Undefined Variable f as i am trying to extract the features

Question

Muzafar Pandit 2012-10-11

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https://ww2.mathworks.cn/matlabcentral/answers/50513-undefined-variable-f-as-i-am-trying-to-extract-the-features

clc
clear
close all
format compact
face_folder='pos/';
folder_content=dir([face_folder,'*']);
face=size (folder_content,1);
for k=3:face
    k1=k-2
    string=[face_folder,folder_content(k,1).name];
    image=imread(string);
    f(k1,:)=cohog(image);
end
face=face-2;
x=[f(1:face,:);]';
    y=[ones(face,1);]';
save testinria x y face

This is my code through which i want to extract the features, but while executing the program it shows error as

Undefined variable f.

I have defined f within the for loop, then why it's showing this error, please help me to rectify the error.

Error in feature1 (line 15)
x=[f(1:face,:)]';

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Image Analyst 2012-10-11

编辑：Image Analyst 2012-10-11

Are you absolutely 100% sure it entered the loop?

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Answer 1

Walter Roberson 2012-10-11

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在 MATLAB Online 中打开

Because you do not initialize f, you would get that error if dir() returned no more than 2 results.

Note: you should not be assuming that the first two entries should be discarded. dir() does not sort the results, and just returns whatever order is returned by the operating system. In turn, the operating system does not sort the results and returns whatever order is used by the file system. Different file system types on the same machine may return different orders. For example some filesystem types sort in strict ASCII order and so may sort a filename that begins with a space before they sort the '.' directory.

Also, using a variable named "image" will conflict with the image() function.

Adjusted code:

folder_content = dir([face_folder,'*']);
folder_content([folder_content.isdir]) = [];
face = length(folder_content);
f = [];
for k = 1 : face
  string = fullfile(face_folder,folder_content(k).name);
  thisimage = imread(string);
  f(k, :) = cohog(thisimage);
end

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Answer 2

Image Analyst 2012-10-11

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You need to preallocate f before the loop - at least one row. In your line it doesn't know how many columns there are and it needs to if you're going to assign it like that.

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Image Analyst 2012-10-11

So, assuming it entered the loop like he says it definitely did, why didn't it create the array f? Instead of saying it was not defined yet, why didn't it just define/create it? I couldn't run the code because I don't have cohog.

My guess would be that it never did enter the loop so by the time it gets to x=[f..... there is no f yet. But he says it was defined in the loop. I wonder if Muzafar knows how to use the debugger.

By the way Muzafar, don't use "image" as the name of your variable - that will blow away the "image()" function that is built in to MATLAB.

Muzafar Pandit 2012-10-12

thanks dear

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Answer 3

Muzafar Pandit 2012-10-12

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Thanks Dear,actually it had a path problem, but you gave me the idea as you pointed "whether it enters the loop or not", and obviously it wasn't entering the loop as face value was 0; thanks guys