Function Repository Resource:

KeyIndex

Source Notebook

Create an association indexed by a given key from a list of associations

Contributed by: Richard Hennigan (Wolfram Research)

ResourceFunction["KeyIndex"][{a₁,a₂,…},key]

gives an Association that has keys a_i[key] and values a_i with key dropped.

ResourceFunction["KeyIndex"][key]

represents an operator form of ResourceFunction["KeyIndex"] that can be applied to an expression.

Details and Options

Each of the a_i can be either an Association or a List of rules.

In ResourceFunction["KeyIndex"][expr,key] for non-atomic expr, the expr can have any head, not just List.

ResourceFunction["KeyIndex"][Dataset[…],…] is also supported when the dataset is formed from a list of associations.

ResourceFunction["KeyIndex"][assoc,…] is equivalent to ResourceFunction["KeyIndex"][Values[assoc],…] when assoc is an Association.

ResourceFunction["KeyIndex"] has the following options:

"DropKey"

whether to drop key from each of the a_i

MergingFunction

determine how a_i and a_j should be combined when a_i[key]⩵a_j[key]

MissingBehavior

how to handle a_i that do not have key

The values for MissingBehavior determine how to handle a_i that do not have the given key:

discard a_i that lack the given key

include a_i under the key Missing["KeyAbsent",key] in the result

Examples

Basic Examples (5)

Index a list of associations by a given key:

In[1]:=

Out[1]=

Index lists of rules:

In[2]:=

Out[2]=

Use a combination of lists and associations:

In[3]:=

Out[3]=

Use the operator form:

In[4]:=

Out[4]=

In[5]:=

Out[5]=

Index a Dataset by a given key:

In[6]:=

Out[6]=

In[7]:=

Out[7]=

Scope (2)

The head of expr doesn’t necessarily have to be List:

In[8]:=

Out[8]=

If expr is an Association, KeyIndex uses Values[expr]:

In[9]:=

Out[9]=

Options (7)

DropKey (2)

Drop the given key from each of the resulting associations:

In[10]:=

Out[10]=

Leave the given key in the results:

In[11]:=

Out[11]=

MergingFunction (3)

By default, only the first value will be used when there is a collision:

In[12]:=

Out[12]=

Specify how collisions should be handled:

In[13]:=

Out[13]=

Combine values with Merge:

In[14]:=

Out[14]=

MissingBehavior (2)

The default behavior of KeyIndex is to discard associations that lack the given key in the result:

In[15]:=

Out[15]=

Don’t apply special rules if the key is missing:

In[16]:=

Out[16]=

Applications (2)

Create a new Dataset from an existing one that is indexed by a particular key:

In[17]:=

Out[17]=

In[18]:=

Out[18]=

Look up data using the new index:

In[19]:=

Out[19]=

Create groups by a given key:

In[20]:=

Out[20]=

Get the set matching a given classification:

In[21]:=

Out[21]=

Properties and Relations (2)

KeyIndex[expr,key] is similar to GroupBy[expr,Key[key]]:

In[22]:=

Out[22]=

In[23]:=

Out[23]=

In[24]:=

Out[24]=

Similar behavior can be obtained by setting options:

In[25]:=

Out[25]=

The default behavior of KeyIndex does additional post-processing of the data:

In[26]:=

Out[26]=

In[27]:=

Out[27]=

Possible Issues (1)

Each item must be an Association or a List of rules:

In[28]:=

Out[28]=

In[29]:=

Out[29]=

In[30]:=

Out[30]=

Related Links

Version History

1.0.0 – 22 May 2019

Related Resources

License Information

This work is licensed under a Creative Commons Attribution 4.0 International License