Question

I know I can sum over multiple nodes with numeric values.
How could I do a “boolean sum” over a set of nodes? For example:

<a>
  <b>false</b>
  <b>false</b>
  <b>true</b>
  <b>false</b>
</a>

How could I get the boolean OR of all the <b> node values? (which should be ‘true’).

Answer 1

Use:

boolean(/*/b[. = 'true'])

This produces the boolean value of the expression:

 /*/b[. = 'true']

and is true exactly when the above expression selects at least one node — that is, when there is a b that is a child of the top element and whose string value is the string 'true.

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In case you want also to calculate the “boolean product” (using and), do:

not(/*/b[. = 'false'])