# 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:
•  n levels 1 through n Infinity levels 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 , 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:

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Find the first non-integer:

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

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Pick an element from an Association:

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