How can i replace a certain element in a list with an element from another list according to a condition

Question

I am new to python and I want to build a function that updates part of a list according to a condition.

here is an example of what I want:

List1=[1,2,3,4,10,5,9,3,4]
List2=[2,4,6,8]

I want to update List1 to be [2,4,6,8,10,5,9,6,8], and here is my code to do that:

x = [1, 2, 3, 4, 10, 5, 9, 3, 4]
y = [2, 4, 6, 8]


def update_signal(gain):
    for i in range(0, len(y)):
        for j in range(0, len(x)):
            if x[j] == y[i] / gain:
                x[j] = y[i]
            elif x[j - 1] == y[i] / gain:
                break


update_signal(2)  # for this example only gain =2
print("x=", x)
print("y=", y)

the expected output is:

x=[2,4,6,8,10,5,9,6,8] 
y=[2,4,6,8]

what it actually prints is:

x= [8, 8, 6, 8, 10, 5, 9, 6, 8]
y= [2, 4, 6, 8]

so, what am I doing wrong to make this function behave like this?

You're processing x multiple times. So after you replace an element, you'll consider the new element when checking the condition against a different element of y. — Barmar
– Barmar, Commented Mar 12, 2020 at 7:00
Maybe you should change the order of the loops, and make x the outer loop. — Barmar
– Barmar, Commented Mar 12, 2020 at 7:03
And break out of the inner loop whenever you perform a replacement. — Barmar
– Barmar, Commented Mar 12, 2020 at 7:03

Alexander Lekontsev · Accepted Answer · 2020-03-12 07:23:20Z

1

Maybe something like this?

def update_signal(gain):
    return [item * gain if item * gain in y else item for item in x]

answered Mar 12, 2020 at 7:23

Alexander Lekontsev

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2 Comments

Boseong Choi Over a year ago

It may different if y's elements are not divided by gain. For example, y = [1, 3, 5] and gain = 2.

jizhihaoSAMA Over a year ago

This is maybe not OP expected.

Boseong Choi · Accepted Answer · 2020-03-12 07:24:22Z

1

I assume that your algorithm means:

let i is x's arbitrary element.
let j is y's arbitrary element.
if i is same with j / gain, replace i to j

(If I am wrong, point it please. Then I'll fix for it.)

x = [1, 2, 3, 4, 10, 5, 9, 3, 4]
y = [2, 4, 6, 8]


def update_signal(gain):
    convert_table = {i / gain: i for i in y}
    x[:] = [convert_table.get(i, i) for i in x]


update_signal(2)  # for this example only gain =2
print('x = ', x)
print('y = ', y)

output:

x =  [2, 4, 6, 8, 10, 5, 9, 6, 8]
y =  [2, 4, 6, 8]

answered Mar 12, 2020 at 7:24

Boseong Choi

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Comments

Elwin Wong · Accepted Answer · 2020-03-12 07:50:28Z

1

Please try the code below:

x = [1, 2, 3, 4, 10, 5, 9, 3, 4]
x_copy = x[:]
y = [2, 4, 6, 8]

def update_signal(gain):
    for i in range(0, len(y)):
        cond = y[i] / gain
        for j in range(0, len(x)):
            if x[j] == cond:
                x_copy[j] = y[i]
            elif x[j - 1] == cond:
                continue


update_signal(2)  # for this example only gain =2
print("x=", x_copy)
print("y=", y)

answered Mar 12, 2020 at 7:50

Elwin Wong

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3 Comments

Todd Over a year ago

Your continue statement isn't needed since it's the last line of code in the loop anyway ;-) it looks like this one works though @Marcus

Elwin Wong Over a year ago

@Todd Yes, you're right. But since I still don't understand what the asker means of the condition judgment elif x[j - 1] == cond:, I keep it in my answer.

Elwin Wong Over a year ago

The results are the same whether to declare globla or not. list is global variable in Python. So there is no need to declare global x_copy here since x_copy is a list.

Itamar Mushkin · Accepted Answer · 2020-03-12 11:03:13Z

1

x=[1,2,3,4,10,5,9,3,4]
y=[2,4,6,8]

def update_signal(gain):

    for i in range(len(x)):
        for j in range(len(y)):
            if y[j]//x[i]==gain:
                x[i]=y[j]
                break

edited Mar 12, 2020 at 11:03

Itamar Mushkin

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answered Mar 12, 2020 at 9:39

Abhishek

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