why is my plot not showing anything

clc
k = 8.9870e+009;
q1 = .000000024820;
q2 = .000005200901;
for r = [.10,.15,.20,.25,.30,.40];
    f = (k*q1*q2)/r.^2
end
plot(r,f)
hold on
axis([0 .5 0 .2])

i want to plot f vs r

[Tanmay Varshney]

//Please tell why my graph is not plotting//

v0 = 6.2;

g = 9.81;

theta = 45*(pi/180);

y0 = 1.5;

Vx = v0* cos(theta);

t = linspace(0,2);

x = Vx * t ;

y = y0 + v0 * t * sin(theta) - 0.5 * g * t.^2;

for i=1:size(x,2)

if (i>1 && y(i)<=0)

break;

end

plot(x(i),y(i))

xlabel ('Horizontal Distance(m)')

ylabel ('Vertical Distance(m)')

hold on;

pause(0.05);

end

[Walter Roberson]

plot() does not add lines to points that have already been drawn in previous calls. plot() only draws lines when during a single call there are two or more finite coordinates in a row.

Your plot() call is only drawing one point at a time, not two or more, so it is not going to draw lines.

• you can record all of the output and plot after the calculation loop. This is recommended
• you can leave the plot call but add a marker such as '+' so that it will draw each point
• instead of plot() you could switch to animatedline() and addpoints()

Answer 1

[Oleg Komarov]

You are overwriting f at every iteration and in the end you just have one point.

plot(r,f)

will plot thus one point. If you look carefully you'll find it.

Try this, the vectorized code:

k  = 8.9870e+009;
q1 = .000000024820;
q2 = .000005200901;
r  = [.10,.15,.20,.25,.30,.40];
f  = (k*q1*q2)./r.^2
plot(r,f)
hold on
axis([0 .5 0 .2])

Answer 2

[Thomas]

try

k = 8.9870e+009;

q1 = .000000024820;

q2 = .000005200901;
 r = [.10,.15,.20,.25,.30,.40];

for count=1:numel(r)

f(count) = (k*q1*q2)/r(count).^2  % save f as array

end

plot(r,f)

hold on

axis([0 .5 0 .2])

You were not saving f as array and hence were only plotting one f and one r pair..

You can avoid the loop totally as well using:

 f = (k*q1*q2)/r.^2;
  plot(r,f)

[lowcalorie]

ok thanks

Answer 3

[rajdeep singh]

What is r is a function of random numbers? Like data points

[Walter Roberson]

Oleg's and Thomas's solutions do not care how you create r and do not care whether r is sorted.

The one thing to watch out for if r is not sorted is that because by default plot() connects points that are adjacent in data order, you would get a lot of lines back and forth across the screen. sorting on r prevents that problem.

for idx=1:10 
  r(idx) = rand() ;
  f(idx) = (k*q1*q2)/r(idx).^2;
end 

[sr, sidx] = sort(r) ;
sf = f(sidx);

   plot(sr, sf)