Issues in For loop

Hi All,

I ma facing issues while reading data and saving in to desired fashion. Here is an example, I need to save the temperature and salinity from a large dataset in every 120th raw for which i wrote the code as shown below.....is there any error? as I am recieving the entire data filed instead of every 120th one....

for i=1:38976;

if(j==120);

if(temp(i)> 0 && temp(i) < 30 );

temp(i)=temp(i) ;

else

temp(i)=NaN;

end

if(sal(i)> 30 && sal(i)< 42);

sal(i)=sal(i);

else

sal(i)=NaN;

end

j=1;

else

j=j+1;

end

end

Answer 1

[Joseph Cheng]

if we break down your for loop:

 for i=1:38976;
     %this if statement will be done if j==120
     %an okay statement here but you can do this with mod(i,120)==0 as you
     %already have a counter
        if(j==120); 
            if(temp(i)> 0 && temp(i) < 30 );
            temp(i)=temp(i);  %this isn't doing anything as its over writting itself
            else
            temp(i)=NaN; %changes i to nan... okay statement but its only working on j==120
            end
            %temp statements apply to the sal if statement so its sal
            %portion has been cut out.
            j=1; %reset j to 1.
        else
            j=j+1; %increment j if j~=120
       end
 end
 

so since you're not stashing the items into another array when the index is a multiple of 120 you'll obviously get the whole array back as you did nothing but look at the input array and make it a Nan if the temp is between 0 and 30.

you can quickly retrieve the data your input variables through indexing like

temp_120=temp(120:120:end); % as that'll start with 120 increment by 120 till the end of the array.
temp_120(~(temp_120>0 & temp_120<30))=Nan; %change the items which are not between 0 and 30 to nans

like here:

x = [ 1 2 3 4 5 10 2 3 4 5 18 2 3];
x_4 = x(2:4:end)
x_4 =

     2     4    10     3     5     2
x_4(~(x_4>3 & x_4<10))=nan
x_4 =
   NaN     4   NaN   NaN     5   NaN

Answer 2

[Alan Stevens]

Have you intialised j (i.e. set j = 1;) before going into the for i = 1:38976 loop?