What tomSym cannot do - Revision history

Per at 10:25, 27 July 2011

2011-07-27T10:25:19Z

← Older revision		Revision as of 05:25, 27 July 2011
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	There are a few things that are possible in Matlab code, but which tomSym currently cannot do. Below we list the most common issues, and their work-arounds.		There are a few things that are possible in Matlab code, but which [[tomSym]] currently cannot do. Below we list the most common issues, and their work-arounds.

	===Variable-size matrixes===		===Variable-size matrixes===

Per: /* Conditional execution */

2011-07-27T10:03:50Z

Conditional execution

← Older revision		Revision as of 05:03, 27 July 2011
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	===Conditional execution===		===Conditional execution===

			TomSym expressions cannot be used in if statements.

			>> toms x
			>> if x>=0; y=x; else; y=-x; end
			??? The following error occurred converting from tomSym to logical:
			Error using ==> logical
			Conversion to logical from tomSym is not possible.

			This is because the <tt>if</tt>-statement is evaluated before the solver is called,
			so the symbolic expression has no numeric value.

			The work-around is to use the ifThenElse function.

			>> y = ifThenElse(x>=0,x,-x)
			y = tomSym(1x1):
			(if x>=0 then x else -x)

			Note that the ifThenElse function usually has a sharply discontinuous derivative.
			It is therefore common that the numeric solver fails to find a solution to problems
			defined using this function. It may be necessary to re-write the problem in a way
			that is easier for the solver to handle.

	===N-dimensional arrays for N > 2===		===N-dimensional arrays for N > 2===

Per: /* Replacing values in arrays */

2011-07-27T09:54:45Z

Replacing values in arrays

← Older revision		Revision as of 04:54, 27 July 2011
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	===Replacing values in arrays===		===Replacing values in arrays===

			It is not possible to change a value in an array into a tomSym expression.

			>> toms x
			>> v = [3 1 4];
			>> v(2) = x;
			??? The following error occurred converting from tomSym to double:
			Error using ==> tomSym.double at 11
			Cannot convert symbolic tomSym object to double.

			When using tomSym, it is best to always create the entire vector in one operation.

			>> v = [v(1) x v(2)]
			v = tomSym(1x3):
			[3 x 1]

			The <tt>sparse</tt> function can be used to create a matrix that contains a lot of zeroes,
			but has tomSym expressions in some positions.

	===Conditional execution===		===Conditional execution===

Per: /* Indexes into double arrays */

2011-07-27T09:46:03Z

Indexes into double arrays

← Older revision		Revision as of 04:46, 27 July 2011
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	ans = tomSym(1x1):		ans = tomSym(1x1):
	lookup([3 1 4],i)		lookup([3 1 4],i)

			The <tt>lookup</tt> function does not have a derivative, which can be a problem if we are optimizing with respect to the
			lookup index. In that case, it is better to use interpolation. Interpolation also works with non-integer values for <tt>i</tt>.

			>> interp1(1:3,v,i,'spline')
			ans = tomSym(1x1):
			ppval($struct,i)

	===Replacing values in arrays===		===Replacing values in arrays===

Per: /* Indexes into double arrays */

2011-07-27T09:42:10Z

Indexes into double arrays

← Older revision		Revision as of 04:42, 27 July 2011
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	===Indexes into double arrays===		===Indexes into double arrays===

			It is not possible to use a tomSym symbol as an index into a non-tomSym array.

			>> toms i
			>> v = [3 1 4];
			>> v(i)
			??? Error using ==> tomSym.subsindex at 8
			Cannot use a tomSym object directly as an index (due to Matlab syntax rules).
			- Use lookup or submatrix instead. (See "help lookup")

			For this to work, tomSym would have to intercept every array lookup (even when you are not using tomSym at all).
			This would slow down Matlab in general, so instead tomSym provides the lookup function. <tt>lookup(v,i)</tt> does
			exactly what <tt>v(i)</tt> would do if it worked.

			>> lookup(v,i)
			ans = tomSym(1x1):
			lookup([3 1 4],i)

	===Replacing values in arrays===		===Replacing values in arrays===

Per: /* Limits */

2011-07-26T14:33:25Z

Limits

← Older revision		Revision as of 09:33, 26 July 2011
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	This means that expressions such as <tt>a*log(a)</tt> can not be used if <tt>a</tt> might become zero. In this case it might be prudent to add a constraint <tt>a >= 1e-8</tt> (or similar) to help the solver stay out of trouble.		This means that expressions such as <tt>a*log(a)</tt> can not be used if <tt>a</tt> might become zero. In this case it might be prudent to add a constraint <tt>a >= 1e-8</tt> (or similar) to help the solver stay out of trouble.

			[[Category:tomSym]]

Per: Created page with "There are a few things that are possible in Matlab code, but which tomSym currently cannot do. Below we list the most common issues, and their work-arounds. ===Variable-size mat..."

2011-07-26T14:32:28Z

Created page with "There are a few things that are possible in Matlab code, but which tomSym currently cannot do. Below we list the most common issues, and their work-arounds. ===Variable-size mat..."

New page

There are a few things that are possible in Matlab code, but which tomSym currently cannot do. Below we list the most common issues, and their work-arounds.

===Variable-size matrixes===

The size of a tomSym object cannot depend on the value of another tomSym object. This has to do with the fact that numeric solvers typically allocate memory only at the start of an optimization run, and then expect the number of unknowns to remain constant. (This may change in the future.)

===Indexes into double arrays===

===Replacing values in arrays===

===Conditional execution===

===N-dimensional arrays for N > 2===

TOMSYM works with scalars, vectors, and two-dimensional matrices. Although Matlab can handle arrays of any dimension, TOMSYM cannot. The work-around is to use tomArray, which transforms tomSym symbols into arrays of any dimension. (TomArray also allows for a GAMS-style indexing which is a bit different from Matlab syntax, but convenient when formulating optimization problems involving high-dimension arrays.)

Note that TOMSYM supports <tt>interpn</tt> with high-dimensional numeric data, although the symbolic inputs must be of dimension less than two.

===Limits===

TOMSYM evaluates expressions by generating matlab code. (It does some mathematical simplification to improve efficiency, but only to a limited extent.) If an expression evaluates to 0/0 or 0*Inf, then NaN (Not a Number) will be returned. When the solver encounters a NaN, it will often just quit (although some solvers include back-tracking algorithms.)

This means that expressions such as <tt>a*log(a)</tt> can not be used if <tt>a</tt> might become zero. In this case it might be prudent to add a constraint <tt>a >= 1e-8</tt> (or similar) to help the solver stay out of trouble.