Asked by Edward Lisa
on 18 May 2018 at 14:10

Hi! I just started my Matlab course and i have run into a little problem which i hope you can help me with. I am suppose to write a function which integrates a function e^(-t^2) from 0 to some x which is a variable in the function. I did that like this:

function y = simpson_method(x)

y1 = exp(-0.^2); y2 = exp(-(x).^2);

c = 0; d = 0; incr = 0.1;

for i = 1 : (x - 0)/incr - 0.5

y = exp(-(0 + i*incr).^2);

if i/2 == floor(i/2)

d = d + y; continue else c = c + y; continue end end

y = incr/3 * (y1 + 4*c + 2*d + y2);

The function integrates correctly and gives me a desired output for every x. But the problem is when i try to plot it:

x = 0:0.1:15; y = simpson_method(x); plot(x,y)

It gives me the wrong graph which i find weird because all the outputs are correct. I get this which is nonsense. Whats happening?

Answer by KALYAN ACHARJYA
on 21 May 2018 at 13:02

Edited by KALYAN ACHARJYA
on 21 May 2018 at 13:10

Shown Graph the x ranges are different

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## 5 Comments

## Adam (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401422-why-is-my-matlab-plot-wrong#comment_569371

What is the right graph?

And what are the values of y that you get out?

## Edward Lisa (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401422-why-is-my-matlab-plot-wrong#comment_569387

This is the right plot and my function outputs y as very close to 0 when x=0 and reaches 0.884.. when x goes larger which is exactly the plot of the function when integrated on https://www.integral-calculator.com/ with input e^(-x^2) from 0 to your chosen upper limit.

## Stephen Cobeldick (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401422-why-is-my-matlab-plot-wrong#comment_569391

@Edward Lisa: can you please confirm the ambiguous syntax:

## Edward Lisa (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401422-why-is-my-matlab-plot-wrong#comment_569400

Its e^(-(t^2))

## Adam (view profile)

Direct link to this comment:https://www.mathworks.com/matlabcentral/answers/401422-why-is-my-matlab-plot-wrong#comment_570199

If your y values are correct I can see no reason at all why plot would not give the correct answer so your y values cannot be correct if you are literally just calling plot.

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