I am trying to write a program to solve a quadratic equation using loops and braces. see the images for the exact problem and my attempt

Here is the exact problem:

Here is my attempt:

The answers I get are: y1=5.750000e+000 and y2=-238. However, I'm not convinced by this because I should be getting more than 2 answers, shouldn't I...? i.e. for=-9:0.5:9 generates a 1x37 array. Many thanks if anyone can spot the problems!

Answer 1

[Andrei Bobrov]

t = -9:.5:9;
n = numel(t);
out = zeros(n,1);
for jj = 1:n
    if t(jj) < 0
        out(jj) =  3*t(jj)^2 + 5;
    else
        out(jj) = -3*t(jj)^2 + 5;
    end
end

or

t = (-9:.5:9)';
out = -3*sign(t).*t.^2 + 5;

[Matthew Lintern]

Cheers for this. I solved the problem in the end and this code (Andrei Bobrov) doesn't look dissimilar to my own, so its good to see I got it in the end. Also thanks for the vectorized form! Cheers for the help.

Answer 2

[Torsten]

You get back y1, evaluated at x=-0.5, and y2, evaluated at x=9.

Do you know why ?

1. Use only one variable (y) instead of y1 and y2.

2. Save the values for t and y in an array. Otherwise (like above), you will overwrite all values previously calculated. Then plot y against t to see whether you succeeded.

Best wishes

Torsten.

[Matthew Lintern]

I have no idea why the program only takes x=-0.5 and x=9. It just doesn't make sense to me....

[Torsten]

Now look at what the program does:

It sets t to -9 first, enters the first if-statement and sets y1 to 3*(-9)^2+5. The second if-statement is false.

In the next step, t is -8.5. Again, the program enters the first if-statement and overwrites the value for y1 obtained for t=-9 by 3*(-8.5)^2+5. The second if-statement is false.

...

In the ...-step, t is -0.5. Again, the program enters the first if-statement and overwrites the value for y1 obtained for t=-1 by 3*(-0.5)^2+5. The second if-statement is false.

In the next step, t=0. The first if-statement is false. Thus y1 keeps the value 3*(-0.5)^2+5. Now the program enters the second if-statement and sets y2 to -3*0^2+5.

In the next step, t=0.5. The first if-statement is false - y1 keeps its value 3*(-0.5)^2+5. The second if-statement is true, thus the value for y2 obtained for t=0 is overwritten by -3*(0.5)^2+5.

...

For t=9, the first if-statement is wrong. y2 keeps its value obtained for t=-0.5. The second if-statement is true. Thus the value for y2 obtained for t=8.5 is overwritten by -3*(8.5)^2+5.

Thus when the program leaves the for-loop, y1=3*(-0.5)^2+5 and y2=-3*9^2+5.

Best wishes

Torsten.