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

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Salvatore Manfredi D'Angelo 2021-7-22

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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 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 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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Answer 1

Konrad 2021-7-22

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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 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