Matlab to Python converter
|Table 1. SMOP performance, measured on fujitsu AH552 running linux 3.8.0-19|
- With less than five thousands lines of python code
- SMOP does not pretend to compete with such polished products as matlab or octave. Yet, it is not a toy. There is an attempt to follow the original matlab semantics as close as possible. Matlab language definition (never published afaik) is full of dark corners, and SMOP tries to follow matlab as precisely as possible.
- There is a price, too.
- The generated sources are matlabic, rather than pythonic, which means that library maintainers must be fluent in both languages, and the old development environment must be kept around.
- Should the generated program be pythonic or matlabic?
For example should array indexing start with zero (pythonic) or with one (matlabic)?
I beleive now that some matlabic accent is unavoidable in the generated python sources. Imagine matlab program is using regular expressions, matlab style. We are not going to translate them to python style, and that code will remain forever as a reminder of the program’s matlab origin.
Another example. Matlab code opens a file; fopen returns -1 on error. Pythonic code would raise exception, but we are not going to do that. Instead, we will live with the accent, and smop takes this to the extreme — the matlab program remains mostly unchanged.
It turns out that generating matlabic` allows for moving much of the project complexity out of the compiler (which is already complicated enough) and into the runtime library, where there is almost no interaction between the library parts.
- Working example: solver.m
We will translate solver.m to present a sample of smop features. The program was borrowed from the matlab programming competition in 2004 (Moving Furniture). For the impatient, it is possible to compile and run the example without installing smop:
$ tar zxvf smop-0.25.4.tar.gz $ cd smop-0.25.4/smop $ python main.py solver.m $ python go.py
To the left is solver.m. To the right is a.py — its translation to python. Though only 30 lines long, this example shows many of the complexities of converting matlab code to python.
01 function mv = solver(ai,af,w) 01 def solver_(ai,af,w,nargout=1): 02 nBlocks = max(ai(:)); 02 nBlocks=max_(ai[:]) 03 [m,n] = size(ai); 03 m,n=size_(ai,nargout=2)
|02||Matlab uses round brackets both for array indexing and for function calls. To figure out which is which, SMOP computes local use-def information, and then applies the following rule: undefined names are functions, while defined are arrays.|
|03||Matlab function size returns variable number of return values, which corresponds to returning a tuple in python. Since python functions are unaware of the expected number of return values, their number must be explicitly passed in nargout.|
04 I = [0 1 0 -1]; 04 I=matlabarray([0,1,0,- 1]) 05 J = [1 0 -1 0]; 05 J=matlabarray([1,0,- 1,0]) 06 a = ai; 06 a=copy_(ai) 07 mv = ; 07 mv=matlabarray()
|04||Matlab array indexing starts with one; python indexing starts with zero. New class matlabarray derives from ndarray, but exposes matlab array behaviour. For example, matlabarray instances always have at least two dimensions – the shape of I and J is [1 4].|
|06||Matlab array assignment implies copying; python assignment implies data sharing. We use explicit copy here.|
|07||Empty matlabarray object is created, and then extended at line 28. Extending arrays by out-of-bounds assignment is deprecated in matlab, but is widely used never the less. Python ndarray can’t be resized except in some special cases. Instances of matlabarray can be resized except where it is too expensive.|
08 while ~isequal(af,a) 08 while not isequal_(af,a): 09 bid = ceil(rand*nBlocks); 09 bid=ceil_(rand_() * nBlocks) 10 [i,j] = find(a==bid); 10 i,j=find_(a == bid,nargout=2) 11 r = ceil(rand*4); 11 r=ceil_(rand_() * 4) 12 ni = i + I(r); 12 ni=i + I[r] 13 nj = j + J(r); 13 nj=j + J[r]
|09||Matlab functions of zero arguments, such as rand, can be used without parentheses. In python, parentheses are required. To detect such cases, used but undefined variables are assumed to be functions.|
|10||The expected number of return values from the matlab function find is explicitly passed in nargout.|
|12||Variables I and J contain instances of the new class matlabarray, which among other features uses one based array indexing.|
14 if (ni<1) || (ni>m) || 14 if (ni < 1) or (ni > m) or (nj<1) || (nj>n) (nj < 1) or (nj > n): 15 continue 15 continue 16 end 16 17 if a(ni,nj)>0 17 if a[ni,nj] > 0: 18 continue 18 continue 19 end 19 20 [ti,tj] = find(af==bid); 20 ti,tj=find_(af == bid,nargout=2) 21 d = (ti-i)^2 + (tj-j)^2; 21 d=(ti - i) ** 2 + (tj - j) ** 2 22 dn = (ti-ni)^2 + (tj-nj)^2; 22 dn=(ti - ni) ** 2 + (tj - nj) ** 2 23 if (d<dn) && (rand>0.05) 23 if (d < dn) and (rand_() > 0.05): 24 continue 24 continue 25 end 25 26 a(ni,nj) = bid; 26 a[ni,nj]=bid 27 a(i,j) = 0; 27 a[i,j]=0 28 mv(end+1,[1 2]) = [bid r]; 28 mv[mv.shape + 1,[1,2]]=[bid,r] 29 end 29 30 30 return mv
- Which one is faster — python or octave? I don’t know.
