Please answer! How to use an if function effectively?

Hi everyone,

I have a set data loaded into the workspace at 4x100, using the if function to analyse a certain colomn within that data to see say if any value is say greater than 2, and then say if a number within that coloum is greater than 2 then set a number on that same line from a different colomn say in the second coloum to a variable.

Eg, say coloum 4 is the colum i want to analyse, to see if any number in that colum is greater than 2, say lines 30, 67 and 80 are greater than 2, i want the function to then set column 2 values from lines 30, 67 and 80 to a variable lets say x. If that makes more sense

Thank you!

[Stephen23]

https://web.archive.org/web/20201110054432/https://www.mathworks.com/matlabcentral/answers/642435-please-answer-how-to-use-an-if-function-effectively

Answer 1

[dpb]

colmn_analyze=4;    % desired column to compare
colmn_target=2;     % desired column to set
valLimit=2;         % limiting value
% engine
ix=(A(:,colmn_analyze)>valLimit);       % logical vector of locations match condition
A(ix,colmn_target)=x;                   % set the target column locations to value of x

NB: x must either be a scalar or a vector of length of TRUE elements in ix in the order to match or will have a number elements mismatch on the assignment.

Read up on logical indexing in the Getting Started doc for a most useful feature in MATLAB.

[Et.B200]

What is a in this text, is it the name that the data is stored under in my workspace?

[James Tursa]

Note: Edited the ; row index to be :

[James Tursa]

@Ethan: A in this context is simply the variable that holds your data. Use whatever your actual variable name is in this code instead of A.

[Et.B200]

I still get an error message, ive inputted the a value as my workplace variable with the data? And thank you James for letting me know its a :, the error message reads that the x variable is undefined

[James Tursa]

Example run:

>> A = 4*rand(5,5) % arbitrary sample data
A =
    2.8242    3.2938    1.7550    1.9591    1.1041
    0.1273    2.7793    1.5262    1.7823    2.7188
    1.1077    1.2684    3.0621    2.5853    2.6204
    0.1847    3.8009    3.1808    2.8375    0.6504
    0.3885    0.1378    0.7475    3.0187    0.4760
>> colmn_analyze=4;    % desired column to compare
>> colmn_target=2;     % desired column to set
>> valLimit=2;         % limiting value
>> x = 10;
>> ix=(A(:,colmn_analyze)>valLimit);       % logical vector of locations match condition
>> A(ix,colmn_target)=x
A =
    2.8242    3.2938    1.7550    1.9591    1.1041
    0.1273    2.7793    1.5262    1.7823    2.7188
    1.1077   10.0000    3.0621    2.5853    2.6204
    0.1847   10.0000    3.1808    2.8375    0.6504
    0.3885   10.0000    0.7475    3.0187    0.4760

So in column 4 the 3rd, 4th, and 5th elements are over the valLimit of 2. So those elements in column 2 get changed to 10 (i.e., x).

[Et.B200]

Ohh sorry, i've just worded my question wrong!! Sorry guys! So I want the output, using your example, to be the numbers 1.2684, 3.8009, and 0.1378, not to change the data set as such, but to output the numbers on the same line as those in another column which suit the value limit.

Many thanks, Ethan

[James Tursa]

Do you mean like this?

colmn_other = 6;
A(ix,colmn_other) = A(ix,colmn_target)