Trying to modify a vector by removing alternate elements

Hi ,

I am learning matlab and I came across a problem on cody:

I need to be able to remove even elements from a 1D vector:

if x= [ 1 8 5 9]

then y = [1 5]

I tried with somethin like this:

n=1;
y=x;
for i = 1:length(x)
    if mod(n,2) == 0
        y(n)=[];         
    end
    n= n+1;
    
end

and I got this error:

Matrix index is out of range for deletion.

Error in for2 (line 5)

y(n)=[];

Answer 1

[Stephen23]

There is no point in defining n when it always has exactly the same value as the loop iteration variable i. Get rid of one of those variables.

Your code is buggy because you did not consider that when you remove elements from a vector it gets smaller, which is clear if you print the vector on each loop iteration. This is what your code does:

1. iteration -> remove nothing [1 8 5 9]
2. iteration -> remove 2nd value [1 5 9]
3. iteration -> remove nothing [1 5 9]
4. iteration -> remove 4th value [1 5 9] !!! EEROR: the vector does not have a 4th element !!!

What do you expect to happen when you try to remove the 4th element of a 3-element vector?

You can resolve this by looping over the vector in reverse:

for n = numel(y):-1:1 % reverse!
    ...
end

Or by getting rid of the loop and writing simpler, more efficient MATLAB code using basic indexing:

y(2:2:end) = []

[Hussain Bhavnagarwala]

Thanks stephen,

the reversing solution worked for me!

I have a few questions:

1) is there a way to print the vector on every iteration in matlab

2) The second solution seems more simple, could you let me know where i can find more about it.

[Stephen23]

1)

for ...
    ...
    disp(y)
end

2)

https://www.mathworks.com/help/matlab/learn_matlab/array-indexing.html

https://www.mathworks.com/help/matlab/math/array-indexing.html

[Subham Kumar]

How do we use the second solution for a 2D array? I want to keep it a 2D array after removing the alternative elements.

[Stephen23]

@Subham Kumar: do you mean like this?:

M = rand(4,3)

M = 4×3

0.2389 0.9783 0.6445 0.2981 0.6712 0.3533 0.8601 0.4258 0.1559 0.3875 0.8520 0.8540

M(2:2:end,:) = []

M = 2×3

0.2389 0.9783 0.6445 0.8601 0.4258 0.1559