using for loop to solve a function

Given the following function ??(??) = 20000 * log(x) − 3x Use a for loop to compute f(x) for 1 ≤ x ≤ 20.

my solution

x = linspace(0,20,20);

for i=1:20
         x2 = x;
         y2 = 20000*log(x)-3*x;
         plot(x2,y2,'r')

am i on the right track plot is blank

Answer 1

[madhan ravi]

I know it is your homwork and appreciate that you have tried something.

I suggest you to do Matlab Onramp course which is interactive and fun to learn MATLAB.

https://www.mathworks.com/help/matlab/matlab_prog/loop-control-statements.html

Loop version:

x = linspace(0,20,20);
              %   ^^------ increase to get a smoother curve    
y2=zeros(size(x)); % preallocate
for k=1:numel(x)
%          x2 = x; ?? does nothing 
y2(k) = 20000*log(x(k))-3*x(k);
% ^^^------ save y2 in each iteration
end
plot(x,y2,'r') % plot outside the loop

Matlab's way (vectorization):

x = linspace(0,20,20); % just three lines of code
y2 = 20000*log(x)-3*x;
plot(x,y2,'r')

[caroline]

thank you! and appreciate the detail explanation of what each line does

[caroline]

what does numel(x) mean

[madhan ravi]

numel(x) gives number of elements in x ,

The best part in MATLAB is whenever you have a doubt just type

help numel % will give you detailed description of the command
doc numel

[caroline]

ok thanks! i also retested the code i posted early and it spat out a figure this time

[caroline]

so i have a while loop and a for loop do u happen to know how i can plot this for loop and while loop plot on the same graph?

[madhan ravi]

use hold on after the first plot command

[caroline]

x = linspace(0,20,100);
j = 0;
while (j<=20)
    y = x.^3-(5.*x).^2+2.^x-(10000.*x);
    j = j+1;
   end
f= plot(x,y,'--r')
hold on 

x1 = linspace(1,20,100);
y2 = zeros(size(x1));
for k=1:numel(x1)
         %x2 = x;
         y2(k) = 20000*log(x1(k))-3*x1(k);
  end 
f = plot(x1,y2,'b')

when i plot both it looks wrong but seperately they look great" image attached"

[madhan ravi]

I got two questions:

1) The original question you asked was about for loops ?

2) Why do you say it’s wrong ? Each graph represents different function.

[caroline]

i just thought it was wrong because when plotted on calc it looked wrong. im not sure if you saw the picture but the graph in blue looks different when graphed seperately

[madhan ravi]

You didn't answer my first question.Plus I don't see any differences in the figure? not sure what do you mean by "i just thought it was wrong because when plotted on calc it looked wrong. " Did you verify the scale?

[caroline]

original question was for loops yes