How can I know what element a value came from in a vector?

I just started using Matlab. I found my minimum value of a given vector, but I need to know what the corresponding value is in another vector. Is there a way to know where the exact element is?

I need to know my "r" value for the cheapest.

    v=600;
    r=[1:.1:100];
    h=(v-2.*pi.*((r.^3)./3))./(pi.*r.^2);
    cost=(400.*(2.*pi.*r.*h))+(600.*(2.*pi.*r.^2));
    cheapest=min(cost)  
   cheapest =
   1.4825e+005

Answer 1

[Walter Roberson]

[cheapest, cheapidx] = min(cost);

Answer 2

[Oleg Komarov]

Use the second output of min:

[cheapest, pos] = min(cost); 
r(pos)

Oleg

Answer 3

[Paulo Silva]

    v=600;
    r=[1:.1:100];
    h=(v-2.*pi.*((r.^3)./3))./(pi.*r.^2);
    cost=(400.*(2.*pi.*r.*h))+(600.*(2.*pi.*r.^2));
    cheapest=min(cost); 
    Minr=r(cost==cheapest) %The r value that minimizes the cost function
    %check if the value is correct on a figure
    clf
    hold on
    plot(r,cost) %draw the cost function
    plot(Minr,cheapest,'rx') %mark the minimum value
    axis([0 10 0 5e5])
    grid on