What is the difference between XSLT attributes when and if

xml, xslt

Solution

Structure of choose is

<xsl:choose>
    <xsl:when test="a">A</xsl:when>
    <xsl:when test="b">B</xsl:when>
    <xsl:when test="c">C</xsl:when>
    <xsl:when test="...">...</xsl:when>
    <xsl:otherwise>Z</xsl:otherwise>
</xsl:choose>

which allows multiple checks and one operation to be performed for the first test evaluating to `true`. `xsl:otherwise` is for performing default actions when none of the checks evaluates to `true`; in particular this facilitates an if-then-else construction (just one `xsl:when` alternative plus an `xsl:otherwise` block).

It always astounded me that `xsl:if` did not allow for an `xsl:else` alternative, but as this is available in `xsl:choose` construct, I guess it was judged not to add it. Maybe the next XSLT version will incorporate an `xsl:else`?

For the rest, the tests in `xsl:when` and in `xsl:if` do exactly the same thing: check the condition.

Note that the structure of `xsl:if` is simply

<xsl:if test="a">A</xsl:if>

A single

<xsl:when test="a">A</xsl:when>

would be invalid: `xsl:when` element is always a child of `xsl:choose`. And `xsl:choose` may have children `xsl:when` and `xsl:otherwise` only.

Problem

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