MATLAB and Simulink resources for Arduino, LEGO, and Raspberry Pi

# Help on for loop

Asked by Mark on 12 Mar 2013

I have an R vector and a U vector. The R vector contains mostly zeros but has a couple of values. The U vector is a shorter length than the R vector and contains no zeros. I want to make a U_final vector which has the values of the U vector at the same locations where the R vector values are non-zero. My attempted code is below. It iterates through fine, but the end result is the last value in U in all locations where R is non-zero rather than all values of U in the appropriate non-zero locations of R. I'm using U = [0.0137;0.0081] and R = [0;0;0;72;0;90]. I'd like the output to be U_final = [0;0;0;0.0137;0;0.0081]

```    for i = 1:length(R)
for j = 1:length(U)
if R(i,1) ~= 0
U_final(i,1) = U(j,1)
else U_final(i,1) == 0
end
end
end```

## 1 Comment

Walter Roberson on 12 Mar 2013

Note that U_final(i,1) == 0 is a comparison statement, not an assignment.

## Products

Answer by Teja Muppirala on 12 Mar 2013

No need to iterate. MATLAB makes this simple with logical indexing.

```U = [0.0137;0.0081];
R = [0;0;0;72;0;90];
```
```U_final = R;
U_final(R~=0) = U
```

But the number of nonzeros in R needs to be equal to the length of U.

## 1 Comment

Mark on 12 Mar 2013

Thanks Teja. I now see it as a quick replacement of the nonzero elements. No need to overcomplicate things is apparently the lesson for the day. ;) cheers!

Answer by Youssef KHMOU on 12 Mar 2013
Edited by Youssef KHMOU on 12 Mar 2013

hi try this , BUT the number of non zeros in R must be the same as the length of U:

```R=zeros(100,1);
R(2:4:55)=rand; % R contains some values ( 14 elements !)
U=rand(14,1); % U is Shorter than R and contains non zeros vals
index=find(R~=0); %locations where R has non zeros .
U_final=zeros(size(R)); % this is optional
U_final(index)=U;
```