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Topic: boolean indexing for subset reassignment
Replies: 3   Last Post: Apr 26, 2014 2:07 AM

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Bill Rowe

Posts: 1,647
Registered: 3/14/08
Re: boolean indexing for subset reassignment
Posted: Apr 21, 2014 5:12 AM
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On 4/20/14 at 4:47 AM, alan.isaac@gmail.com (Alan) wrote:

>Two years ago this answer was given:
>http://mathematica.stackexchange.com/questions/2821/does-mathematica
>-have-advanced-indexing


>Essentially, the answer was that Mathematica did not provide direct
>support for this feature (unlike say NumPy, R, etc.). Work arounds
>were suggested.


>Has the situation changed?

No, there is no built-in function that does just what was
described. But this is a very simple thing to do in Mathematica.

generate an array with some values greater than 15:

In[3]:= a = RandomInteger[{1, 20}, {4, 3}]

Out[3]= {{12, 6, 17}, {4, 17, 19}, {14, 10, 7}, {8, 2, 15}}

generate the array to change:

In[4]:= b = RandomReal[1, {4, 3}]

Out[4]= {{0.6304544421081466, 0.10672523812862322,
0.4822372961689563}, {0.06301096415944252,
0.975158725702236, 0.052202491707093346},
{0.06883790695346681, 0.7890506831190796,
0.8367965617051536}, {0.8641752609839166,
0.6371442124093818, 0.9217402690944818}}

then

In[5]:= m = b Unitize@Clip[a, {1, 15}, {0, 0}]

Out[5]= {{0.6304544421081466, 0.10672523812862322, 0.},
{0.06301096415944252, 0., 0.}, {0.06883790695346681,
0.7890506831190796, 0.8367965617051536},
{0.8641752609839166, 0.6371442124093818,
0.9217402690944818}}

is array b changed to have a 0 wherever the corresponding
element of a is greater than 15





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