# BBC BASIC Programmers' Reference

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returning_20an_20array_20from_20a_20function

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=====Returning an array from a function===== //by Richard Russell, November 2012//\\ \\ Modifying the //contents// of an array (i.e. its //elements//) within a PROC or FN is straightforward, because arrays are passed **by reference** and therefore any modifications to their contents remain intact after the FN or PROC returns.\\ \\ However this necessarily means that the array must first be created (DIMensioned) **outside** the FN or PROC, and this may not always be convenient or desirable. The alternative is to create the array **inside** the function and then to return it to the caller. The conventional way of doing that is to use the RETURN qualifier; for example this procedure creates an //identity matrix// (having ones on the leading diagonal and zeroes elsewhere): \\ DEF PROCidn(RETURN i(), n%) LOCAL i% DIM i(n%, n%) FOR i% = 0 TO n% i(i%,i%) = 1 NEXT ENDPROC The procedure is called with two parameters: the array to be created (which must not have been previously dimensioned) and the required size (maximum row and column index), for example:\\ PROCidn(identity(), 3) This works well, but in one respect it is not particularly 'satisfying'. Normally, when you want to return a value (numeric or string) from a sub-module you would use a **function** (FN) rather than a **procedure** (PROC). A function provides a more natural means to return a value, since it may be assigned directly to a variable:\\ result = FNfunction(parameters) Wouldn't it be nice if you could do the same with an array, in other words create an array within a function and return it to the caller in the same way as a regular function returns a number or a string? In fact, with a little bit of trickery, you **can**!\\ \\ Using the example of a function to create an identity matrix, this is how you would code it:\\ DEF FNidn(n%) LOCAL i%, !^i() DIM i(n%, n%) FOR i% = 0 TO n% i(i%,i%) = 1 NEXT = !^i() Note particularly the use of **!^** in both the LOCAL statement and when the array is returned from the function. The function can then be called as follows:\\ !^identity() = FNidn(3) Again, note the use of an //array pointer// using **!^**.\\ \\ This may be a useful technique when translating from a language that can directly return an array from a function, or simply to create a more elegant code solution.

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returning_20an_20array_20from_20a_20function.1522502377.txt.gz · Last modified: 2018/03/31 14:19 by 127.0.0.1