IDL to numarray mapping
This is intended to be a brief guide for IDL users to the
features of the Python numarray extension. It is incomplete
and may have a few errors.
Feedback is welcome.
February 2002
| IDL |
Python Equivalent |
|
|
| .run |
import (.py assumed)
reload(module) to recompile/re-execute |
| @"filename" |
execfile("filename") |
| exit |
(e.g. ^D) |
| up-arrow (cmd recall) |
up-arrow |
Operators
| <,> (clipping) |
Currently no operator equivalent. The functional
equivalents are: |
| a < 100. |
choose(greater(a, 100.),(a, 100.)) |
| a > 100. |
choose(less(a, 100.),(a, 100.)) or for both
clip(a, min_val, max_val)
[For python 2.1 or greater, the following should work]
choose(a < 100, (a, 100.)) |
| MOD |
% |
| # (matrix multiply) |
multiply.outer() is equivalent for some applications,
matrixmultiply() for others. |
| ^ |
** |
Boolean operators
| and |
and |
| not |
not |
| or |
or |
| xor |
<None> |
Bitwise operators
| and |
& (bitwise and) |
| or |
| (bitwise or) |
| xor |
^ (bitwise xor) |
| not |
~ (bitwise not) |
| ishft(a,1) |
a<<1 (left shift) |
| ishft(a,-1) | a>>1 (right shift) |
Comparison operators
|
(array ufuncs) |
|
| eq |
== or equal() |
| ge |
>= or greater_equal() |
| gt |
> or greater() |
| le |
<= or less_equal() |
| lt |
< or less() |
| ne |
!= or not_equal() |
|
|
The operator versions only work in Python 2.1.
Use the functional form for Python 2.0 |
Slicing and indexing
NOTE: Order of indices is opposite! For a 2-dim array:
a(i,j) is equivalent to a[j,i]
For a given dimension,slices may be specified
| i:j |
i:j (except for different interpretation for j) |
| i:* |
i: |
| *:i |
:i
|
| a(i,*) |
a[:,i] |
|
| no IDL equiv. |
i:j:s (stride, take every s'th element) |
| . |
i:-j |
| . |
empty arrays and slices are permitted (e.g. a[0:0] is
an array of length 0) |
| . |
-i (indexing from end of array) |
| . |
... (fill in unspecified dimensions) |
| . |
NewAxis (add new axis to array shape as part
nbsp;nbsp;nbsp;nbsp;nbsp;nbsp;of subscript to match shapes)
|
NOTE: Slicing generally does not create a new array, but
instead creates a virtual array (non-contiguous in memory)
that points to the same memory locations as the original
array. E.g. b=a[0:10:2] followed by b[0] = 5 will change
a[0] to 5 as well. You must use b = copy(a[0:10:2]) to get
a complete new copy of the data.
This may sound confusing, but it can be very useful if you
are working with big arrays and need to limit memory use.
Unlike IDL, every new slice does not generate a new array in
memory.
Index arrays
| IDL |
numarray |
|
|
|
| newarr = arrayvar(indexarr) |
newarr = arrayvar[indexarr] |
| arrayvar(indexarr) = valuearr |
arrayvar[indexarr] = valuearr |
Array Creation
| fltarr() |
array(seq,[type]) to create from existing sequence |
| dblarr() |
zeros(shape,[type]) to create 0 filled array |
| complexarr() |
ones(shape,[type]) to create 1 filled array |
| intarr() |
|
| longarr() |
|
| bytarr() |
|
| make_array() |
|
|
| strarr() |
lists and tuples are more appropriate
(chararray do exist to handle large fixed length
strings) |
|
| findgen() |
arrayrange(size, [type]) (aka arange() ) |
| dindgen() |
|
| indgen() |
|
| lindgen() |
|
|
| sindgen() |
no direct equivalent, but trivial to duplicate. |
|
| replicate() |
repeat() (more general) |
Array Conversions
| byte(arrayvar) |
arrayvar.astype() |
| fix(arrayvar) |
|
| long(arrayvar) |
|
| float(arrayvar) |
|
| double(arrayvar) |
|
| complex(arrayvar) |
|
Array Manipulation
| reform() |
reshape() [also arrayvar.flat() and ravel(),
or changing the shape attribute] |
| sort() |
argsort() [i.e.,indices needed to sort] |
| arrayvar(sort(arrayvar)) |
sort(arrayvar) [i.e., sorted array] |
| max() |
max() [ufunc] (also argmax()) |
| min() |
min() [ufunc] (also argmin()) |
| where() |
nonzero() |
| arrayvar(where(condition)) |
compress(condition, arrayvar) |
| transpose() |
transpose() |
| [a,b] (array concatenation) |
concatenate() |
In general, Python array manipulation functions are far more
general and powerful than IDL manipulation functions, but
also harder to understand. These include:
choose()
take()
repeat()
resize() |
nonzero()
compress()
reshape()
resize() |
concatenation()
transpose()
diagonal()
identity() |
argmax()
searchsorted()
sort()
argsort() |
Array Operations
IDL:
If size or shape mismatches, result is coerced to smaller of
two and shape of one.
Python:
Shape or mismatch results in error if one array not a
'subset' of other's shape. If subset, 'broadcasting' is
used.
| IDL types |
Python numarray types |
|
|
|
| byte |
Int8 |
|
UInt8 |
| int |
Int16 |
| long |
Int32 |
| float |
Float32 |
| double |
Float64 |
| complex |
Complex64
Complex128 |
Math Functions
|
[Python ufuncts] |
|
| abs() |
absolute(), fabs() |
| acos() |
arccos() |
| alog() | log() |
| alog10() | log10() |
| asin() | arcsin() |
| atan() | arctan() |
| atan(y,x) | arctan2() |
| ceil() | ceil() |
| conj() | conjugate() |
| cos() | cos() |
| cosh() | cosh() |
| exp() | exp() |
| floor() | floor() |
| imaginary() | complexvar.imag (.real for real
component) Python 2.2 only
complexvar.getimag() (complexvar.getreal() for
real) |
| invert() | Matrix module |
| ishft() | right_shift(), left_shift() |
| round() | ??? |
| sin() | sin() |
| sinh() | sinh() |
| sqrt() | sqrt() |
| tan() | tan() |
| tanh() | tanh() |
|
| Other Python ufuncs: |
|
| comparison: |
| greater(), greater_equal(), |
| less(), less_equal(), |
| equal(), not_equal(), |
| bitwise_and(), bitwise_or(), bitwise_xor(), |
| logical_and(), logical_or(), logical_xor(), |
|
| add, subtract, multiply, divide, remainder,
power, negative, hypot |
|
| For binary ufuncs, the following methods exist: |
|
reduce() | apply operator to all elements
in array (add.reduce equivalent
to IDL total()) |
| accumulate() |
generate new array whose elements
equal the operation applied to the
first n elements of the input array
(add.accumulate is equivalent to
an integrate function) |
| outer() |
produce all combinations of results
(multiply.outer(x,y) is equivalent
to an outer product)
|
| fft() |
fft() [FFT module] (not available yet) |
| convol() | (not available yet) |
| randomu() | random(), uniform() [RandomArray
(not available yet)] |
| randomn() | RNG module (not available
yet) |
Programming
| execute() | exec() |
| n_elements() | len() |
| n_params() |
closest equivalent is len(*args). Ability
to supply default values for parameters
replaces some uses of n_params(). |
| size() | shape(), arrayvar.type() |
| wait | time.sleep() |
Last updated: November 5 2002
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