Have you ever needed to do arithmetic on every element of a list? Perform complicated list analysis not available in the existing list transformers? Come take a technical look at 3 methods for manipulating lists in FME, their ease of setup and relative efficiency.

1. List Manipulation in FME
Joanna Hobbins

2. JOANNA HOBBINS
Subject Matter Expert: Content Creation
Global Engineering, Immersive Environment

3. UNMATCHED ON A GLOBAL SCALE
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4. Lists
Lists are everywhere in FME:
• 15 transformers are
dedicated to list manipulation
• Over 80 transformers can
produce lists
But there’s no such thing as a
list data type in FME
Working with
Wait…
What?

5. With a specific naming convention
Lists are simply attributes
As long as the attribute name
contains {#} FME will treat it as a list.
You can use regular attribute
manipulators to create a list.

6. Vocabulary
A Little
Basic listSimple
Complex
Nested
List with
sub-elements
_animals{0} Cat
_animals{1} Dog
_pets{0}.Name Felix
_pets{0}.Type Cat
_pets{1}.Name Fido
_pets{1}.Type Dog
Multi-level list _relationships{0}.pass{0} Contains
_relationships{0}.pass{1} Intersects
_relationships{1}.pass{0} Intersects
_relationships{2}.pass{0} Intersects

7. Processing Every
List Element
There are lots of situations
where you might want to
process every list element
individually.
We’re going to examine
three different methods of
doing so.
The Classic
The Loop
The Python

8. The Classic
Pros
• Easy to setup
• Access to all transformers
Cons
• Loss of geometry (ListBuilder) or
duplication of geometry (Aggregator)
• Special handling required for non-list
attributes
• Does not scale well

9. The Classic
Methodology
1. Create unique ID
2. Explode the list: ListExploder
3. Perform the manipulation(s) on each feature
4. Recombine the list: ListBuilder or Aggregator
Grouping by unique ID
5. Restore geometry/non-list attributes

10. The Loop
Pros
• No issues with
geometry or non list
attributes
• More efficient than
classic method for
shorter lists
Cons
• Requires a linked
custom transformer if
element manipulation
involves blocking
transformers
• Inefficient with large
lists

11. The Loop
Methodology
1. Create a Custom Transformer
2. Create an attribute to hold the current index
3. Check the index is less than the length of the list
4. Perform the manipulation(s) using the current index
5. Increment the index
6. Loop to Step 3

12. Place your screenshot here
The Python
Pros
• Extremely efficient
• No attribute or
geometry issues
Cons
• Requires coding
skills
• Not all
transformers have
an equivalent
python method

13. Place your screenshot here
The Python
Methodology
1. Get the list
attribute
2. Perform the
manipulations
3. Set the list
attribute
4. Expose the
list attribute if
creating a
new list
Is it really that
simple?

14. Place your screenshot here
Gotchas: Types
feature.getAttribute()
will often return
numbers as strings.
You may have to
explicitly cast
variables back and
forth between types.

15. Place your screenshot here
Gotchas: Types
feature.setAttribute()
will not work directly
on numeric lists.
You can either cast
the list to string or
use a for loop.

16. Place your screenshot here
Gotchas: Nested List
Nested lists cannot be
accessed directly.
You can retrieve the
sublist for a specific
element of the main
list, or a specific
element of the sublist
for all elements of the
main list.
To use all elements of
both lists,you need to
loop through the main
list.
But the length of the
list cannot be accessed
directly.

17. Place your screenshot here
Gotchas: Nested List
Hybrid
Solution
• Use a
ListElementCounter
prior to the
PythonCaller
• Use a “for i in
range” loop to
retrieve the subList

18. Benchmarks
Features List Items
Processing Time in Seconds
ListBuilder Aggregator Loop Python
10 10 1.5 0.5 0.5 0.6
10 100 1.5 0.6 0.6 0.6
10 1000 2.8 1.8 2.1 1.0
100 10 2.3 0.7 0.7 0.6
100 100 3.1 1.5 1.7 0.7
100 1000 12.2 9.9 12.8 1.5
1000 10 8.5 2.7 1.8 0.8
1000 100 17.8 12.6 12.3 1.2
1000 1000 101.2 91.5 120.5 6.4
10000 10 73.5 22.8 13.2 1.9
10000 100 156.0 102.4 119.2 6.2
10000 1000 1001.9 916.7 1199.0 53.6

19. Custom Transformers involving lists
• ListAppender
• ListComparer
• ListElementRemover
• ListExpressionEvaluator
• HistogramSummer
• AngularLineJoiner*
• FeatureSegmentator*
• LongestEdgeCalculator*
• OverlapPercentList*
• VisvalinghamSimplifier*
• ListElementFilter
• ListSlicer
• ListStatisticsCalculator
• ListStringReplacer
• ListSubsetEnumerator
• ListZipper
• ListCombiner
• ListElementExtractor
• ListMerger
• ListMissingElementFiller
• ListSampler
• AttributeListExploder
• AttributeListPopulator

20. But when you look at the
vertices, the left edge has the
longest segment.
Longest Edge Calculator
One might reasonably assume
the bottom edge is the longest.

21. Longest Edge Calculator
Uses a variation of the
HistogramSummer to get the
longest cumulative edge
within the Angle Tolerance.

22. VisvalingamSimplifier
Uses a list to store the triplet
triangle areas of
the feature, and the
ListElementRemover to
iteratively reduce vertices.

23. THANK YOU!
www.cae.com |
joanna.hobbins@cae.com