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Text File | 1987-01-14 | 8.7 KB | 266 lines

ARRAYS: SOME QUESTIONS AND ANSWERS by F. Ho; edited by Renee Gentry ========================================================================= The information below has been gathered from notes developed within the Support Department. They appear in Question/Answer format. =========== Q: - How is memory allocated for arrays? A: - Memory is allocated "dynamically" For example: If an array of 100 elements has been declared, a pointer is set from the memvar area to a small chunk of memory area where a reference to the array elements are kept. This array area is 22 bytes wide times the number of elements long. (See figure 1) If the values in the array elements are of type NUMERIC, they are stored within this area in their respective storage element area. (See figure 2) If the values were of type CHARACTER, a pointer is set from the storage element area of that array to any contiguous area of memory that can hold that string. (See figure 3) figure 1: memvar area ------------------ 22 bytes | | <--long--> ------------------ |-----------| | | |-----------| ------------------ |-----------| Array(100) | | |-----------| | ------------------ |-----------| | | Array(100) | -----------> |-----------| ------------------ |-----------| | | |-----------| | | figure 2: 22 bytes long ------------------ | 1) | Array(2) = 75 ------------------ Array(3) = 40 | 2) 75 | | ------------------ Array(100) | 3) 40 | | ------------------ | | 4) | ------------------ | | figure 3: --------------- 22 bytes long ---> | "Hello There" | ------------------ | --------------- | 1) | ------------ ------------------ | 2) 75 | | ------------------ Array(1) = "Hello There" Array(100) | 3) 40 | Array(4) = "Waz Happenin..." | ------------------ | | 4) | ----- ------------------ | | | | ------------------- ------------> | "Waz Happenin..." | ------------------- ========== Q: - How do arrays respond to the TYPE() function? A: - i - a TYPE() done on an array name will return "A" for "Array" e.g. - declare ARRAY[10] - ? type("ARRAY") - * will return "A" - ii - a TYPE() done on an array element will return "U" for "Undefined" - regardless of any values assigned to it e.g. - ARRAY[1] = 25 - ARRAY[2] = "Hello" - ? type("ARRAY[1]") - * will return "U" - ? type("ARRAY[2]") - * will return "U" - iii- on the other hand, if the array elements are assigned to variables, a TYPE() function done on the respective variable will return the appropriate type: e.g. - ARRAY[1] = 25 - ARRAY[2] = "Hello" - A = ARRAY[1] - B = ARRAY[2] - ? type("A") - * will return "N" - ? type("B") - * will return "C" ========== Q: - How would you collapse a declared array? A: - Like all other variables, you can: 1 - RELEASE the array e.g. release ARRAY 2 - assign it a false value e.g. ARRAY = .F. *3 - if the array was declared in a lower procedure, a return to the higher calling procedure destroys out the array =========== Q: - How would you create a 2-dimensional array in Clipper? A: - First, a background note on how arrays are actually "declared" or "dimensioned" by any computer languages: - when one issues the commands: DIM ARRAY(10) - in BASIC DECLARE ARRAY[10] - in CLIPPER - a single array dimension of 10 elements is allocated. - Or when one issues the command: DIM ARRAY(3,5) - in BASIC for a two-dimensional array - a SINGLE array of 15 (3*5) elements is allocated. The only thing that makes it work like it's two- dimensional is a built-in function in BASIC that does the proper POSITIONAL-ALLOCATION for the element concerned. This "position-allocator" function is what's missing in Clipper. But, you can get around this by making your own UDF which accomplishes the same thing. Below is an example on how to declare a 2-dimensional array: ---------------------------------------------------------------------- clear declare ARRAY[3*5] && or declare ARRAY[15] ARRAY[DIM2(2,3)] = "TEST" && DIM2 is our pos-alloc function ? ARRAY[DIM2(2,3)] && should print "TEST" quit function DIM2 && our position-allocator function parameters X,Y return(((X - 1) * 5) + Y) ---------------------------------------------------------------------- The above can be generalised as: clear ITEMS = 3 ELEMENTS = 5 declare ARRAY[ITEMS * ELEMENTS] ARRAY[DIM2(2,3)] = "TEST" ? ARRAY[DIM2(2,3)] quit function DIM2 parameters X,Y return(((X - 1) * ELEMENTS) + Y) ----------- - the above declaration will set up a single dimensional array of 15 elements: ARRAY 01 02 - - - 14 15 - but the POS-ALOC function can make it access the table as if it were: 1 2 3 -------------- ARRAY 1 | 01 | 06 | 11 | -------------- 2 | 02 | 07 | 12 | -------------- 3 | 03 | 08 | 13 | -------------- 4 | 04 | 09 | 14 | -------------- 5 | 05 | 10 | 15 | -------------- - so that, the command: ARRAY[DIM2(2,3)] = "TEST" - will store "TEST" at location: ARRAY[DIM2(2,3)] or ARRAY[(((2-1) * 5) + 3)] or ARRAY[8] ========== Note: - You CANNOT do a GET into an array element. Doing so will return a runtime error message of "Type conflict in subscript" ,or it will just go right through the read. @ 12,12 get ARRAY[1] && invalid read - workaround: XX = ARRAY[1] @ 12,12 get XX read or, at least initialize the array element you want to do a GET on: ARRAY[1] = space(10) @ 12,12 get ARRAY[1] read ========== * eof *