XSLT Cookbook

Duplicating a String N Times

Problem

You need to duplicate a string N times, where N is a parameter. For example, you might need to pad out a string with spaces to achieve alignment.

Solution

A nice solution is a recursive approach that doubles the input string until it is the required length while being careful to handle cases in which $count is odd:

<xsl:template name="dup">
     <xsl:param name="input"/>
     <xsl:param name="count" select="1"/>
     <xsl:choose>
          <xsl:when test="not($count) or not($input)"/>
          <xsl:when test="$count = 1">
               <xsl:value-of select="$input"/>
          </xsl:when>
          <xsl:otherwise>
               <!-- If $count is odd append an extra copy of input -->
               <xsl:if test="$count mod 2">
                    <xsl:value-of select="$input"/>
               </xsl:if>
               <!-- Recursively apply template after doubling input and 
               halving count -->
               <xsl:call-template name="dup">
                    <xsl:with-param name="input" 
                         select="concat($input,$input)"/>
                    <xsl:with-param name="count" 
                         select="floor($count div 2)"/>
               </xsl:call-template>     
          </xsl:otherwise>
     </xsl:choose>
</xsl:template>

Discussion

The most obvious way to duplicate a string $count times is to figure out a way to concatenate the string to itself $count-1 times. This can be done recursively by the following code, but this code will be expensive unless $count is small, so it is not recommended:

<xsl:template name="slow-dup"> <xsl:param name="input"/> <xsl:param name="count" select="1"/> <xsl:param name="work" select="$input"/> <xsl:choose> <xsl:when test="not($count) or not($input)"/> <xsl:when test="$count=1"> ...