using if/else statements to find molecular weight of compounds given the info from a string

I'm sure there are more elegant ways of doing this, but does anyone have any suggestions to make this work? Any compound using the atoms C,H,O,S,N should be calculable. Full disclosure: I'm doing this for my intro programming class, but we are encouraged to seek help.

Here's where I am code wise:

    % Calculate the mol weight of given compounds given the formula as a string
    function output = molweight(str)
    %calculates the molecular weight of a coumpound given its atomic structure
    %in a string

if isempty(str)
    error('Invalid use of molweight');
end
str = upper(str) %convert to uppercase
%Identify characters in the string

k=input('What compund do you want to check?')
i=sscanf(k, '%s',1)
if (i == C) 
    i=scanf(k,'*%s %f')
    if isempty(i)
        mass=12.0107
    else
    mass=12.0107*i
    end
    %multiply carbon value by the number following

elseif (i==H)
    i=scanf(k,'*%s%f')
    if isempty(i)
        mass=1.0079
    else
    mass=1.0079*i
    end

      %If H is found 
  elseif (k==O)
       i=scanf(k,'*%s%f')
      if isempty(i)
          mass=15.999
      else
      mass=15.999*i
      %O
  elseif (k == N)
        i=scanf(k,'*%s%f')
      if isempty(i)
          mass=14.007
      else
      mass=14.007*i
      %N
  elseif (k == S)
         i=scanf(k,'*%s%f')
      if isempty(i)
          mass=32.065
      else
      mass=32.065*i
      %S
  else
      disp('Invalid use of molwt')
  end

I know this will only calculate the first letter of the string, but was thinking of going through systematically and calculating each letter after that. Cumbersome! There must be a better way... Thanks for any help!

[Richard Zapor]

The word it's is "it is" and is not possessive of it. Please remember this rule and correct your comment line of "it's atomic structure" to "its atomic structure". The word its is possessive and there is no such thing as its'.

[Mary]

For shame...you are right. Thanks for noticing that.

Answer 1

[Richard Zapor]

The related Cody Challenge was solved by Alfonso in one line using regexprep, strread, and str2num. To see his solution solve the Challenge using the above and then any other Cody Challenge solved will allow user to see all solutions of previously solved challenges. Your Prof is guaranteed to be flummoxed by the Alfonso solution.

[Mary]

He'll appreciate it, I'm sure. Thank you very much! Now I'll dissect each solution.

Answer 2

[Richard Zapor]

I created a Cody Challenge Molecular Wt based on the question. Watch the Challenge as significant improvements are expected.

My initial answer to the processing is:

function mass=atomic_wt(eqn)
 wtmap=zeros(83,1);
 wtmap( 'CHONS'/1 )= [12 1 16 14 32];
 mass=0;
 ELE=regexp(eqn,'[CHONS]'); % Locate all elements

   for i=1:length(ELE)
    wt=wtmap(eqn(ELE(i)));
    if i==length(ELE) 
     if ELE(end)==length(eqn) % single atom  CH3OH  methanol
      mass=mass+wt;
     else % Case of final numeric  CH4
      qty=str2num(eqn(ELE(i)+1:end));
      mass=mass+wt*qty;
     end
    elseif ELE(i)+1==ELE(i+1) % single atom intra eqn
     mass=mass+wt;
    else % Element followed by a qty  allow >9
     qty=str2num(eqn(ELE(i)+1:ELE(i+1)-1));
     mass=mass+wt*qty;
    end
   end

end

Answer 3

[Walter Roberson]

hint:

table1 = {'C', 'H', 'O', 'N', 'S'};
table2 = [12.0107, 1.0079, 15.999, 14.007, 32.065];

for K = 1 : length(table1);
  fprintf('%2s %g\n', table1{K}, table2(K));
end

[Mary]

Thank you!