support date math with mutate2

[tolot27]

Currently, mutate2 accepts date strings and can do some math with it, i.e. subtracting seconds but it does not return date-formatted strings (see #159 (comment)).

Even if it can do some math if the expression contains a numeric value, it cannot compute the difference (for instance) between dates; although, it detects the date types:

echo -e "date\n2021-08-26" | csvtk mutate2 -e '"2021-08-28"-$date' -n res
[ERRO] Cannot transition token types from MODIFIER [-] to TIME [2021-08-26 00:00:00 +0200 CEST]

It can compare dates, already:

echo -e "date\n2021-08-26" | csvtk mutate2 -e '"2021-08-28"> $date' -n res
date,res
2021-08-26,true

[shenwei356]

I do some tests and believe the package doe not support date computation.

[tolot27]

According to https://github.com/Knetic/govaluate/blob/master/MANUAL.md#types it converts dates to float64 automatically. Hence, it should be able to do some math with it. And it works, indeed:

echo -e "date\n2021-08-26" | csvtk mutate2 -e '$date - 86400' -n res
date,res
2021-08-26,1629842400.00

But providing two dates does not seem to work because of some inconsistencies.

[san-r]

So, we can operate on the basis of adding/subtracting 'seconds' from a date.

The above operation gave us 1629842400.00, which the Epoch Converter at https://www.epochconverter.com/ shows as 2021-08-24 in GMT.

The Excel formula to convert Unix timestamp to Date at https://exceljet.net/formula/convert-unix-time-stamp-to-excel-date also gives us 2021-08-24.

But, in the previous post one day was subtracted from 2021-08-26, which should be 2021-08-25. Then why is the Unix timestamp to date conversion of the result of operation showing 2021-08-24 ?

Being able to calculate the difference in days between two dates directly in csvtk would indeed be very useful.

[san-r]

The following is done using miller's go port:

If we have a csv file test.txt as:

name,date1,date2
A,2020-09-20,2020-08-30
B,2020-01-31,2020-01-20

Then the following command:

mlr --c2p --from test_date.txt put "$date_diff = (strptime($date1, \"%Y-%m-%d\") - strptime($date2, \"%Y-%m-%d\"))/86400"

gives:

name date1      date2      date_diff
A    2020-09-20 2020-08-30 21
B    2020-01-31 2020-01-20 11

Something similar may be implemented for csvtk.

[shenwei356]

Sorry I can't make a promise I'll add this. Because mutate2 relies on a third-party package that has not been updated for > 4 years.

[san-r]

I've noticed one custom function len() in csvtk. If another custom function DATEDIF() could be made available in csvtk it would be really nice since it is needed very frequently while analyzing data.

Usage: https://exceljet.net/excel-functions/excel-datedif-function

[shenwei356]

Let's leave this to v0.25.0, after finishing #172