# Syntax example

**URL:** <https://mworks.discourse.group/t/syntax-example/380>\
**Category:** Support\
**Created:** [September 6, 2017, 3:13pm UTC](https://mworks.discourse.group/t/syntax-example/380 "2017-09-06T15:13:59Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cstawarz](https://yyz1.discourse-cdn.com/flex031/user_avatar/mworks.discourse.group/cstawarz/32/3_2.png) [@cstawarz](https://mworks.discourse.group/u/cstawarz)\
**Post date:** [September 6, 2017, 3:13pm UTC](https://mworks.discourse.group/t/syntax-example/380/1 "2017-09-06T15:13:59Z")

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Hi Chris,

I found today an issue I couldn’t easily solve within MWorks.

I have a bunch of lists: varAList, varBList,varCList,varDList  
and a variable I use as an index: N.

I’d like to do two things from the XML/MWEL:

1. index the list with the variable, or if the list is length 1, use the first element.  
Python example:  
if len(varList) == 1:  
var = varList[0]  
else  
var = varList[N]  
What I currently do is something like: (python)  
def assign\_list\_or\_one(varListStr, indexStr, varStr):  
vL = mw.getvar(varListStr)  
if len(vL) == 1:  
out = vL[0]  
else:  
out = vL[mw.getvar(indexStr)]  
mw.setvar(varListStr, out)  
(xml)  
\<action=“run\_python\_string” code=“assign\_list\_or\_one(‘varList’, ‘N’, ‘var’)”/\>
2. check to make sure all lists are the same length, or length 1  
There’s not really an easy way to do this without temp vars in XML/MWEL.

First, do you see an easier way to do (1), perhaps within the XML/MWEL, without making the code too unreadable?

If I do 10-20 of the calls into python in (1), do you think that will be a speed issue? (i.e. do you have a sense of how much overhead time run\_python\_string takes to call into and out of python, and how much time mw.getvar() / setvar() take for scalars or reasonable-sized lists?)

And this is the kind of thing that would be easier if there was a Python MW experiment language. But perhaps the current run\_python\_string solution is the best compromise.

Mark

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**Author:** ![cstawarz](https://yyz1.discourse-cdn.com/flex031/user_avatar/mworks.discourse.group/cstawarz/32/3_2.png) [@cstawarz](https://mworks.discourse.group/u/cstawarz)\
**Post date:** [September 7, 2017, 3:38pm UTC](https://mworks.discourse.group/t/syntax-example/380/2 "2017-09-07T15:38:57Z")

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Hi Mark,

> First, do you see an easier way to do (1), perhaps within the XML/MWEL, without making the code too unreadable?

How about this?

```
varList[[0, index][size(varList) > 1]]

```

That expression will work in XML or MWEL. In MWEL, you could create a [macro](https://mworks.github.io/documentation/0.8/mwel/index.html#macros) for it:

```
%define first_or_nth(list, index) list[[0, index][size(list) > 1]]

```

> If I do 10-20 of the calls into python in (1), do you think that will be a speed issue? (i.e. do you have a sense of how much overhead time run\_python\_string takes to call into and out of python, and how much time mw.getvar() / setvar() take for scalars or reasonable-sized lists?)

I didn’t know, so I wrote a simple timing test (attached). The test computes the elapsed time per assignment, averaged over 1000 assignments, for lists of length 1, 10, 100, 1000, and 10000.

Without Python, using the `first_or_nth` macro defined above, I get the following results on my Mac Pro:

```
Average elapsed time per assignment:
          1-item list: 30.774 us
         10-item list: 33.078 us
        100-item list: 35.654 us
       1000-item list: 53.526 us
      10000-item list: 204.227 us

```

With Python, using the equivalent of your `assign_list_or_one` function, I get the following:

```
Average elapsed time per assignment:
          1-item list: 62.106 us
         10-item list: 64.307 us
        100-item list: 74.065 us
       1000-item list: 166.576 us
      10000-item list: 1180.75 us

```

For lists with 100 items or fewer, the Python version takes about twice as long. With 1000 and 10000 items, the Python version takes about 3 and 6 times longer, respectively. The multiplier gets larger with larger lists, because the entire MWorks list is converted into a Python list by `getvar`.

> And this is the kind of thing that would be easier if there was a Python MW experiment language.

I think I’ve made the case that (1) isn’t any harder without Python. What is the “easy” way to do (2)?

Chris

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<div class="post-metadata">

**Author:** ![cstawarz](https://yyz1.discourse-cdn.com/flex031/user_avatar/mworks.discourse.group/cstawarz/32/3_2.png) [@cstawarz](https://mworks.discourse.group/u/cstawarz)\
**Post date:** [September 7, 2017, 3:38pm UTC](https://mworks.discourse.group/t/syntax-example/380/3 "2017-09-07T15:38:57Z")

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Attachment: [run\_python\_timing.zip](https://mworks.discourse.group/uploads/short-url/4VnJ1FOkDNFCVj3QW4mG7rPvFYK.zip) (1.73 KB)

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<div class="post-metadata">

**Author:** ![cstawarz](https://yyz1.discourse-cdn.com/flex031/user_avatar/mworks.discourse.group/cstawarz/32/3_2.png) [@cstawarz](https://mworks.discourse.group/u/cstawarz)\
**Post date:** [July 19, 2022, 9:52pm UTC](https://mworks.discourse.group/t/syntax-example/380/4 "2022-07-19T21:52:34Z")

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