What is wrong with my if statement?

Hello all,

I'm not sure how to phrase my if statement so that when the value of y at the point where x is equal to dto, the statement will run.

Currently it stops at the beginning of the if statement so i know that thats where the problem lies.

Below is my code for the relevant parts:

if max(x)>=Ox %&& (y(x==max(x))<=Oy)
                   dto = find(x < Ox,1,'last')
                   tto = dto/v0x
                   if y(x==dto)<=Oy %%this is where it stops

                   xlim([0, Ox+Ot+0.5])
                   ylim([0, max(y)+0.5])
                   hold on
                   
                   for i=1:length(tto)
                       plot(x(i),y(i),'ko')
                       pause(0.05)
                   end 
                   elseif y(x==dto)>Oy
                        for i=1:length(tFinal)
                       plot(x(i),y(i),'ko')
                       pause(0.05)
                        end
                   end 
               end

Thanks in advance! :3

[Diaa]

Can you put some values of x, y, ox, ot and oy that reproduce your problem?

[Ryan Fedeli]

^ This would be helpful.

Also I want to add that since you're using the find function, dto is defined as an index, so it's an integer. If x does not contain integers, this would be one reason why it never runs

[Anon]

so the x:

x = linspace(0,11.8817,100)

this was calculated using initial speed and time of flight

the y:

y0 = 2
v0y = 5.7358
t = linspace(0,1.4505,100) %these values are used to calculate the y value below:
g = -9.81
y = y0 + v0y.*t+0.5*g*t.^2 %this is the y value plotted

Ox = 7

Oy = 5

Ot = 0.5

[Anon]

ah okay i didnt know about the find function only working for integers - what function allows me to find any value in an array

[Daniel Pollard]

You've used logical indexing to get y where x=dto:

if y(x==dto)<=Oy %%this is where it stops

x is an array of values from 0 to 11.8817, and when I print dto, it returns 59, so the if statement isn't satisfied. So it goes to the elseif, where you say

for i=1:length(tFinal)

where, at least in the code you've given us, tFinal isn't defined. So it does nothing.

[Ryan Fedeli]

Well the find function works for any type of array. It returns the index (an integer number) of the desired value. So in the array:

p = [2.0 4.2 6.0 8.4 10.0];
index = find(p==6.0)

find will return 3. 3 is the place in the array p where p = 6.0.

So to find the value at the index given by find, all you need is

value = p(index)

[Anon]

the tFinal value is the last number in:

t = linspace(0,1.4505,100)

so it would be 1.4505s for tFinal

[Anon]

Okay that make sense Ryan thank you! I'll try to rewrite that bit

[Anon]

I have changed the value for the dto function thank you Ryan so now i ca get past the If statement howvever i only have two points plotted with this for loop:

                for i=1:length(tto)
                       plot(x(i),y(i),'ko')
                       pause(0.05)
                       disp('here now')
                   end