How to change specific numbers to 0

So here is my current code:

A = [ 1 2 3 4 5 6 7 8 9];
B = [ 1 2 3 4];
A(~B)=0;
disp(A);

     1     2     3     4     5     6     7     8     9

Basically I want A to be equal to [ 1 2 3 4 0 0 0 0 0 ]. But for some reason it won't take ~B, where as it will work when it is just B. Anyone know a solution to this or why this is the case? Thanks in advance!

[Stephen23]

"But for some reason it won't take ~B, where as it will work when it is just B. "

Compare:

B = [1,2,3,4]

B = 1×4

1 2 3 4

~B

ans = 1×4 logical array
   0   0   0   0

The first is a numeric array, the second is a logical array. Now revise what kind of indexing MATLAB supports:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

[William]

@Stephen23 thank you for pointing me in a direction that will help my understanding of this code in general

Answer 1

[Daniel]

There are two ways you can select elements of an array in MATLAB.

1. Pass in an array of valid indices. "Valid" means that they're within the range of indices to the array.
2. Pass in an array of logical values, of the same size as the array.

a = (1:10)*2; % Demo array. This has 10 elements, indexed 1 through 10, values 2, 4, 6, ... 20.

% First method
a(1)

ans = 2

a(10)

ans = 20

a(end) % Special case: "end" refers to the last element in the array

ans = 20

a([5 7 2])

ans = 1×3

10 14 4

% Second method
mod(a,4) == 0 % Notice that this returns a "logical" data type at the output, as will any comparison

ans = 1×10 logical array
   0   1   0   1   0   1   0   1   0   1

a(mod(a,4)==0)

ans = 1×5

4 8 12 16 20

~ is a logical operator, so ~B is an array of logicals. Since it's a logical array, it has to have the same size as A, which it doesn't. Therefore the line of code fails.

One alternative method would be to write:

A(5:end) = 0;

That will select and modify elements 5 through 9 (the last element) of A.

Answer 2

[John D'Errico]

For some reason? Learn to use ismember instead. What you think makes sense does not work in MATLAB. And that means you need to start back with the tutorials, to learn MATLAB syntax. Sorry. Time to start at the beginning.

[William]

Ok, I wasn't going to respnond to you until you came back 30 minutes later on a friday night and edited your response to be condesending. I am an undergrad 21 year old student who just tries to learn this code with no instruction from my school. You have commented on multiple of my questions just being an a jerk and honestly not helpful. I will be the first to tell you I am not good at coding or will claim to be the smartest in the room. But I try ok. I try to be adequete, and sometimes I don't always reach that. But I'm trying. So please, next time you leave a reply on someone's profile just understand that people come from different backrounds and don't always get the same level of teaching.

[Daniel]

Regarding starting at the beginning, speaking as a MathWorker, you probably have access to some MATLAB fundamentals courses if you go to matlab.mathworks.com through your university account and click on "Online Training". You may find the MATLAB Onramp and/or MATLAB Fundamentals to be helpful.