create new column based on condition

[paul18fr]

I do not see any condition?

If i correctly understand what you're writting (index means row), then the following snippet works.

Of course you can replace "Matrix" by the corresponding " A", "B" and "C"

import numpy as np
import time

Nmax = 10_000
r, c = 1_000_000, 3

Matrix = np.arange(r*c).reshape(r, c)

# for all the matrix except the first row
t0=time.time()
D = np.zeros((r, 1))
i = np.arange(1, r)
D[i, 0] = Matrix[i-1, 2] - Matrix[i, 0] + Matrix[i, 1]

# only for the first row
i = 0
D[i, 0] = Matrix[i, 2] + Matrix[i, 0] - Matrix[i, 1]
D = D.astype(int)
t1 = time.time()
print(f"Duration = {t1-t0}")

print(f"D = {D}")

Output:
D = [[      1]
 [      3]
 [      6]
 ...
 [2999991]
 [2999994]
 [2999997]]