using for loop to solve a function

2019 1 27
1 个回答
更新时间：2019 1 28
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0 个投票

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

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回答（1 个）

madhan ravi
madhan ravi 2019-1-27
编辑：madhan ravi 2019-1-27

0 个投票

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')

11 个评论
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caroline
caroline 2019-1-27
thank you! and appreciate the detail explanation of what each line does
caroline
caroline 2019-1-27
编辑：madhan ravi 2019-1-27
what does numel(x) mean
madhan ravi
madhan ravi 2019-1-27
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
caroline 2019-1-27
ok thanks! i also retested the code i posted early and it spat out a figure this time
caroline
caroline 2019-1-27
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
madhan ravi 2019-1-27
use hold on after the first plot command
caroline
caroline 2019-1-27
编辑：madhan ravi 2019-1-27
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
madhan ravi 2019-1-27
编辑：madhan ravi 2019-1-27
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
caroline 2019-1-27
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
madhan ravi 2019-1-28
编辑：madhan ravi 2019-1-28
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
caroline 2019-1-28
original question was for loops yes

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caroline
caroline
2019-1-27

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caroline
2019-1-28

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