XPath format-number with number of decimal places based on a variable?

Question

I have an XML document where the number of decimal places a particular xs:decimal should be reported in is held in a sibling node. I'm currently struggling to find a simple way to output this via the format-number function.

I can build a picture string with some other functions, but this seems terribly long-winded for what should be (at least imo) a relatively straightforward and common task.

e.g. what I'm currently doing is something like this:

<xsl:value-of
 select="format-number(myNode/DecimalValue,
         concat('#0.', 
                string-join(for $i in 1 to myNode/DecimalPlaces return '0'))"
/>

Is there a better way?

Solution 1

Very good question! That usually means, I don't know the answer but I hope someone else does as this is a pain for me too.

Anyway, i did some searching and I think the round-half-to-even function might do the trick (http://www.xqueryfunctions.com/xq/fn_round-half-to-even.html)

Your code would become:

<xsl:value-of 
  select="
    round-half-to-even(
      myNode/DecimalValue
    , myNode/DecimalPlaces
    )
  "
/>

Now off for a little tangent: For people that are using XSLT 1.1 or lower and XPath 1, you could use this:

<xsl:value-of 
  select="
    concat(
      substring-before(DecimalValue, '.')
    , '.'
    , substring(substring-after(DecimalValue, '.'), 1, DecimalPlaces -1)
    , round(
        concat(
          substring(substring-after(DecimalValue, '.'), DecimalPlaces, 1)
        ,   '.'
        ,   substring(substring-after(DecimalValue, '.'), DecimalPlaces+1)
        )
      )
    )
  "
/>

Of course, this code is worse than the original, but if anyone knows how to solve the original question for XPath 1 and has a better idea than this, I'd love to hear that. (More and more often, I wish the world would have skipped XML altogether and moved immediately to JSON)

Solution 2

<!-- use a generous amount of zeros in a top-level variable -->
<xsl:variable name="zeros" select="'000000000000000000000000000000000'" />

<!-- …time passes… -->
<xsl:value-of select="
  format-number(
     myNode/DecimalValue,
     concat('#0.', substring($zeros, 1, myNode/DecimalPlaces))
  )
" />

You can abstract it away into a template:

<!-- template mode is merely to prevent collisions with other templates -->
<xsl:template match="myNode" mode="FormatValue">
  <xsl:value-of select="
    format-number(
      DecimalValue, 
      concat('#0.', substring($zeros, 1, DecimalPlaces))
    )
  " />
</xsl:template>

<!-- call like this -->
<xsl:apply-templates select="myNode" mode="FormatValue" />

You can also make a named template and use the XSLT context node when calling it. Depends a bit on your input document and needs if this is feasible for you.

<xsl:template name="FormatValue">
  <!-- same as above -->
</xsl:template>

<!-- call like this -->
<xsl:for-each select="myNode">
  <xsl:call-template name="FormatValue" />
</xsl:for-each>