By using the Levels facility, the user need not know how Quantum deals with trailer card data internally. However, there are occasions when it may be necessary to edit or tabulate the data without using levels. To do this, it is necessary to know more about how trailer cards are processed.

Quantum deals with trailer cards in a number of ‘reads’. Cards are read into the appropriate rows of the C array until:

or

To produce useful tables, you need to know which cards are currently in the C array.

Quantum has four reserved variables (thisread, allread, firstread and lastread) which it uses to keep track of which cards it has read for each respondent.

The array called thisread is used to check which cards have been read in during the current read. thisread1 will be true (or 1) if a card type 1 has just been read in; thisread2 will be true if a card 2 has just been read, and so on.

There are nine such variables (thisread1 to thisread9) available unless extra card types have been specified using the max= option In this case, these variables will be numbered 1 to max; if there are 13 cards, you will have thisread1 to thisread13. For further details on max=, see Highest card type number.

allread notes which cards have been read in so far for this questionnaire. If cards 1, 2 and 3 have been read so far, allread1, allread2 and allread3 will all be true. Additionally, each cell of allread will contain the number of cards of the given type read in — for example, if two cards of type 3 have been read, allread3 will be true and it will contain the number 2.

There are nine allread variables available unless extra card types have been specified with max=.

The variables firstread and lastread become true when the first and last cards in a record have been read in.

You can use these variables in your program to associate specific parts of the edit or tabulation section with specific types of data. For example:

For example, look at the contents of the C array and the values of thisread, allread, firstread and lastread. Suppose the record has five cards: 1, 2, 2, 2 and 3 of 80 columns each. The first ‘read’ places card 1 in c(101,180) and the first card 2 in c(201,280). The second card 2 is not read into the array yet because it has the same card type as the previous card. As this is the start of a new respondent, firstread is true (or 1), and because cards 1 and 2 have been read, thisread1, thisread2, allread1 and allread2 are also true.

The second ‘read’ deals only with the second card 2 since it is followed by another card of the same type. thisread2 is true, as are allread1 and allread2. Also, allread2 contains the value 2 because you have read in 2 card 2s so far. Note that thisread1 is now false (or 0) as no card 1 was read this time.

On the third and final ‘read’ the third card 2 is read into c(201,280) and card 3 is copied into c(301,380). lastread is true because you have reached the end of the record, thisread2 and thisread3 are true because you have just read cards 2 and 3, and allread1, allread2 and allread3 are true because this record contains cards 1, 2 and 3. allread2 now contains the value 3 because there were 3 card 2s altogether.

The chart below summarizes the cards read and the variables which will be true after each read.

If Quantum reads a record in which the repeated cards are out of sequence, it inserts blanks cards of the appropriate types wherever necessary to force the cards into the correct sequence. For example, if the record contains the cards 1, 2, 4, 3, 4, 4 in that order, Quantum generates a completely blank card 3 when it reads the first card 4. The record is then processed as if it contained cards 1, 2, 3, 4, 3, 4, 4.