For loop with Else statement

Hi all! I am running a loop of the form:

RR=flip(1:100);   
CC=0;
threshold=3000;
for i=1:100
  if CC<threshold
      C_old=CC;
      CC=sum(RR(1:i));    
  else
      CC=C_old;
      RR(i)=0;
  end
end

The problem is when the loop passes through the else statement it automatically increase "i" by 1. Which leads to skipping values of the vector "RR". How can I fix this?

[Guillaume]

it automatically increase "i" by 1

It certainly doesn't so if that really happens it's because you have written code that explicitly does it.

We would need to see the actual code for us to tell you what is happening.

[Miroslav Mitev]

Hi @Guillaume, I've added the code.

Answer 1

[Guillaume]

Your code is behaving exactly as expected. It may not do what you want but that's because you've made a mistake.

The best way for you to understand what is happening is to use the debugger to step through your code one line at a time and see how the variables evolve.

• At i = 37, CC is 2970, so the if is true, C_old is set to 2970 and CC becomes 3034.
• At i = 38, the if is now false, hence CC gets assigned C_old so is set back to 2970. RR(38) is set to 0
• At i = 39, since CC is now again 2970 the if is true, C_old is set again to 2970 (the same value it already had), CC becomes 3220
• At i = 40, the if is now false, hence CC gets assigned C_old which is still 2970. RR(40) is set to 0.
• At i = 41, see step 39
• and so on ... every even i set RR(i) to 0, every odd i, CC is 2970.

I have actually no idea what your code is trying to achieve. My feeling is that you probably don't need a loop.

[Miroslav Mitev]

Thanks. I fixed the issue by using another if loop instead of else statement. It works fine now:

RR=flip(1:100);   
CC=0;
threshold=3000;
for i=1:100
  if CC<threshold
      C_old=CC;
      CC=sum(RR(1:i));    
  end
  if CC>threshold
      CC=C_old;
      RR(i)=0;
  end
end

[Guillaume]

As said, a loop is not necessary:

RR = flip(1:100);
CC = cumsum(RR);
RR(CC > threshold & CC(find(CC < threshold, 1, 'last')) + RR > threshold) = 0;

Note that you may want to change either the > or the < into >= or <= respectively.

Answer 2

[KALYAN ACHARJYA]

for i=1:100
   if b(i)>1
      do something
   else
      do something else
   end
end

No, this cant, except "do something else" statement include i=i+1, other any other i increment statement.

For Example

b=-5:1:100
  c=1;
  for i=1:100
     if b(i)>1
        disp(b(i));
     else
        c=c+1;
     end
  end

[Miroslav Mitev]

Hi, @KALYAN, please see the code in the question above.

[KALYAN ACHARJYA]

@Miroslavi value is not skipped (I checked it by disp(i))

RR=flip(1:100);   
CC=0;
threshold=3000;
for i=1:100
  if CC<threshold
        disp(i);
      C_old=CC;
      CC=sum(RR(1:i));    
    else
      disp(i);
      CC=C_old;
      RR(i)=0;
    end
end

Answer 3

[Dennis]

My guess is that you want to set every value in RR to 0 after the cumulative sum reaches 3000.

Here is why this does not work:

CC is the sum of RR(1:i), once CC reaches 3001 you enter your else statement. In your else statement:

        CC=C_old; % you set CC to zero
        RR(i)=0;

In the next iteration of your loop CC will initially be 0. Hence it enters your if statement:

CC=sum(RR(1:i));    %this will be >3001

So basically from here on your loop will alternate between if and else.

Now i am not completly sure what you want to do, if my assumption was correct that you want to set values in RR to 0 after the sum reaches a specific value you can try this code:

RR=100:-1:1;
A=cumsum(RR);
RR(A>3000)=0;