FirstPosition

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`FirstPosition`

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FirstPosition[expr,pattern]

gives the position of the first element in expr that matches pattern, or Missing["NotFound"] if no such element is found.

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FirstPosition[expr,pattern,default]

gives default if no element matching pattern is found.

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FirstPosition[expr,pattern,default,levelspec]

finds only objects that appear on levels specified by levelspec.

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FirstPosition[pattern]

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

Details and Options

In FirstPosition[expr,pattern,default], default is evaluated only if no element matching pattern is found.
The expression expr can be a list, an association, or any other type of expression. In the case of an association, FirstPosition gives a key.
The default level specification for FirstPosition is {0,Infinity}, with Heads->True.
A part specification {} returned by FirstPosition represents the whole of expr.
FirstPosition[list,pattern,default,{1},Heads->False] finds positions only of objects that appear as complete elements of list.
FirstPosition uses standard level specifications:
n levels 1 through n

Infinity levels 1 through Infinity

{n} level n only

{n₁,n₂} levels n₁ through n₂
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 0 corresponds to the whole expression.
With the default option setting Heads->True, FirstPosition includes heads of expressions and their parts.
FirstPosition traverses expressions in depth‐first order.
FirstPosition[pattern][expr] is equivalent to FirstPosition[expr,pattern].

Examples

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Basic Examples (3)Summary of the most common use cases

Find the first position at which b occurs:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-v4f

Out[1]=1

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-nz7

Out[2]=2

In[3]:=3

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https://wolfram.com/xid/0d6fke9ziq-j9ssb0

Out[3]=3

Find the first position at which x to any power appears:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-fwf

Out[1]=1

Find the first position in an Association:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-fpp3ld

Out[1]=1

Scope (4)Survey of the scope of standard use cases

Use a position operator:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-836t6x

Out[1]=1

Find the first position with a prime value:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-dqvhf

Out[1]=1

Specify a custom default value:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-bswqzv

Out[1]=1

Look for 3 anywhere:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-8z0iuc

Out[1]=1

Look for only a 3 that appears as an element of the outermost list:

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-547dz3

Out[2]=2

Options (1)Common values & functionality for each option

Heads (1)

By default, FirstPosition will search expression heads:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-szy

Out[1]=1

Use the option Heads->False to exclude them:

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-ld9

Out[2]=2

Properties & Relations (3)Properties of the function, and connections to other functions

The default value is only evaluated if needed:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-ikdop5

Out[1]=1

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-f6ohyb

Out[2]=2

Use Extract to extract parts based on results from FirstPosition:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-v58

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-nju

Out[2]=2

In[3]:=3

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https://wolfram.com/xid/0d6fke9ziq-e1i

Out[3]=3

FirstPosition returns only the first position:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-18dbdy

Out[1]=1

Position returns all positions:

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-odqxnc

Out[2]=2

FirstPosition can be implemented with a combination of Position and Replace:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-nnbjkg

Out[1]=1

Out[1]=1

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-iftb30

Out[2]=2

Out[2]=2

Possible Issues (1)Common pitfalls and unexpected behavior

FirstPosition searches based on patterns, which may not be the same as numerical equality:

In[1]:=1

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https://wolfram.com/xid/0d6fke9ziq-srm

Out[1]=1

Write a pattern using Condition to force searching based on numerical equality:

In[2]:=2

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https://wolfram.com/xid/0d6fke9ziq-1u062z

Out[2]=2

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