattern which matches an expression only if test yields True when applied to the expression 
Another way to constrain patterns.
The construction
pattern/;condition allows you to evaluate a condition involving pattern names to determine whether there is a match. The construction
pattern?test instead applies a function
test to the whole expression matched by
pattern to determine whether there is a match. Using
? instead of
/; sometimes leads to more succinct definitions.
With this definition matches for x_ are tested with the function NumberQ. 
The definition applies only when p has a numerical argument.
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Here is a more complicated definition. Do not forget the parentheses around the pure function. 
The definition applies only in certain cases.
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Except[c]  a pattern which matches any expression except c 
Except[c,patt]  a pattern which matches patt but not c 
Patterns with exceptions.
This gives all elements except 0.
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This picks out integers that are not 0.
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Except[c] is in a sense a very general pattern: it matches
anything except
c. In many situations you instead need to use
Except[c, patt], which starts from expressions matching
patt, then excludes ones that match
c.