BBC BASIC Programmers' Reference

by Richard Russell, January 2023

The SUM() and MOD() functions return the sum and modulus (square-root of the sum of the squares) respectively of a numeric array, and they are much faster than performing the calculations yourself in a loop. However you have no control over which elements are operated on: it is always the entire array (remember that in BBC BASIC array indices are zero-based).

To find the sum or modulus of a subset of the elements you can use the user-defined functions below. To illustrate their use, this code calculates the Standard Deviation of elements 1 to 8 inclusive of an array:

      DIM array(9)
      array() = 1,2,3,4,5,6,7,8,9,10
      mean = FNsum(array(),1,8) / 8
      array() -= mean
      PRINT FNmod(array(),1,8) / SQR(8)
      array() += mean

This prints 2.29128785, being the Standard Deviation of the values 2, 3, 4, 5, 6, 7, 8 and 9.

Here are the functions. Please be aware that they temporarily modify the array, so do not access it in a timer interrupt or other asynchronous event handler:

      DEF FNsum(a(), I%, N%)
      IF DIM(a())<>1 ERROR 0, "Array must be one-dimensional"
      IF I% < 0 OR N% < 1 OR I% + N% - 1 > DIM(a(),1) ERROR 0, "Invalid array limits"
      LOCAL p%% : p%% = ^a(I%) - 5 
      LOCAL ?p%%, p%%!1 : ?p%% = 1 : p%%!1 = N% : PTR(a()) = p%%
      = SUM(a())
 
      DEF FNmod(a(), I%, N%)
      IF DIM(a())<>1 ERROR 0, "Array must be one-dimensional"
      IF I% < 0 OR N% < 1 OR I% + N% - 1 > DIM(a(),1) ERROR 0, "Invalid array limits"
      LOCAL p%% : p%% = ^a(I%) - 5
      LOCAL ?p%%, p%%!1 : ?p%% = 1 : p%%!1 = N% : PTR(a()) = p%%
      = MOD(a())