index exceeds matrix dimensions??

This is part of a loop that I'm trying to run and get an error:

c = zeros(1,20); 

for b=1:20 
    a = 1; 

      while dH_df(a,b)<0.80 %line 199
        a = a+1;
      end

      c(b) = a; 
      dH_df(c(b),b); 
  end

The error is:

"??? Index exceeds matrix dimensions.

Error in ==> filename at 199 while dH_df(a,b)<0.80"

-I tried a few things, but I still can't see where the error stems from A few clarifications: -this loop is part of a bigger program that calculates dH_df

-dH_df is a 31x20 matrix with values ranging from 0.4700 to 1.1100

-the loop is supposed to record the position of values ranging 0.80 to 0.85 from dH_df and record the row position in "c" - I'm having trouble setting up that range as well

Any suggestions will be really appreciated. Thank you.

Answer 1

[Matt Fig]

while a<=size(dH_df,1) && dH_df(a,b)<0.80 %line 199

If you want to find the row position of values between 0.80 and 0.85,

[r,c] = find(dH_df>.8 & dH_df<.85);

r has the row position, c has the column position.

[Walter Roberson]

Yup. If _all_ the rows in one column satisfied the condition then the original code would try to access past the end of the column.

Answer 2

[Sean de Wolski]

does dH_df of that line have any values <0.8? If it doesn't 'a' will grow to be bigger than the matrix and it would throw that error.

You could replace the while loop with

[a,a] = max(dH_df>=0.80,[],2);

and it will find 'a' for each row all at once (assuming each row has a value less than 0.8, else it'll return the first column.)

Answer 3

[Annie M]

Thank you, this:

[r,c] = find(dH_df>.8 & dH_df<.85);

happened to work best with what I'm trying to achieve. The remaining solutions work well, but I would have to state my range differently to avoid the loop trying to access past the end of the column. Thank you very much for everyone's help.