Finding molar mass with MATLAB - arrays

Question

Alrighty, so I have a pretty 'simple' problem on my hands. I am given two inputs for my function: a string that gives the formula of the equation and a structure that contains the information I need and looks like this:
Name
Symbol
AtomicNumber
AtomicWeight
To find the molecular weight, I have to take all of the elements in the formula, find their total mass and add them all together. For example, let's say that I have to find the molecular weight of oxygen. The formula would look like:
H2,O
The molecular weight will thus be
2*(Hydrogen's weight) + (Oxygen's weight), which evaluates to 18.015.
There will always be a comma separating the different elements in a formula. What I am having trouble with right now, is taking the number out of the string(the formula). I feel like I'm over-complicating how I am going about extracting it. If there's a number, I know it can be in positions 2 or 3 (depending on the element name). I tried to use isnumeric, I tried to do some really weird, coding stuff (which you'll see below), but I am having difficulties.
test case:
mass5 = molarMass('C,H2,Br,C,H2,Br', table)
mass5 => 187.862
table:
Name Symbol AtomicNumber AtomicWeight
'Carbon' 'C' 6 12.0110000000000
'Hydrogen' 'H' 1 1.00800000000000
'Nitrogen' 'N' 7 14.0070000000000
'Oxygen' 'O' 8 15.9990000000000
'Phosphorus''P' 15 30.9737619980000
'Sulfur' 'S' 16 32.0600000000000
'Chlorine' 'Cl' 17 35.4500000000000
'Bromine' 'Br' 35 79.9040000000000
'Sodium' 'Na' 11 22.9897692800000
'Magnesium' 'Mg' 12 24.3050000000000
My code so far is:
function[molar_mass] = molarMass(formula, information)
Names = []; %// Creates a Name array
[~,c] = size(information); %Finds the rows and columns of the table
for i = 1:c %Reads through the columns
Molecules = getfield(information(:,i), 'Name'); %Finds the numbers in the 'Name' area
Names = [Names {Molecules}];
end
Symbols = [];
[~, c2] = size(information);
for i = 1:c2 %Reads through the columns
Symbs = getfield(information(:,i), 'Symbol'); %Finds the numbers in the 'Symbol'
Symbols = [Symbols {Symbs}];
end
AN = [];
[~, c3] = size(information);
for i = 1:c3 %Reads through the columns
Atom = getfield(information(:,i), 'AtomicNumber'); %Finds the numbers in the 'AtomicWeight' area
AN = [AN {Atom}];
end
Wt = [information(:).AtomicWeight];
formula_parts = strsplit(formula, ','); % cell array of strings
total_mass = 0;
multi = [];
atoms = [];
Indices = [];
for ipart = 1:length(formula_parts)
part = formula_parts{ipart}; % Takes in the string
isdigit = (part >= '0') & (part <= '9'); % A boolean array
atom = part(~isdigit); % Select all chars that are not digits
Indixes = find(strcmp(Symbols, atom));
Indices = [Indices {Indixes}];
mole = atom;
atoms = [atoms {mole}];
natoms = part(isdigit); % Select all chars that are digits
% Convert natoms string to numbers, default to 1 if missing
if length(natoms) == 0
natoms = '1';
multi = [multi {natoms}];
else
natoms = num2str(natoms);
multi = [multi {natoms}];
end
end
multi = char(multi);
multi = str2num(multi); %Creates a number array with my multipliers
f=56;
Molecule_Wt = Wt{Indices};
duck = 62;
total_mass = total_mass + atom_weight * multi;
end
Thanks to Bas Swinckels I can now extract the numbers from the formulas, but what I'm struggling with now is how to pull out the weights associated with the symbols. I created my own weight_chart, but strcmp won't work there. Neither will strfind or strmatch. What I want to do is find the formulas in my input, in the chart. Then index it from that index, to the column (so add 1 I believe). How do I find the indices though? I'd prefer to find them in the order the strings appear in my input, since I can then apply my 'multi' array to it.
Any help/suggestions would be appreciated :)

Answer 1

Given the string, you can pull out the part that is a digit character with the isstrprop function. Then use that to address your string to get just those characters, then cast that as a double with str2double.
PartialString = 'H12';
Subscript = str2double (PartialString (isstrprop (PartialString, 'digit')));

Answer 2

This should get you started, there is still some parts that need to be filled in:
formula_parts = strsplit(formula, ','); % cell array of strings
total_mass = 0;
for ipart = 1:length(formula_parts)
part = formula_parts{ipart}; % string like 'H2'
isdigit = isstrprop(part, 'digit'); % boolean array
atom = part(~isdigit); % select all chars that are not digits
natoms = part(isdigit); % select all chars that are digits
% convert natoms string to int, default to 1 if missing
if length(natoms) == 0
natoms = 1;
else
natoms = num2str(natoms);
end
% calculate weight
atom_weight = lookup_weight(atom); % somehow look up value in table
total_mass = total_mass + atom_weight * natoms;
end
See this old question about how to extract letters or digits from a string.