- Doing reliable performance measurements is notoriously hard, and is of low priority for me now. Instead, I wrote a simple driver go.m and go.py and rewrote rand so that python and octave versions run the same code. Then I ran the above example on my laptop. The results are twice as fast for the python version. What does it mean? Probably nothing. YMMV.
ai = zeros(10,10); af = ai; ai(1,1)=2; ai(2,2)=3; ai(3,3)=4; ai(4,4)=5; ai(5,5)=1; af(9,9)=1; af(8,8)=2; af(7,7)=3; af(6,6)=4; af(10,10)=5; tic; mv = solver(ai,af,0); toc
Work in progress below this line
Running the test suite:
$ cd smop $ make check
lei@dilbert ~/smop-github/smop $ python main.py -h SMOP compiler version 0.25.1 Usage: smop [options] file-list Options: -V --version -X --exclude=FILES Ignore files listed in comma-separated list FILES -d --dot=REGEX For functions whose names match REGEX, save debugging information in "dot" format (see www.graphviz.org). You need an installation of graphviz to use --dot option. Use "dot" utility to create a pdf file. For example: $ python main.py fastsolver.m -d "solver|cbest" $ dot -Tpdf -o resolve_solver.pdf resolve_solver.dot -h --help -o --output=FILENAME By default create file named a.py -o- --output=- Use standard output -s --strict Stop on the first error -v --verbose
- Base-one indexing
- Following fortran tradition, matlab starts array indexing with one, not zero. Correspondingly, the last element of a N-element array is N, not N-1.
- C_CONTIGUOUS and F_CONTIGUOUS data layout
- Matlab matrix elements are ordered in columns-first, aka F_CONTIGUOUS order. Numpy arrays are C_CONTIGUOUS by default, with some support for F_CONTIGUOUS arrays. Instances of matlabarray are F_CONTIGUOUS except if created empty, in which case they are C_CONTIGUOUS.
- Auto-expanding arrays
Matlab arrays are auto-magically resized on out-of-bounds update. Though deprecated, this feature is widely used in legacy code. Supporting this feature is one of the main reasons behind creation of the dedicated matlabarray class. If we chose the pythonic option — smop arrays directly mapped to ndarrays — any array update that could not be proven to be safe, should have been enclosed in try-except-resize-retry. It would not look any better.
In fortran, the pattern should be somehow (how exactly?) detected in compile-time. In python __setitem__ hides try-catch, with resize called inside catch. Is try-catch in fortran?
In numpy out-of-bounds assignment is an error. In smop, out-of-bounds assignment is supported for row and column matrices and their generalizations having shape
[1 1 … N … 1]
These arrays may be resized along their only non-singular dimension. For other matrices, new columns can be added to F_CONTIGUOUS arrays, and new rows can be added to C_CONTIGUOUS arrays.
Finally, scalar array of any dimension, having shape
[1 1 … 1]
can be resized along any dimension.
- Update to create
In matlab, arrays may be created by updating a non existent array, as in the example:
>>> clear a >>> a(17)=42
This unique feature is not supported by smop, but can be worked around by inserting assignments into the original matlab code:
>>> a= >>> a(17_=42
SMOP assumes that the input is syntactically correct and passes some test suite.
01 ok = 0 01 def solver_(c): 02 if c 02 if c: 03 ok = f00 03 ok = f00() .. code:: matlab
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