I keep getting ??? Error using ==> plus Matrix dimensions must agree. Error in ==> dewpoint at 6 f(T,RH)=((​a*T)./(b+T​))+log(RH/​100)

T=[20 25 30 35];
RH=30:10:100; %relative humidity percentages
[Td] = dewpoint(T,RH)

the function file is...

function [Td] = dewpoint(T,RH)
   %dewpoint at varius temps and humidities
   %   Detailed explanation goes here
   a=17.27;
   b=237.7; % Celcius
   f(T,RH)=((a*T)./(b+T))+log(RH/100);
   Td=(b*f(T,RH))/(a-f(T,RH));

Answer 1

[Jacob Ward]

Your T has 4 elements (1x4) while your RH has 8 (1x8). When you try to add them together in your function, MATLAB doesn't know how to add these two matrices because they are not the same size.

Try changing the line defining RH to RH = 30:20:90 so that it has 4 elements like T and your problem should go away.

[Michele Thompson]

I'm supposed to evaluate this function at intervals of 10, so I can't change the RH input. Could I use a for loop instead?

Answer 2

[Walter Roberson]

Vectorize.

function [Td] = dewpoint(T,RH)
  %dewpoint at varius temps and humidities
  %   Detailed explanation goes here
  a = 17.27;
  b = 237.7; % Celcius
  [gT, gRH] = ndgrid(T, RH);
  f = ((a*gT)./(b+gT))+log(gRH/100);
  Td = (b*f) ./ (a-f);