MolecularWeight.java

Below is the syntax highlighted version of MolecularWeight.java from §4.4 Symbol Tables.

/******************************************************************************
 *  Compilation:  javac MolecularWeight.java
 *  Execution:    java MolecularWeight
 *  Dependencies: ST.java StdIn.java elements.txt
 *
 *  Reads in a list of elements and their molecular weights.
 *  Prompts the user for the molecular description of a chemical
 *  formula and prints out its molecular weight.
 *
 *  % java MolecularWeight
 *  H2.O
 *  Molecular weight of H2.O = 18.020000
 *  N.H4.N.O3
 *  Molecular weight of N.H4.N.O3 = 80.060000
 *
 ******************************************************************************/

public class MolecularWeight {

    public static void main(String[] args) {

        // symbol table whose key = element symbol and value = molecular weight
        ST<String, Double> st = new ST<String, Double>();

        // read in elements symbols and their atomic weights
        In in = new In("elements.csv");
        while (in.hasNextLine()) {
            String line = in.readLine();
            String[] fields = line.split(",");
            String symbol = fields[2];
            double weight = Double.parseDouble(fields[3]);
            st.put(symbol, weight);
        }

        // read in chemical formulas from stdin, and print out molecular weight
        while (StdIn.hasNextLine()) {
            double total = 0.0;
            String line = StdIn.readLine();
            String[] fields = line.split("\\.");   // period is a special regexp char
            for (int i = 0; i < fields.length; i++) {

                // parse Si5 into element and number of atoms
                int j;
                for (j = 0; j < fields[i].length(); j++) {
                    if (Character.isDigit(fields[i].charAt(j))) break;
                }
                String symbol = fields[i].substring(0, j);
                double weight = st.get(symbol);

                // add weight to running total
                String atoms = fields[i].substring(j, fields[i].length());
                if (atoms.length() == 0) total += weight;
                else                     total += Integer.parseInt(atoms) * weight;
            }


            StdOut.printf("Molecular weight of %s = %f\n", line, total);
        }


    }
}