Mark Needham

Thoughts on Software Development

Neo4j: Cypher – Rounding of floating point numbers/BigDecimals

without comments

I was doing some data cleaning a few days ago and wanting to multiply a value by 1 million. My Cypher code to do this looked like this:

with "8.37" as rawNumeric 
RETURN toFloat(rawNumeric) * 1000000 AS numeric
 
╒═════════════════╕
│"numeric"        │
╞═════════════════╡
│8369999.999999999│
└─────────────────┘

Unfortunately that suffers from the classic rounding error when working with floating point numbers. I couldn’t figure out a way to solve it using pure Cypher, but there tends to be an APOC function to solve every problem and this was no exception.

I’m using Neo4j 3.2.3 so I downloaded the corresponding APOC jar and put it in a plugins directory:

$ ls -lh plugins/
total 3664
-rw-r--r--@ 1 markneedham  staff   1.8M  9 Aug 09:14 apoc-3.2.0.4-all.jar

I’m using Docker so I needed to tell that where my plugins folder lives:

$ docker run -v $PWD/plugins:/plugins \
    -p 7474:7474 \
    -p 7687:7687 \
    -e NEO4J_AUTH="none" \
    neo4j:3.2.3

Now we’re reading to try out our new function:

with "8.37" as rawNumeric 
RETURN apoc.number.exact.mul(rawNumeric,"1000000") AS apocConversion
 
╒════════════════╕
│"apocConversion"│
╞════════════════╡
│"8370000.00"    │
└────────────────┘

That almost does what we want, but the result is a string rather than numeric value. It’s not too difficult to fix though:

with "8.37" as rawNumeric 
RETURN toFloat(apoc.number.exact.mul(rawNumeric,"1000000")) AS apocConversion
 
╒════════════════╕
│"apocConversion"│
╞════════════════╡
│8370000         │
└────────────────┘

That’s more like it!

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Written by Mark Needham

August 13th, 2017 at 7:23 am

Posted in neo4j

Tagged with , ,