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$HistoryLength

$HistoryLength
specifies the number of previous lines of input and output to keep in a Mathematica session.

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The default setting for $HistoryLength is Infinity.

Values of In[n] and Out[n] corresponding to lines before those kept are explicitly cleared.

Using smaller values of $HistoryLength can save substantial amounts of memory in a Mathematica session.

Basic Examples (2)

The default is to save the entire history:

Change the setting to save no history:

This means the previous input can no longer be used directly:

By assigning to a variable you can still refer to previous results:

$Line In Out

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Mathematica Session History

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