Loops

Home > Books > Solutions Developer Guide > DPML > Loops

Loop Processing

DPML provides a single loop instruction, while.

while

The structure of the while statement is:

<while>
  <cond>
    <instr />
  </cond>
  <seq>
    <instr />
  </seq>
</while>

When program flow enters the <while> block the <cond> block is executed first. If the execution of instructions in this block results in the resource this:cond being set to the value true then the instructions contained in the <seq> block are executed in order. Next the <cond> block is executed again and the process continues until the resource this:cond is set to the value false. As in any programming language, care should be taken not to create an infinite loop!

This example DPML program illustrates the use of the while statement.

<idoc>
  <!-- Copies all items that cost over 1.00 from inventory to basket -->
  <instr>
    <type>copy</type>
    <operand>
      <inventory>
        <item name="cheese" price="1.23" />
        <item name="ham" price="1.50" />
        <item name="brocolli" price="0.75" />
        <item name="tiger roll" price="0.90" />
        <item name="onion marmalade" price="3.45" />
      </inventory>
    </operand>
    <target>var:inventory</target>
  </instr>
  <instr>
    <type>copy</type>
    <operand>
      <basket />
    </operand>
    <target>var:basket</target>
  </instr>
  <while>
    <cond>
      <instr>
        <type>xpatheval</type>
        <operator>
          <xpath>count(item)</xpath>
        </operator>
        <operand>var:inventory</operand>
        <target>this:cond</target>
      </instr>
    </cond>
    <seq>
      <instr>
        <type>copy</type>
        <operand>var:inventory#xpointer(item[1])</operand>
        <target>var:item</target>
      </instr>
      <if>
        <cond>
          <instr>
            <type>xpatheval</type>
            <operator>
              <xpath>@price &gt; '1.00'</xpath>
            </operator>
            <operand>var:item</operand>
            <target>this:cond</target>
          </instr>
        </cond>
        <then>
          <instr>
            <type>stm</type>
            <operator>
              <stm:group xmlns:stm="http://1060.org/stm">
                <stm:append xpath="/basket">
                  <stm:param />
                </stm:append>
              </stm:group>
            </operator>
            <operand>var:basket</operand>
            <param>var:item</param>
            <target>var:basket</target>
          </instr>
        </then>
      </if>
      <instr>
        <type>stm</type>
        <operator>
          <stm:group xmlns:stm="http://1060.org/stm">
            <stm:delete xpath="item[1]" />
          </stm:group>
        </operator>
        <operand>var:inventory</operand>
        <target>var:inventory</target>
      </instr>
    </seq>
  </while>
  <instr>
    <type>copy</type>
    <operand>var:basket</operand>
    <target>this:response</target>
  </instr>
</idoc>