MATLAB: Explain this please

#logical #orMATLAB

x = 2;

e = 2.713;

c = 5 + (x < 2 || (e < pi));

c = 6

How does the value of c=6 after the code is executed? 2 is not less than 2, while e is not less than pi, so that is throwing me off. Thank you!

Best Answer

c = 5 + (x < 2 || (e < pi));

Let's look at each section of the code inside the outer parentheses in turn.

x < 2

Since x is equal to 2, x < 2 is false.

(e < pi)

Since e is less than pi, this returns true. [By the way, you can use exp(1) to return the value of the constant e.]

(false || true)

This results in true.

So what is 5 + true? For many intents and purposes in MATLAB, you can think of false like 0 and true like 1. Indexing is the main exception; you can't index into an array with 0 but you can with false. [Search the documentation for "logical indexing" for more information on this.] So 5 + true is essentially 5 + 1 which results in 6.

Related Solutions

MATLAB: Eigenvalues and Eigenvectors of Symbolic Matrix

syms E t 
H = [E -t -t -t -t 0 0 0 0;-t E 0 0 0 -t -t 0 0;-t 0 E 0 0 0 0 -t -t;...
    -t 0 0 E 0 -t 0 -t 0; -t 0 0 0 E 0 -t 0 -t; 0 -t 0 -t 0 E 0 0 0;...
    0 -t 0 0 -t 0 E 0 0; 0 0 -t -t 0 0 0 E 0; 0 0 -t 0 -t 0 0 0 E];
[V,D] = eig(H)

gives:

V =
 
[  0, -1, -1,                               0,                               0,                               0,                               0,                                 2,                                 2]
[ -1,  0,  0, (E + 2^(1/2)*t)/(2*t) - E/(2*t), (E + 2^(1/2)*t)/(2*t) - E/(2*t), (E - 2^(1/2)*t)/(2*t) - E/(2*t), (E - 2^(1/2)*t)/(2*t) - E/(2*t), E/(2*t) - (E - 2*2^(1/2)*t)/(2*t), E/(2*t) - (E + 2*2^(1/2)*t)/(2*t)]
[ -1,  0,  0, E/(2*t) - (E + 2^(1/2)*t)/(2*t), E/(2*t) - (E + 2^(1/2)*t)/(2*t), E/(2*t) - (E - 2^(1/2)*t)/(2*t), E/(2*t) - (E - 2^(1/2)*t)/(2*t), E/(2*t) - (E - 2*2^(1/2)*t)/(2*t), E/(2*t) - (E + 2*2^(1/2)*t)/(2*t)]
[  1,  0,  0, E/(2*t) - (E + 2^(1/2)*t)/(2*t), (E + 2^(1/2)*t)/(2*t) - E/(2*t), E/(2*t) - (E - 2^(1/2)*t)/(2*t), (E - 2^(1/2)*t)/(2*t) - E/(2*t), E/(2*t) - (E - 2*2^(1/2)*t)/(2*t), E/(2*t) - (E + 2*2^(1/2)*t)/(2*t)]
[  1,  0,  0, (E + 2^(1/2)*t)/(2*t) - E/(2*t), E/(2*t) - (E + 2^(1/2)*t)/(2*t), (E - 2^(1/2)*t)/(2*t) - E/(2*t), E/(2*t) - (E - 2^(1/2)*t)/(2*t), E/(2*t) - (E - 2*2^(1/2)*t)/(2*t), E/(2*t) - (E + 2*2^(1/2)*t)/(2*t)]
[  0,  0,  1,                               0,                              -1,                               0,                              -1,                                 1,                                 1]
[  0,  1,  0,                              -1,                               0,                              -1,                               0,                                 1,                                 1]
[  0,  1,  0,                               1,                               0,                               1,                               0,                                 1,                                 1]
[  0,  0,  1,                               0,                               1,                               0,                               1,                                 1,                                 1]
 
 
D =
 
[ E, 0, 0,             0,             0,             0,             0,               0,               0]
[ 0, E, 0,             0,             0,             0,             0,               0,               0]
[ 0, 0, E,             0,             0,             0,             0,               0,               0]
[ 0, 0, 0, E + 2^(1/2)*t,             0,             0,             0,               0,               0]
[ 0, 0, 0,             0, E + 2^(1/2)*t,             0,             0,               0,               0]
[ 0, 0, 0,             0,             0, E - 2^(1/2)*t,             0,               0,               0]
[ 0, 0, 0,             0,             0,             0, E - 2^(1/2)*t,               0,               0]
[ 0, 0, 0,             0,             0,             0,             0, E - 2*2^(1/2)*t,               0]
[ 0, 0, 0,             0,             0,             0,             0,               0, E + 2*2^(1/2)*t]

See documentation for eig also in its symbolic version.

MATLAB: Does the cellfun fails

"...however if I try to apply the same command again as..."

You have failed to take into account that inbetween running these two commands you have changed the data. So while the first cell array only contained scalar numeric (which is a bad way to store numeric data), the second time you call cellfun the cell array now also contains char vectors (lots of 'false', to be precise). Of course when you compare 'false'==1 then the output will be a vector like this:

>> 'false'==1
ans =
     0     0     0     0     0

and that is clearly not a scalar value, so your cellfun usage will fail unless you use 'uni',false and some more complex processing, or wrap the first argument function in any or something similar. Your third example give this error message

Function 'subsindex' is not defined for values of class 'cell'

because when you use the option 'UniformOutput',false then cellfun outputs a cell array, which cannot be used for indexing. So your attempt to use the output as an index will fail.

Here is a simpler approach using a numeric array (which it should be anyway):

>> m = cell2mat(a);
>> id0 = m==0;
>> id1 = m==1;
>> a(id0) = {'false'};
>> a(id1) = {'true'};
>> a
a = 
    'false'    'true'     [    2]    [    3]    [    4]    [    5]    [    6]    [    7]    [    8]    [    9]    [   10]    [   11]
    'true'     'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'
    [    2]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    3]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    4]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    5]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    6]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    7]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    8]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [    9]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [   10]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    [   11]    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'true'     'false'    'false'    'false'

Note that your row and column headers indicate that you should perhaps be using a table, or only storing the actual data (not the headers) in a simple numeric array. Then you could do this task very simply with some basic indexing:

>> c = {'false','true'};
>> b = c(1+m(2:end,2:end))
b = 
    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'false'    'false'    'false'    'false'
    'true'     'true'     'true'     'false'    'false'    'true'     'false'    'true'     'false'    'false'    'false'

Best Answer

Related Solutions

MATLAB: Eigenvalues and Eigenvectors of Symbolic Matrix

MATLAB: Does the cellfun fails

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