Function Repository Resource:

AddIndices

Source Notebook

Pairs each element of a list with its index

Contributed by: Sander Huisman

ResourceFunction["AddIndices"][{e₁,e₂,…}]

pairs each e_i with its index i forming the list {{1,e₁},{2,e₂},…}.

ResourceFunction["AddIndices"][{e₁,e₂,…},i_min]

pairs each e_i with its index starting from i_min forming the list {{i_min,e₁},{i_min+1,e₂},…}.

ResourceFunction["AddIndices"][{e₁,e₂,…},i_min,di]

uses step di forming the list {{i_min,e₁},{i_min+di,e₂},…}.

Details and Options

ResourceFunction["AddIndices"] is frequently used when a large List of items needs to be transformed, but the original indices must be preserved.

Examples

Basic Examples (1)

Add indices to a List of elements:

In[1]:=

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Scope (2)

Start with the index 4:

In[2]:=

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The starting index and the step can be symbolic. Start the index at x:

In[3]:=

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The step can also be symbolic:

In[4]:=

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Applications (2)

Use Select to choose palindromic words while retaining their indices:

In[5]:=

s = {"anna", "has", "a", "red", "racecar", ",", "wow", "!"};
s = ResourceFunction["AddIndices"][s];
s = Select[s, Last/*PalindromeQ]

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Sort the strings and retain their original indices:

In[8]:=

s = {"anna", "has", "a", "red", "racecar", ",", "wow", "!"};
s = ResourceFunction["AddIndices"][s];
SortBy[s, Last]

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Properties and Relations (2)

The functionality of AddIndices can be achieved using MapIndexed:

In[11]:=

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AddIndices is roughly 4 times faster:

In[12]:=

$ele = RandomReal[{0, 1}, 10^5]; {First[RepeatedTiming[MapIndexed[{#2[[1]], #1} &, ele];]], First[RepeatedTiming[ResourceFunction["AddIndices"][ele];]]}$

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Publisher

SHuisman

Version History

1.0.0 – 03 July 2019

Related Resources

License Information

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