15.7.10.1 PCOMPUTE

/PCOMPUTE &_postcompute_ =EXPR(_expression_)

The PCOMPUTE subcommand, which must precede the first TABLE command, defines computed categories. It is optional and may be used any number of times to define multiple postcomputes.

Each PCOMPUTE defines one postcompute. Its syntax consists of a name to identify the postcompute as a PSPP identifier prefixed by ‘&’, followed by ‘=’ and a postcompute expression enclosed in EXPR(…). A postcompute expression consists of:

[_category_]

This form evaluates to the summary statistic for category , e.g. [1] evaluates to the value of the summary statistic associated with category 1. The category may be a number, a quoted string, or a quoted time or date value. All of the categories for a given postcompute must have the same form. The category must appear in all the CATEGORIES list in which the postcompute is used.

[_min_ THRU _max_][LO THRU _max_][_min_ THRU HI]MISSINGOTHERNM

These forms evaluate to the summary statistics for a category specified with the same syntax, as described in previous section (see CTABLES Explicit Category List). The category must appear in all the CATEGORIES list in which the postcompute is used.

SUBTOTAL

The summary statistic for the subtotal category. This form is allowed only if the CATEGORIES lists that include this postcompute have exactly one subtotal.

SUBTOTAL[_index_]

The summary statistic for subtotal category index , where 1 is the first subtotal, 2 is the second, and so on. This form may be used for CATEGORIES lists with any number of subtotals.

TOTAL

The summary statistic for the total. The CATEGORIES lsits that include this postcompute must have a total enabled.

_a_ + _b__a_ - _b__a_ * _b__a_ / _b__a_ ** _b_

These forms perform arithmetic on the values of postcompute expressions a and b. The usual operator precedence rules apply.

_number_

Numeric constants may be used in postcompute expressions.

(_a_)

Parentheses override operator precedence.

A postcompute is not associated with any particular variable. Instead, it may be referenced within CATEGORIES for any suitable variable (e.g. only a string variable is suitable for a postcompute expression that refers to a string category, only a variable with subtotals for an expression that refers to subtotals, …).

Normally a named postcompute is defined only once, but if a later PCOMPUTE redefines a postcompute with the same name as an earlier one, the later one take precedence.

The following syntax and output shows how PCOMPUTE can compute a total over subtotals, summing the “Frequent Drivers” and “Infrequent Drivers” subtotals to form an “All Drivers” postcompute. It also shows how to calculate and display a percentage, in this case the percentage of valid responses that report never driving. It uses PPROPERTIES (see CTABLES PPROPERTIES) to display the latter in PCT format.

CTABLES
    /PCOMPUTE &all_drivers=EXPR([1 THRU 2] + [3 THRU 4])
    /PPROPERTIES &all_drivers LABEL='All Drivers'
    /PCOMPUTE &pct_never=EXPR([5] / ([1 THRU 2] + [3 THRU 4] + [5]) * 100)
    /PPROPERTIES &pct_never LABEL='% Not Drivers' FORMAT=COUNT PCT40.1
    /TABLE=freqOfDriving BY gender
    /CATEGORIES VARIABLES=freqOfDriving
                             [1 THRU 2, SUBTOTAL='Frequent Drivers',
                              3 THRU 4, SUBTOTAL='Infrequent Drivers',
                              &all_drivers, 5, &pct_never,
                              MISSING, SUBTOTAL='Not Drivers or Missing'].

		Male	Female
		S3a. GENDER:
		Count	Count
1. How often do you usually drive a car or other motor vehicle?	Every day	2305	2362
Several days a week	440	834
Frequent Drivers	2745	3196
Once a week or less	125	236
Only certain times a year	58	72
Infrequent Drivers	183	308
All Drivers	2928	3504
Never	192	348
% Not Drivers	6.2%	9.0%
Don't know	3	5
Refused	9	10
Not Drivers or Missing	204	363