How do I plot points coming from a for loop without using vectors?

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Salvatore Manfredi D'Angelo
评论： Salvatore Manfredi D'Angelo 2021-7-22
Hi, I am new to Matlab so excuse my ignorance. I am trying to make a code that evaluates a definite integral from 0 to infinity for different values of two parameters, which I called v,L in the script below. Then I want to plot these definite integrals versus values of, say, L, which increases by one in each cycle. Why aren't values of L on the horizontal axis equally spaced as they should be? Is there something wrong with the plot function? If so, is there a way to plot points coming from a for loop as they get out, without using vectors? Thanks in advance.
syms x;
double L;
double v;
f=L*exp(-v*x^2);
L=0;
v=1;
for i=0:50
L=L+i;
k=int(f,x,0,inf);
plot(L,k,'o');
hold on
end
hold off
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Salvatore Manfredi D'Angelo
Salvatore Manfredi D'Angelo 2021-7-22
Hi, thanks for your reply! I was actually testing out the code before letting double L take on decimal values on an interval
Konrad
Konrad 2021-7-22
I think what Stephen refers to is that
double L; % = double('L')
doesn't declare a variable (as in other languages), but type-casts the character 'L' to type double, which is simply the number 76.

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Konrad
Konrad 2021-7-22
Hi,
values on the x-axis are not equally spaced because on every interation you increase L by i, which itself is increased by 1 on every interation:
1st interation: L = 0+0 = 0
2nd L = 0+1 = 1
3rd L = 1+2 = 3
4th L = 3+3 = 6
...
but you can just use i as your x-parameter for the plot function:
plot(i,k,'o');
> "is there a way to plot points coming from a for loop as they get out, without using vectors?"
you allready do that using hold on
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Konrad
Konrad 2021-7-22
I'm not really familiar with the symbolic math toolbox and I don't know int() for integrals, sorry.
But here would be a numeric solution:
ffactory = @(L,v)@(x)L*exp(-v*x.^2); % function factory returning f as an anonymous function
v = 1;
for i = 0:50
f = ffactory(i,v);
k = integral(f,0,inf);
plot(i,k,'o');
hold on;
end
Salvatore Manfredi D'Angelo
Salvatore Manfredi D'Angelo 2021-7-22
Thank you! This solved my issue. integral() is numeric and int() is symbolic then, I will keep in mind

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