The Alternative Csound Reference Manual
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# tablewa

tablewa -- Writes tables in sequential locations.

## Description

These opcodes read and write tables in sequential locations to and from an a-rate variable. Some thought is required before using them. They have at least two major, and quite different, applications which are discussed below.

## Syntax

kstart tablewa kfn, asig, koff

## Performance

kstart -- Where in table to read or write.

kfn -- i- or k-rate number of the table to read or write.

asig -- a-rate signal to read from when writing to the table.

koff -- i- or k-rate offset into table. Range unlimited - see explanation at end of this section.

In one application, these are intended to be used in pairs, or with several tablera opcodes before a tablewa -- all sharing the same kstart variable.

These read from and write to sequential locations in a table at audio rates, with ksmps floats being written and read each cycle.

tablera starts reading from location kstart. tablewa starts writing to location kstart, and then writes to kstart with the number of the location one more than the one it last wrote. (Note that for tablewa, kstart is both an input and output variable.) If the writing index reaches the end of the table, then no further writing occurs and zero is written to kstart.

For instance, if the table's length was 16 (locations 0 to 15), and ksmps was 5. Then the following steps would occur with repetitive runs of the tablewa opcode, assuming that kstart started at 0.

Run Number | Initial kstart | Final kstart | Locations Written
--- | --- | --- | ---
1 | 0 | 5 | 0 1 2 3 4
2 | 5 | 10 | 5 6 7 8 9
3 | 10 | 15 | 10 11 12 13 14
4 | 15 | 0 | 15

This is to facilitate processing table data using standard a-rate orchestra code between the tablera and tablewa opcodes. They allow all Csound k-rate operators to be used (with caution) on a-rate variables - something that would only be possible otherwise by ksmps = 1, downsamp and upsamp.

| Several cautions
| The k-rate code in the processing loop is really running at a-rate, so time dependent functions like port and oscil work faster than normal - their code is expecting to be running at k-rate. This system will produce undesirable results unless the ksmps fits within the table length. For instance a table of length 16 will accommodate 1 to 16 samples, so this example will work with ksmps = 1 to 16.

Both these opcodes generate an error and deactivate the instrument if a table with length < ksmps is selected. Likewise an error occurs if kstart is below 0 or greater than the highest entry in the table - if kstart = table length.

- kstart is intended to contain integer values between 0 and (table length - 1). Fractional values above this should not affect operation but do not achieve anything useful.

- These opcodes are not interpolating, and the kstart and koff parameters always have a range of 0 to (table length - 1) - not 0 to 1 as is available in other table read/write opcodes. koff can be outside this range but it is wrapped around by the final AND operation.

- These opcodes are permanently in wrap mode. When koff is 0, no wrapping needs to occur, since the kstart++ index will always be within the table's normal range. koff not equal to 0 can lead to wrapping.

- The offset does not affect the number of read/write cycles performed, or the value written to kstart by tablewa.

- These opcodes cannot read or write the guardpoint. Use tablegpw to write the guardpoint after manipulations have been done with tablewa.

## Examples

    kstart   =       0

    lab1:
      atemp  tablera ktabsource, kstart, 0  ; Read 5 values from table into an
             ; a-rate variable.

      atemp  =       log(atemp)  ; Process the values using a-rate
             ; code.

      kstart tablewa ktabdest, atemp, 0   ; Write it back to the table

    if ktemp  0 goto lab1      ; Loop until all table locations
             ; have been processed.

The above example shows a processing loop, which runs every k-cycle, reading each location in the table ktabsource, and writing the log of those values into the same locations of table ktabdest.

This enables whole tables, parts of tables (with offsets and different control loops) and data from several tables at once to be manipulated with a-rate code and written back to another (or to the same) table. This is a bit of a fudge, but it is faster than doing it with k-rate table read and write code.

Another application is:

    kzero = 0
    kloop = 0

    kzero tablewa 23, asignal, 0  ; ksmps a-rate samples written
           ; into locations 0 to (ksmps -1) of table 23.

    lab1: ktemp table kloop, 23  ; Start a loop which runs ksmps times,
           ; in which each cycle processes one of
     [ Some code to manipulate ]  ; table 23's values with k-rate orchestra
     [ the value of ktemp. ]  ; code.

     tablew ktemp, kloop, 23  ; Write the processed value to the table.

    kloop = kloop + 1      ; Increment the kloop, which is both the
           ; pointer into the table and the loop
    if kloop < ksmps goto lab1  ; counter. Keep looping until all values
           ; in the table have been processed.

    asignal   tablera 23, 0, 0  ; Copy the table contents back
           ; to an a-rate variable.

koff -- This is an offset which is added to the sum of kstart and the internal index variable which steps through the table. The result is then ANDed with the lengthmask (000 0111 for a table of length 8 - or 9 with guardpoint) and that final index is used to read or write to the table. koff can be any value. It is converted into a long using the ANSI floor() function so that -4.3 becomes -5. This is what we would want when using offsets which range above and below zero.

Ideally this would be an optional variable, defaulting to 0, however with the existing Csound orchestra read code, such default parameters must be init time only. We want k-rate here, so we cannot have a default.

## Credits

Author: Robin Whittle
Australia

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