This nested for loop goes on forever?

Question: A ball is launched from a height of 12m. It should land in a pool that has a length of 22m and is 9m away from the wall. Find the minimum and maximum initial velocities for the ball to land in the pool. The ramp is angled at 1 radian such that vy = 0.54v and vx = 0.84v at time zero. The ball will not be airborned for more than 3 seconds.

So I tried to write a loop for it. My nested for loop goes on forever...can someone please help?

s= 'DEAD';

k = 'ALIVE';

v0=0:0.01:100;
t=0:0.01:3;

for r=1:length(v0);
    v00=v0(r);
    vxx=v0(r)*0.84;
    vyy=v0(r)*0.54;

      pxi=vxx.*t;

      for j = 1:length(pxi);

          if pxi(j)<9;
          fprintf('%s', s);

          elseif pxi(j)>31;
          fprintf('%s', s);

          else
          fprintf('%s', k);

          end;

          j=j+1;

      end;

end;

PLEASE help! I think my approach may be wrong. If anyone can provide some pseducode :/ I suck at matlab!

Answer 1

[Walter Roberson]

Your loop does not go on forever. On the other hand, length(v0) will be over 100/0.01 which is 10000, and those 10000 velocities will each loop over the 301 times, so your inner loop is going to execute over 3 million times.

[Blah Blah]

Is there another way of doing this? ><'

[Walter Roberson]

Create a function that determines the distance for a given velocity. Find the zero of (dist(velocity) - 9)^2 to determine the close end. Find the zero of (dist(velocity) - 31)^2 to determine the further end.

Answer 2

[Jan]

The code should be cleaned up a bit. E.g. the loop counter should not be incremented manually inside the loop:

for j = 1:10
  disp(j)
  j = j + 1;  % *useless* and therefore a waste of time
end

v00 and vyy are never used in your code, so omit them.

Suggestion:

s  = 'DEAD';
k  = 'ALIVE';
v0 = 0:0.01:100;
t  = 0:0.01:3;

for r = 1:length(v0)
  vxx = v0(r) * 0.84;
  pxi = vxx .* t;
  for j = 1:length(pxi)
    if pxi(j) >= 9 && pxi(j) <= 31
      fprintf('%s', k);
    else
      fprintf('%s', s);
    end
  end
end

Or even shorter:

s  = {'DEAD', 'ALIVE'};
v0 = 0:0.01:100;
t  = 0:0.01:3;
for r = 1:length(v0)
  pxi = v0(r) * 0.84 .* t;
  index = (pxi >= 9 && pxi <= 31) + 1;
  fprintf('%s\n', s{index});
end

And a tick faster:

    index = (abs(20 - pxi) <= 11) + 1;

However, you will still have 3 million strings in the command window and I cannot imagine a situation where this is useful.