How does ismember and assert work in this code?

Previously, I raised a question of how to generate a matrix of only masses; provided that a matrix A is given that contains the object list and a matrix B that contains the objects + their masses. I am new to MATLAB and I want to know how the code works. Thank you!

A = ["C"; "A"; "E"; "F"; "I"]

A = 5×1 string array
    "C"
    "A"
    "E"
    "F"
    "I"

B = ["A", 1;"B", 34;"C" 56;"D" 32;"E",11;"F",8;"G", 7;"H",9;"I" 77]

B = 9×2 string array
    "A"    "1" 
    "B"    "34"
    "C"    "56"
    "D"    "32"
    "E"    "11"
    "F"    "8" 
    "G"    "7" 
    "H"    "9" 
    "I"    "77"

[ism,idx] = ismember(A,B(:,1));
assert(all(ism))
result = B(idx,:)

result = 5×2 string array
    "C"    "56"
    "A"    "1" 
    "E"    "11"
    "F"    "8" 
    "I"    "77"

Answer 1

[Voss]

[ism,idx] = ismember(A,B(:,1)); returns

• ism, a logical array the same size as A saying whether each element of A is in B(:,1), i.e., ism is true where the corresponding element of A exists in B(:,1) and ism is false where the corresponding element of A is not in B(:,1)
• idx, the index in B(:,1) where the first instance of each element of A exists, or 0 if that element of A does not exist in B(:,1)

Examples to illustrate:

x = [10;50;70];
y = [10;30;50;70];
[ism,idx] = ismember(x,y)

ism = 3×1 logical array
   1
   1
   1

idx = 3×1

1 3 4

x = [10;50;70];
y = [10;30;10;30;50;70];
[ism,idx] = ismember(x,y)

ism = 3×1 logical array
   1
   1
   1

idx = 3×1

1 5 6

x = [10;20;70];
y = [10;30;10;30;50;70];
[ism,idx] = ismember(x,y)

ism = 3×1 logical array
   1
   0
   1

idx = 3×1

1 0 6

assert is used in this case to make sure that all elements of A exist in B(:,1). If that's not the case, an error is thrown, execution of the code stops, and subsequent commands are not executed.

assert(all(ism))

Error using assert
Assertion failed.

References:

https://www.mathworks.com/help/matlab/ref/double.ismember.html

https://www.mathworks.com/help/matlab/ref/assert.html

[Masha]

Thank you!

[Voss]

You're welcome!