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How to Use XPath Number Operators and Functions

While XPath is primarily a language for navigating and selecting nodes, it also includes a powerful set of operators and functions for performing mathematical calculations. These are essential for creating complex filters in predicates [...] or for computing values in technologies like XSLT.

Let's explain the standard arithmetic operators and numeric functions in XPath and provide practical examples of how to use them to query an XML document.

XPath Arithmetic Operators​

XPath provides five basic arithmetic operators for use in expressions.

OperatorDescriptionExample
+Addition6 + 4 (returns 10)
-Subtraction6 - 4 (returns 2)
*Multiplication6 * 4 (returns 24)
divDivision8 div 4 (returns 2)
modModulo (remainder)8 mod 3 (returns 2)
note

You must use div for division and mod for modulo, as / and % have other meanings in XPath.

XPath Numeric Functions​

XPath includes several built-in functions for rounding and aggregation.

FunctionDescriptionExample
round()Rounds a number to the nearest integer.round(4.6) (returns 5)
ceiling()Rounds a number up to the nearest integer.ceiling(4.1) (returns 5)
floor()Rounds a number down to the nearest integer.floor(4.9) (returns 4)
sum()Calculates the sum of a node-set.sum(//price)
note

Crucial Note on sum(): Unlike the other functions, sum() operates on a node-set (a collection of nodes). It converts the text value of each node in the set to a number and returns their total.

Practical Examples​

All examples will refer to the following XML document:

<bookstore>
  <book>
    <title>Everyday Italian</title>
    <price>30.00</price>
    <stock>15</stock>
  </book>
  <book>
    <title>Harry Potter</title>
    <price>29.99</price>
    <stock>10</stock>
  </book>
  <book>
    <title>Learning XML</title>
    <price>39.95</price>
    <stock>7</stock>
  </book>
</bookstore>

Using Operators in Predicates​

Arithmetic operators are most commonly used inside predicates [...] to filter nodes based on a calculation.

Example: Select all books where the price multiplied by the stock is greater than 400.

//book[price * stock > 400]

Output: this selects the <book> for "Everyday Italian", because 30.00 * 15 = 450, which is greater than 400.

<book>
    <title>Everyday Italian</title>
    <price>30.00</price>
    <stock>15</stock>
</book>
<book>
    <title>Everyday Italian</title>
    <price>30.00</price>
    <stock>15</stock>
</book>

Example: Select all books with an odd number of items in stock.

//book[stock mod 2 != 0]

Output: this selects the <book> elements for "Learning XML" (stock 7) and "Everyday Italian" (stock 15).

<book>
    <title>Learning XML</title>
    <price>39.95</price>
    <stock>7</stock>
</book>

Using Functions in Predicates​

Rounding functions are useful for filtering based on integer values.

Example: Select all books where the rounded price is exactly 30.

//book[round(price) = 30]

Output: this selects the <book> for "Harry Potter", because round(29.99) is 30.

<book>
    <title>Harry Potter</title>
    <price>29.99</price>
    <stock>10</stock>
</book>

Example: Select all books where the price, when rounded down, is 29.

//book[floor(price) = 29]

Output: this also selects the <book> for "Harry Potter", because floor(29.99) is 29.

<book>
    <title>Harry Potter</title>
    <price>29.99</price>
    <stock>10</stock>
</book>

Using Functions to Return a Value​

Functions like sum() are often used not to select nodes, but to return a single, calculated value. This is especially common in XSLT.

note

This expression doesn't select a node; it returns a single number.

Example: Calculate the total value of all books in stock.

sum(//book/price * //book/stock)

Output: this expression would be evaluated by an XPath processor to return the number 1029.85 (which is (30.00 * 15) + (29.99 * 10) + (39.95 * 7)).

note

You can also use sum() inside a predicate for advanced filtering.

Example: Select all books with a price higher than the average price.

//book[price > sum(//price) div count(//book)]

Output: this selects the <book> for "Learning XML", as its price (39.95) is higher than the average price of all books.

Conclusion​

XPath's number operators and functions transform it from a simple node-selection language into a powerful query language capable of performing calculations.

  • Use arithmetic operators (+, -, *, div, mod) inside predicates ([...]) to filter nodes based on calculations.
  • Use rounding functions (round(), ceiling(), floor()) to simplify comparisons with decimal values.
  • Use sum() to perform aggregations on node-sets, either to return a final value or for advanced filtering within a predicate.

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