How to substring a variable in Assign message Policy?

[asaischintala]

I have extracted a value from an XML via XPath and set it to a variable in an Extract Variable Policy.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ExtractVariables async="false" continueOnError="false" enabled="true" name="EV-Value-1">
    <DisplayName>EV-Value-1</DisplayName>
    <Source>request</Source>
    <XMLPayload stopPayloadProcessing="false">
        <Namespaces>
            <Namespace prefix="ns0">abc:def:xyz</Namespace>
        </Namespaces>
        <Variable name="foo" type="string">
            <XPath>//ns0:Params//ns0:var1</XPath>
        </Variable>
    </XMLPayload>
</ExtractVariables>

So variable "foo" now has a value "XY".

How do I split "XY" into "X" and "Y" and assign to two different variables?

I'm using Assign Message Policy, but for some reason substring doesn't work as expected.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage continueOnError="false" enabled="true" name="AM-SplitValues">
    <DisplayName>AM-SplitValues</DisplayName>
    <AssignVariable>
        <Name>VarX</Name>
        <Ref>foo</Ref>
        <Template>{substring(foo,0,1)}</Template>
    </AssignVariable>
    <AssignVariable>
        <Name>VarY</Name>
        <Ref>foo</Ref>
        <Template>{substring(foo,1,2)}</Template>
    </AssignVariable>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
    <AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>

VarX and VarY are empty after this
However, in trace I can see the variable "foo" has the value "XY".


I referred to
https://www.googlecloudcommunity.com/gc/Apigee/AssignMessage-substring-not-working/td-p/73601
and
https://docs.apigee.com/api-platform/reference/message-template-intro

Can anyone please suggest on how to best handle this?

@dchiesa1  @API-Evangelist @sidd-harth Please advise

[dchiesa1]

This works for me, 

<AssignMessage name='AM-Assign-Variables-via-Substring'>
  <!-- To use numeric values within substring, -->
  <!-- specify IgnoreUnresolvedVariables as true, -->
  <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
  
  <AssignVariable>
    <Name>foo</Name>
    <Value>XY</Value>
  </AssignVariable>
  
  <AssignVariable>
    <!-- when using Template, omit the Ref element -->
    <Name>part1</Name>
    <Template>{substring(foo,0,1)}</Template>
  </AssignVariable>
  
  <AssignVariable>
    <Name>part2</Name>
    <Template>{substring(foo,1,2)}</Template>
  </AssignVariable>
  
</AssignMessage>

If you want to avoid using IgnoreUnresolvedVariables , then you can use symbolic names. 

<AssignMessage name='AM-Assign-Variables-via-Substring-2'>
  <!-- You can use variable names within substring. -->
  <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>

  <AssignVariable>
    <Name>foo</Name>
    <Value>XY</Value>
  </AssignVariable>

  <AssignVariable>
    <Name>zero</Name>
    <Value>0</Value>
  </AssignVariable>

  <AssignVariable>
    <Name>one</Name>
    <Value>1</Value>
  </AssignVariable>

  <AssignVariable>
    <Name>two</Name>
    <Value>2</Value>
  </AssignVariable>

  <AssignVariable>
    <!-- when using Template, omit the Ref element -->
    <Name>part1</Name>
    <Template>{substring(foo,zero,one)}</Template>
  </AssignVariable>

  <AssignVariable>
    <Name>part2</Name>
    <Template>{substring(foo,one,two)}</Template>
  </AssignVariable>

</AssignMessage>

[asaischintala]

Hi @dchiesa1 
What if I had to use a variable that has been set in a previous policy?

[dchiesa1]

A context variable is just a context variable.  It should just work.  Have you tried it?

[asaischintala]

@dchiesa1 

How to read the variable that is set in EV policy, inside an AM policy?

Is there anything wrong with the below usage of Ref and Template?

<AssignVariable>
        <Name>VarX</Name>
        <Ref>foo</Ref>
        <Template>{substring(foo,0,1)}</Template>
    </AssignVariable>

[dchiesa1 · Accepted solution]

You don't use Ref.  If variable foo has been previously set, then you can do this: 

    <!-- REQUIRED -->
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
    <AssignVariable>
        <Name>VarX</Name>
        <!-- OMIT
        <Ref>foo</Ref> 
        -->
        <Template>{substring(foo,0,1)}</Template>
    </AssignVariable>

BTW, just for the record, I am repeating myself. I posted this exact information in my original response. 

HAVE YOU TRIED THIS?

[asaischintala]

Thanks a ton @dchiesa1 
This worked... the actual problem I had was with my request node having a comment tag

<P01>
                    <!-- mandatory -->XY
                </P01>

When I changed this to

<P01>XY</P01>

it worked as expected