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I tried to change values in an array when it is in some condition.

For example, I would to like to add 30, when the vlaues are only higher than 10.

So, I tried as follows:

x = np.arange(15)
x[x>10] = x + 30

I shows "ValueError: NumPy boolean array indexing assignment cannot assign 15 input values to the 4 output values where the mask is true".

Also I tried if else and np.where methods but they also does not work. They seem work only when replacing quantity is a certain value such as 30, not an equation as x + 30.

Any idea or help would be really appreciated.

Thank you,

Isaac

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3 Answers 3

Reset to default
3

You can use the in-place operator += to easily achieve this:

x[x>10] += 30
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1 Comment

Thank you very much Graipher,
1

Subset the array before and after the assignment so they have the same length:

x[x > 10] = x[x > 10] + 30
x
# array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 41, 42, 43, 44])

Or use np.add.at:

np.add.at(x, x > 10, 30)
x
# array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 41, 42, 43, 44])

Use np.where, you can do:

np.where(x > 10, x + 30, x)
# array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 41, 42, 43, 44])
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1 Comment

Thank you very much Psidom.
1

Remember a Boolean array can also be given numerical operations

x += (x > 10) * 30
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1 Comment

Thank you very myuch FHTMitchell.

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