ArrayRules

SparseArray can accept a list of rules to form a sparse array. These rules hold the indices and values of nonzero elements. In the following example, the element at position {2,1} has the value 5. ArrayRules generates the rules for a sparse array.

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ArrayRules returns the rules that represent the sparse array. It returns a rule of blank patterns {_,_} to show the default value.

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When a scalar is added to the sparse array, the default value is changed. For example, here the default value is 5.

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ArrayRules can take a second argument, which is the default value shown on output. Here, a default rule with value 5 is used. Because there is only one element with this value, the list of rules is much longer.

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ArrayRules is a useful way to get information about a sparse array such as the non-default elements or the default value. This example shows how to get the indices of the non-default elements.

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