# Issues in For loop

1 view (last 30 days)
vivek narayanan on 23 Jun 2021
Edited: Joseph Cheng on 23 Jun 2021
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

Joseph Cheng on 23 Jun 2021
Edited: Joseph Cheng on 23 Jun 2021
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
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

Alan Stevens on 23 Jun 2021
Have you intialised j (i.e. set j = 1;) before going into the for i = 1:38976 loop?

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