Convert a string containing a fraction into a number in a fraction format

I need to convert a string containing a fraction (for example, x='4/5') into a number. Every time I do this, the fraction is evaluated and the answer is always x=.8, but I need the number to stay as a fraction. I can't use the rats function, because I need to be a number (I'm multiplying it with another number). Is there a way I can keep x in a fraction format--or convert it from a decimal to a fraction if that's not possible?

Answer 1

[Akira Agata]

if your string is always '(numbers)/(numbers)' format, the numerator and denominator of the fraction can be extracted by using regexp function. Here is an example.

str = '423/523';
c = regexp(str,'[0-9]+','match');
numerator = str2double(c{1});
denominator = str2double(c{2});

Answer 2

[Stephen23]

Much simpler and faster is to use sscanf and simple two-element numeric vectors to represent the fractions:

>> x = sscanf('6/4','%d/%d')
x =
   6
   4
>> y = x .* [5;1] % x * 5/1
y =
   30
    4
>> z = x .* y %  6/4 * 30/4
z =
   180
    16

Finding the lowest terms fraction is easy using gcd (compare Steven Lord's answer):

>> z / gcd(z(1),z(2))
ans =
   45
    4

As is getting the (possibly approximately) equivalent decimal fraction:

>> z(1)/z(2)
ans =  11.250

Thus you get to trivially avoid the pointless complexity and slowness of using symbolic toolbox, and can use basic mathematical operations on the numerator or denominator, or both together.

Answer 3

[Steven Lord]

You could convert it into a symbolic variable. However be aware that the fraction contained in the symbolic variable may be equivalent but not identical to the fraction written in the string, as the x variable created in the example below shows.

>> s = '6/4';
>> x = sym(s)
x =
3/2

>> y = 5*x
y =
15/2

>> z = x*y
z =
45/4