## Summary

SPSS uses a handful of hidden variables for keeping track of things. These so called **system variables** have names starting with "$" and can be useful in some cases. The most important one is $casenum, the number shown next to each case in the data editor window.

## SPSS $casenum

SPSS $casenumIn the left outline of the data editor window (under `$casenum`

. To some extent we can use `$casenum`

in normal SPSS commands. We can also convert it into a regular variable. This can be used to **create a unique row id** if this is absent. The syntax below demonstrates this.

## SPSS Syntax Example 1

***1. Create test data.**

data list free/age.

begin data

14 19 29 30 31

end data.

***2. Use $casenum in regular command.**

compute first_case = $casenum eq 1.

exe.

***3. Convert $casenum to normal variable.**

compute id = $casenum.

exe.

## SPSS Select If and $casenum

Using `SELECT IF`

with `$casenum`

can be tricky. This is because SPSS uses `$casenum`

to keep track of cases *during* procedures. If cases are deleted during a procedure, their `$casenum`

may shift to a different case. Like so, `SELECT IF $casenum`

GT 1. **will delete all cases** in the data. What happens, is this

- SPSS starts at the first case. Since its
`$casenum`

is not greater than one, it's deleted. - The second case now shifts up one row and is thus assigned
`$casenum`

1. - Since the second case's
`$casenum`

is not greater than one (anymore), it's deleted as well. - Now the third case shifts up and gets
`$casenum`

1. And so on.

## SPSS $time

A second system variable that may sometimes be useful is $time. This is hold the current date and time. As with $casenum, it can be used straight away, for example in `DATEDIFF`

. Alternatively, its value can be passed into a regular variable as demonstrated by the final syntax example.

## SPSS Syntax Example 2

***1. Create single case test data.**

data list free/now.

begin data

0

end data.

***2. Compute now = current date and time.**

compute now = $TIME.

exe.

***3. Display now as datetime variable.**

formats now(datetime22).

## Final Note

SPSS uses more system variables than `$casenum`

and `$time`

. Consult the command syntax reference for a complete overview.

## THIS TUTORIAL HAS 1 COMMENT:

## By Natalka Patsiurko on January 17th, 2016

Hi Ruben,

still using your tutorials and still find them very useful. In this one I again found an answer to my question. Just wanted to say thanks!