[FIXED] Check numerically if numbers in array start with given digits

Issue

I have a numpy array of integers a and an integer x. For each element in a I want to check whether it starts with x (so the elements of a usually have more digits than x, but that's not guaranteed for every element).

I was thinking of converting the integers to strings and then checking it with pandas

import pandas as pd
import numpy as np

a = np.array([4141, 4265, 4285, 4, 41656])
x = 42

pd.Series(a).astype(str).str.startswith(str(x)).values   # .values returns numpy array instead of pd.Series

[False  True  True False False]

This works and is also quite performant, but for educational purpose I was wondering if there is also an elegant solution doing it numerically in numpy only.

Solution

You can get the number of digits using the log10, then divide as integer:

# number of digits of x
n = int(np.ceil(np.log10(x+1)))

# number of digits in array
n2 = np.ceil(np.log10(a+1)).astype(int)

# get first n digits of numbers in array
out = a//10**np.clip((n2-n), 0, np.inf) == x

output: array([False, True, True, False, False])

Answered By - mozway