Translator – Arrays

Arrays have subscripts as in A(I) and B(3,X). For now there will be no limitation on the number of subscripts that an array can have (there will be a limitation once the internal language is defined, but this limitation will be sufficiently large). Take the example A(X+5,Y*2,Z) for a three dimension array element where the tokens will be parsed as:

A( X + 5 , Y * 2, Z )

This includes tokens for the commas separating the subscripts and the closing parentheses. Also note that in this example, the subscripts can be expressions. This array element expression will be translated to RPN as:

X 5 + Y 2 * Z A(

Note that the subscript expressions are first with the array at the end. When this array element is processed at run-time, the expressions are processed as discussed before. The run-time stack will contain the values X+5, Y*2 and Z (Z on top of the stack) when execution reaches the array name.

The instruction code for the array will contain the number of subscripts, so when the array element offset is evaluated, it will know that there are 3 subscripts on the stack (in reverse order) that need to be popped. Once the offset is calculated, the value of the array element will be obtained from memory and pushed back onto the stack. This value will then be the operand for the rest of expression the array element is in.

There is a problem with arrays during translation, and that is that the Translator will not know the actual number of subscripts that a particular array will have. Therefore, the subscript checking will be delayed for the Encode, which will use the Dictionary to obtain this information. Remember that the purpose of the Translator is to rearrange the incoming program code into RPN format, doing as much error checking as possible.