[Python] How to efficiently multiply all non-diagonal elements by a constant in a pandas...

I have a square cost matrix stored as a pandas DataFrame. Rows and columns represent positions [i, j], and I want to multiply all off-diagonal elements (where i != j) by a constant c, without using any for loops for performance reasons.

Is there an efficient way to achieve this in pandas or do I have to switch to numpy and then back to pandas to perform this task?

Example

import pandas as pd

# Sample DataFrame
cost_matrix = pd.DataFrame([
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
])

# Constant
c = 4

# Desired output
# 1 8 12
# 16 5 24
# 28 16 9

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