TABLE FILE CAR
PRINT COUNTRY
TABPAGENO NOPRINT
AND COMPUTE NUM/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE (IF TABPAGENO NE LAST TABPAGENO THEN NUM+11 ELSE NUM);
END
-EXIT

 

The result for the first row is 14 , i.e. 'if' and 'else' are both entered ...

TABLE FILE CAR
PRINT COUNTRY MODEL
TABPAGENO 
AND COMPUTE MODEQ/I1=IF MODEL EQ LAST MODEL THEN 1 ELSE 0;
AND COMPUTE TP/I1=IF TABPAGENO EQ LAST TABPAGENO THEN 1 ELSE 0;
AND COMPUTE NUM/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE (IF TABPAGENO NE LAST TABPAGENO THEN NUM+11 ELSE NUM);
END

i think if you run this snip, you'll see why.

I don't see why ...

 TABLE FILE CAR
PRINT
COUNTRY
AND COMPUTE VAL1/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE IF COUNTRY NE LAST COUNTRY THEN VAL1 + 11 ELSE VAL1;
AND COMPUTE VAL2/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE IF TABPAGENO NE LAST TABPAGENO THEN VAL2 + 11 ELSE VAL2;
END
-RUN
-EXIT
 

It is evident for the first row that "model ne last model" so else should not be entered ... Maybe I misunderstand something ...

Certainly seems to be something wrong in the internal matrix.

 
TABLE FILE CAR 
PRINT COUNTRY 
TABPAGENO NOPRINT
COMPUTE LSTMODEL/A50=LAST MODEL;
AND COMPUTE NUM/I10 = IF MODEL NE LSTMODEL THEN 3 ELSE 
                       IF TABPAGENO NE LAST TABPAGENO THEN NUM+11 ELSE NUM;
END
 

This is fixed with adding sorts:

TABLE FILE CAR
PRINT COUNTRY
TABPAGENO NOPRINT
AND COMPUTE NUM/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE IF TABPAGENO NE LAST TABPAGENO THEN (NUM+11) ELSE NUM;
BY MODEL BY TABPAGENO
END
-EXIT

Here's what I surmise is happening, in execution of Martin's original code.

TABPAGENO is unique: You can reference it as a verb object, but its value is (obviously) not determinable until formatting of the output pages is in motion. Ultimately, page-break points (and hence the page-number of a row in the matrix) may depend on conditional subhead and subfoot lines, which in turn depend on matrix columns, both the accumulated verb-object columns and the Computed columns in the matrix.

Space is allocated in the internal matrix for the TABPAGENO column, and for any COMPUTE columns that depend on it, and for their LAST values -- but they have to remain unevaluated (zero or blank) until the page-break calculation phase, after all other computes are evaluated.

The bug seems to be two-fold.

First, the Computes that depend on TABPAGENO (and/or LAST TABPAGENO) are being evaluated twice -- once (prematurely) along with "normal" computes, and again (deservedly) at page-formatting time -- such that the first evaluation polutes values used in the second evaluation.

Second, the pushdown of matrix columns into their LAST-value columns for "normal" columns (such as MODEL) is being done (prematurely) before the computes in the page-formatting phase.

As a result...

In the first evaluation of
NUM/I10 = IF MODEL NE LAST MODEL THEN 3 ELSE (IF TABPAGENO NE LAST TABPAGENO THEN NUM+11 ELSE NUM);
for the first row, LAST MODEL is blank, so MODEL and LAST MODEL are unequal, and NUM gets the value 3.

Then, in the second evaluation (after TABPAGENO is evaluated), MODEL and LAST MODEL are equal (both contain "V12XKE AUTO"), while TABPAGENO and LAST TABPAGENO are unequal, so NUM gets "NUM+11" which is 3+11=14. (QED)