FirstCase

FirstCase[{e1,e2,},pattern]

gives the first ei to match pattern, or Missing["NotFound"] if none matching pattern is found.

FirstCase[{e1,},patternrhs]

gives the value of rhs corresponding to the first ei to match pattern.

FirstCase[expr,pattern,default]

gives default if no element matching pattern is found.

FirstCase[expr,pattern,default,levelspec]

finds only objects that appear on levels specified by levelspec.

FirstCase[pattern]

represents an operator form of FirstCase that can be applied to an expression.

Details and Options

  • The first argument to FirstCase need not have head List and can be an association.
  • FirstCase[expr,pattern:>rhs] evaluates rhs only when the pattern is found.
  • In FirstCase[{e1,e2,},pattern,default], default is only evaluated if none of the ei match.
  • FirstCase[pattern][expr] is equivalent to FirstCase[expr,pattern].
  • FirstCase uses standard level specifications:
  • nlevels 1 through n
    Infinitylevels 1 through Infinity
    {n}level n only
    {n1,n2}levels n1 through n2
  • The default value for levelspec in FirstCase is {1}.
  • A positive level n consists of all parts of expr specified by n indices.
  • A negative level -n consists of all parts of expr with depth n.
  • Level -1 consists of numbers, symbols, and other objects that do not have subparts.
  • Level 0 corresponds to the whole expression.
  • With the option setting Heads->True, FirstCase looks at heads of expressions and their parts.
  • FirstCase traverses the parts of expr in a depth-first order, with leaves visited before roots.

Examples

open allclose all

Basic Examples  (3)

Find the first element in a list matching an integer:

Find the first non-integer:

Return the x from inside the first f[x_] matched:

Pick an element from an Association:

Scope  (7)

Use the operator form of Cases:

Find the first order pair:

Find its sum:

Pick out the first nonzero element:

Use :> to evaluate only after explicit elements have been found:

Specify a custom default value:

Examine the elements of a matrix using a level specification:

Pick elements from nested associations:

Options  (1)

Heads  (1)

Find a symbol among the elements of an expression:

Also include the head of the expression in the search:

Properties & Relations  (4)

The default value is only evaluated if needed:

FirstCase returns only the first match:

Cases returns all matches:

FirstCase can be implemented with a combination of Cases and Replace:

FirstCase is similar to SelectFirst except that it uses a pattern instead of a function:

Select the first list that has a sum of elements less than 10:

Use SelectFirst to get the same result:

Possible Issues  (1)

A rule transforms the matching element into its right side:

Use HoldPattern to treat the rule itself as a pattern:

Introduced in 2014
 (10.0)