Changing undefined values of an array

seems to work fine The operation you're looking for is numpy.add.at. Crucially, this does an unbuffered addition at the indicies specified, whereas F[i] += f uses an internal buffer.
However, ufunc.at is notorious for being non-optimal. If your arrays are sufficiently large and rectangular, it might be worth to do a small loop and use bincount. Example timings:

In [43]: n = 10**5
    ...: m = 10**6
    ...: I = np.random.randint(n, size=m)
    ...: f = np.random.rand(m, 3)

In [44]: %%time
    ...: F = np.zeros((n, 3))
    ...: np.add.at(F, I, f)
Wall time: 624 ms

In [45]: %%time
    ...: F2 = np.zeros((n, 3))
    ...: for dim in range(3):
    ...:     F2[:,dim] += np.bincount(I, f[:,dim], n)t
Wall time: 94 ms

In [46]: np.allclose(F, F2)
Out[46]: True

fixed the issue. Will look into that further You can only handle the change of one input at a time. Here is the problem code:

this.setState({ un: event.target.un, pw: event.target.pw });

<input name="un" value={this.state.un} onChange={this.handleChange} />
<input name="pw" value={this.state.pw} onChange={this.handleChange} />

this.setState({ [event.target.name]: event.target.value });

around this issue
Is there any way to pass the array to a function so that the original array does not get changed while performing any operation in the function?

around this issue You're showing two lines being logged by console.log(dataSplit). The first line has the expected 23 fields, but the second line is just an empty string. When you split the empty string, you get an array containing a single empty string, ['']. Your problem is happening when you process the second line.
The first test if (dataSplit[1] === '') fails because there is no dataSplit[1]. Then it does if (dataSplit[7] !== "Yes"). Since there is no dataSplit[7], its value is undefined. The test succeeds, and then it logs undefined.

if (lineArray[i].trim() === "") {
    continue;
}

should help you out
I'd like to know if the two labeled situations are undefined behaviors or not. I believe that situation 1 is not a undefined behavior (once I use data only inside f),