numpy.cumsum

numpy.cumsum(a, axis=None, dtype=None, out=None)

Return the cumulative sum of the elements along a given axis.

Parameters:

a : array_like

Input array or object that can be converted to an array.

axis : int, optional

Axis along which the cumulative sum is computed. The default (axis = None) is to compute the cumsum over the flattened array. axis may be negative, in which case it counts from the last to the first axis.

dtype : dtype, optional

Type of the returned array and of the accumulator in which the elements are summed. If dtype is not specified, it defaults to the dtype of a, unless a has an integer dtype with a precision less than that of the default platform integer. In that case, the default platform integer is used.

out : ndarray, optional

Alternative output array in which to place the result. It must have the same shape and buffer length as the expected output but the type will be cast if necessary.

Returns:

cumsum : ndarray.

A new array holding the result is returned unless out is specified, in which case a reference to out is returned.

Notes

Arithmetic is modular when using integer types, and no error is raised on overflow.

Examples

>>> a = np.array([[1,2,3],[4,5,6]])
>>> np.cumsum(a)
array([ 1,  3,  6, 10, 15, 21])
>>> np.cumsum(a,dtype=float)     # specifies type of output value(s)
array([  1.,   3.,   6.,  10.,  15.,  21.])
>>> np.cumsum(a,axis=0)      # sum over rows for each of the 3 columns
array([[1, 2, 3],
       [5, 7, 9]])
>>> np.cumsum(a,axis=1)      # sum over columns for each of the 2 rows
array([[ 1,  3,  6],
       [ 4,  9, 15]])