Help with quadriatic formula?

Hi everyone,

We are to create a function using the editor to solve a quadratic formula. The hard part is the user is to be able to input just the equation ie: Ax^2+Bx+C=0 (not the values of ABC). (Below is what I have so far)

____***_*%The user input the formula they want solved_____

    function [xpos, xneg] = Roots(equation)

    %Quadratic equation format is = Ax^2 + Bx + C = 0

    A = strtok(equation, 'x');
    B = 
    C = 

    %This is the discriminant

    D = B .^2 - 4 .* A .* C;

    %These are the equation to find the positive and negative values of x

    xpos = (-B + sqrt(D)) ./ (2 .* A)

    xneg = (-B - sqrt(D)) ./ (2 .* A)

    end

[Matt Kindig]

To clarify, your input is something like this following, correct?

equation = '3x^2 + 5x + 12=0'

[Daniel Shub]

There are lots of parsers for quadratic equations based on regular expressions. This might be easier than using strtok

[dpb]

Oh, indeed! The OPs instructor is, however, apparently not amenable to alternate solutions than those suggested based on his comments later on.

Answer 1

[dpb]

You showed enough I'll do more than normally would on first response...

s=inputdlg('Enter quadratic: ','Quadratic Solver Input',1,{'ax^2+bx+c'});

[MAB]

Thanks for the help. Now, I am able to extract the first number usinf the strtok, but how do I get the the other 2 numbers?

[dpb]

How carefully are you controlling the format of the input the user provides? If the structure of the equation can be assumed to match a given format it's possible to simply do something like

>> s='3x^2+5x+1';
>> c=sscanf(s,'%f x^2 + %f x + %f')'
c =
   3     5     1
>>

If you need to or want to use strtok() to parse substrings, simply use the optional "remainder" output and a while loop to return subsequent tokens until it becomes the empty string.

[t,r]=strtok(s);
% process token appropriately here
while ~isempty(r)
  [t,r]=strtok(s);
  % process the next token appropriately here
end

[MAB]

I was able to talk to the instructor today and he gave a little more clarification for the problem.

he says the user is going to enter the string as follows as a string:

'4x^2+2x+3=0'

Then we are to use strtok() to pull the values for A B & C.

ie A = strtok(s , 'x') will pull out the 4 from the string. The above strtok example is the only way he has showed us and he normally doesn't want us to deviate far from that, but I do not see how using it in that format I can pull out B and C? Im sure I could use the [t,r]=strtok(s,...) but he says we are not allowed to use conditional statements like while and for loops.

[dpb]

That's just evil imo... :)

I don't see any good way to actually parse any of the other numeric values after the first with strtok() only--it just doesn't parse regularly based on the rules strtok() uses.

You could do a two-step process of first separating the terms based on defining the delimiter string as '+=' as the first pass then delimiter "x" on that token for a and b. But c is still an oddball that then parses on the first pass owing to using += for the primary delimiter.

Sotoo,

delim='+=';
[a,r]=strtok(s,delim);  a=strtok(a,'x');
[b,r]=strtok(r,delim);  b=strtok(b,'x');
c=strtok(r,delim);

Of course, all are still strings, not numeric; you still need a str2num() around each or sscanf() or somesuch to get the actual numeric value.

All in all, I'm not impressed by the instructions received...

[MAB]

Below is the code I got but like you said I still have to use str2num() to get actually values for my ABC.

%  The user input the formula they want solved
function [xpos, xneg] = Roots(s)

%Quadratic equation format is = Ax^2 + Bx + C = 0
M = strtok(s,'x');
[a,b] = strtok(s, '+-');
T = strtok(b,'x')
[c,d]=strtok(b, '+-');
H = strtok(d, '=');
A = str2num(M)
B = str2num(T)
C = str2num(H)

%This is the discriminant 
D = ((B) .^2) - (4 .* A .* C)

%These are the equation to find the positive and negative values of x
xpos = (-B + sqrt(D)) ./ (2 .* A)

xneg = (-B - sqrt(D)) ./ (2 .* A)

end

This code works by the way. The str2num is what I was needing to change my string values to numeric values.

[dpb]

Well, it'll be interesting to hear if the instructor actually can come up with a realistic parser under the given rules himself...keep us posted.

[MAB]

OK, Im back again because I realized that if there is no number in front the x, my function will not work. So I am looking for a way to use the strtok without using an if statement to determine that there is a 1 whether it be negative or positive as the coefficient.

ie: -x^2 - x +10 = 0

I want to be able to pull out a -1 for A and B.

[dpb]

The form doesn't fit the required input of an explicit constant.

Only way to deal with it will be to special-case it.