The variable "X" might be used before it is defined

Is something wrong because of that red line? The program can't find that file?

How can I solve this problem? Thanks

[Walter Roberson]

You do not show us any code that uses a variable "X".

[João Carvalho]

the variable is "g1_land"

[per isakson]

What's in g1_land.txt? I guess load g1_land.txt fails. Is it a lowercase L or a one?

[Walter Roberson]

Duplicated by later https://www.mathworks.com/matlabcentral/answers/363470-the-variable-g1_land-might-be-used-before-it-is-defined-error and requires merging

[João Carvalho]

In "g1_land.txt" there are three columns with numbers. It is a lowercase L.

[per isakson]

I would try to interactively run

data = load( c:\mydata\folder\g1_land.txt', '-ascii' );

with your full file specification in the command window.

Answer 1

[OCDER]

Using load without an output is essentially "poofing" variables into your function and forcing the rest of the code to "guess" what variable got poofed in. That's what the warning message is telling you: The variable "g1_land" might be used before it's defined.

To fix this:

g1_land = load('g1_land.txt');  %Now, you explicitly told the computer what g1_land is.
                                "Hey Matlab, I'm importing data into a variable called g1_land, 
                                 so no surprises!" 
ano1 = g1_land(:, 1);

Note that if g1_land.txt is formatted incorrectly, you'll get other errors. There are other ways to import data, like csvread, dlmread, fscanf, textscan, etc.