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vegipete

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Joined: 29/01/2013
Location: Canada
Posts: 1132

Posted: 10:39pm 23 Jun 2020

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The following code works just fine:

dim string c
c = GetLetter
print c
end

function GetLetter() as string length 1
GetLetter = "123"
end function

But should it? Is that the desired behaviour? Can the size of the string returned by the function be specified?

Changing the first line above to

dim string length 1

generates an expected error.

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KeepIS

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Joined: 13/10/2014
Location: Australia
Posts: 1882

Posted: 10:55pm 23 Jun 2020

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I've never used basic like that with a function returning a string of x length, and I would not expect it to function correctly. I'm sure the length parameter was not intended to be used in this way as there is likely a default mem allocation for a string. I think length parameter was mainly for declaring arrays etc.

But as usual I could be totally wrong in my assumptions

Edited 2020-06-24 08:56 by KeepIS

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Geoffg

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Joined: 06/06/2011
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Posted: 05:18am 24 Jun 2020

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First, when you call a function you need the brackets even if there is no argument:

c = GetLetter()

Second, the length qualifier is only used to tell MMBasic how much memory to allocate for the string. This is only important in arrays. To quote from page 58 of the manual:

Quote Strings will default to allocating 255 bytes (ie, characters) of memory for each element and this can quickly use up memory when defining arrays of strings. In that case the LENGTH keyword can be used to specify the amount of memory to be allocated to each element and therefore the maximum length of the string that can be stored. This allocation ('n') can be from 1 to 255 characters.

For example: DIM STRING s(5, 10) will declare a string array with 66 elements consuming 16,896 bytes of memory while:
DIM STRING s(5, 10) LENGTH 20
Will only consume 1,386 bytes of memory. Note that the amount of memory allocated for each element is n + 1 as the extra byte is used to track the actual length of the string stored in each element.

If a string longer than 'n' is assigned to an element of the array an error will be produced. Other than this, string arrays created with the LENGTH keyword act exactly the same as other string arrays. This keyword can also be used with non array string variables but it will not save any memory.

You should use the LEFT$() function to get the first character of the string:

dim string c
c = GetLetter()
print c
end

function GetLetter() as string
GetLetter = LEFT$("123", 1)
end function

Geoff

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vegipete

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Joined: 29/01/2013
Location: Canada
Posts: 1132

Posted: 05:43am 24 Jun 2020

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Thank you for your reply.

First issue, no brackets on function call, was a typo on my part. Apologies.

The second part came from some testing after an error.
Unfortunately, I didn't get the details. A running program halted with an error message along the lines of "insufficient string memory to assign a string" or some such. The indicated error line was a function definition returning a string. At first I thought perhaps the function had been called so many times as to fill up 'string memory' but some testing showed millions of calls to a function returning a string caused no problems.

Since my function only needed to return a single character, I next experimented with forcing the function string length to 1, which led to the above example.

Note that

dim string c length 1

(Oh phooie, another typo in my original post) does work, and creates a string named 'c' that can hold only a single character, regardless of memory consumed.

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GregZone
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Joined: 22/05/2020
Location: New Zealand
Posts: 114

Posted: 06:27am 24 Jun 2020

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Geoffg said To quote from page 58 of the manual:

Quote ...
This keyword can also be used with non array string variables but it will not save any memory.

Geoff

Hi Geoff. I'm a little unclear on your last sentence explaining LENGTH.

Is this because each assignment to a regular string variable deallocates/allocates new storage, thereby always consuming just the current length of the string (+1 byte)?

Or, are you saying that outside of an Array, a string allocation always consumes 256 bytes (length 255+1)?
ie. In effect, the LENGTH keyword, when used outside of an Array, is simply ignored?

Thanks
Greg

TassyJim

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Joined: 07/08/2011
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Posted: 07:20am 24 Jun 2020

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GregZone said
Hi Geoff. �I'm a little unclear on your last sentence explaining LENGTH.

Is this because each assignment to a regular string variable deallocates/allocates new storage, thereby always consuming just the current length of the string (+1 byte)?

Or, are you saying that outside of an Array, a string allocation always consumes 256 bytes (length 255+1)?
ie. In effect, the LENGTH keyword, when used outside of an Array, is simply ignored?

Thanks
Greg

Any string will be allocated 256 bytes.
If you limit to a short length, it will consume 256 bytes
It will also raise an error of you try and store more in a string than it's declared length

So
DIM c$ LENGTH 1
will use 255+1 bytes but trying c$ = "KK" will give an error.

Jim

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Chopperp

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Joined: 03/01/2018
Location: Australia
Posts: 1097

Posted: 12:45pm 24 Jun 2020

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@Jim

Not sure what you mean.

From the CMM2 Manual

Strings will default to allocating 255 bytes (ie, characters) of memory for
each element and this can quickly use up memory when defining arrays of
strings. In that case the LENGTH keyword can be used to specify the
amount of memory to be allocated to each element and therefore the
maximum length of the string that can be stored. This allocation ('n') can be
from 1 to 255 characters.

For example: DIM STRING s(5, 10) will declare a string array with
66 elements consuming 16,896 bytes of memory while:

DIM STRING s(5, 10) LENGTH 20
Will only consume 1,386 bytes of memory.

Using LENGTH should reduce the amount of memory used for the String.

Brian

ChopperP

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10310

Posted: 12:54pm 24 Jun 2020

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Quote Using LENGTH should reduce the amount of memory used for the String.

The minimum unit of memory is 256 bytes. This applies to strings, string arrays and also integer and float arrays. Setting a length on a simple string will control how much data you can save in it but will not change the memory allocated which will always be 256 bytes. The only reason I can see for setting a length on a simple string is if you need to control it in order to copy to or from a string array with a fixed length.

Chopperp

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Location: Australia
Posts: 1097

Posted: 01:40pm 24 Jun 2020

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Should the manual be updated to reflect this?

ChopperP

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