Python - Use of assigning variables

Hi, Please can somebody help me, Please tell me how the below is different.

Scenario 1:

a = 3

b = 2

b + a = answer


Scenario 2

x = -2 * np.random.rand(200,2)

x0 = 1 + 2 * np.random.rand(100,2)

x[100:200, :] = x0

It is understood that a variable cannot be assigned at the end of a statement, So my scenario 1 wouldn’t work - which I understand, however how is it allowed in scenario 2( which i copied from a code tutorial)

Welcome, clinton.

I’ve edited your post for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

See this post to find the backtick on your keyboard. The “preformatted text” tool in the editor (</>) will also add backticks around text.

Note: Backticks are not single quotes.


To answer your question: You are correct about Scenario 1 being incorrect, but Scenario 2 is not doing anything similar to it.

In Scenario 2 the variable x0 has been declared, and is a value. Therefore, in this line:

x[100:200, :] = x0

the values of each of the matrix members between cols 100-200, and all the rows, are given the value of x0.

In this

b + a = answer

answer is never declared to equal a number, and b+a on the left does not become another variable, it becomes a value. Therefore, is invalid.

Hope this helps

Thank you very much for your explanation, it does make a little more sense, However, please excuse me for misunderstanding - but would line 2 in the below code be useless then, because the code is assigning a matrix to x0 in line 2, but then it is being over-written or assigned a new value on line 3 ? so what was the purpose of line 2

x = -2 * np.random.rand(200,2)

x0 = 1 + 2 * np.random.rand(100,2)

x[100:200, :] = x0

Ah, this is a very important thing, when working with a large amount of data. Here is what is going on/should be happening:

Line 1 is telling your program ahead of time: “This variable x is going to be a matrix of size m x n. Prepare this amount of space in memory to store the data.”

Line 2 is just some other matrix, serving as a way to make reading the code slightly easier.

Line 3 is rewriting the values of x from line 1, using the values in x0.

x0 stays the same. It is not over-written. x is over-written, but it has been initialised, to avoid the scenario where you are increasing the size of x by appending new values to it (this is bad for efficiency, as well as memory). Instead, every element of x has already been allocated a place in memory, and the value of the element is just being changed.

Essentially, the above could be written as:

x = -2*np.random.rand(200,2)
x[100:200, :] = 1 + 2*np.random.rand(100,2)

Now, you are only re-writing half of the values of x, but the memory pre-allocation is still useful.

I hope this was clear. It sounded muddled in my head.

Thanks alot, This really clears things up