Convert String to Double

[TimHayes]

In a Feature Class Attribute Table, I have data in a column that is of string data type and need to convert it to a double data type. I have already added an empty column with a double data type on which I can use the Field Calculator.

What is the best way to go about this?

[MichaelRobb]

It should work fine if the featurelcass is in a FileGeodatabase. A Shapefile will not work for what you are looking for as I have mentioned.

Id Import the shapefile into a FileGeodatabase and then your desired <null> will work... and so would the script.

[GoldenJiang]

What's the difference if I save a featureclass in a folder or save it in a FileGeodatabase?

I have been saving data to folders all the time. Hope that doesn't affect my yield data. 

[curtvprice]

What's the difference if I save a featureclass in a folder or save it in a FileGeodatabase?

I have been saving data to folders all the time. Hope that doesn't affect my yield data.

Datasets saved to a folder from ArcGIS tool output are saved in shapefile (feature classes) and .dbf table format.

Geoprocessing considerations for shapefile output—Help | ArcGIS Desktop 

Geoprocessing considerations for shapefile output—Appendices | ArcGIS Desktop   (Pro)

[MichaelRobb2]

Well this is weird.  For a minute I thought I had forgotten how to code in the last 7 years, but quickly realised that my name sake had just helped me out, thanks!!

[MichaelRobb]

Alternatively, you can use the "Show Codeblock" in the field calculator and then use

Output = CDbl( [lFIELDNAME])

[Field] = Output

CDbl converts the string to double.

You can then add more items such as Replace as well to remove items such as periods or decimals if need be

e.g.

Output = CDbl( Replace([FIELDNAME],"."," "))