Displaying and Printing in AMPL

  1. How many decimal places?
  2. Displaying Information
  3. Printing Information
  4. Printing to a File

How many decimal places?

When solving mathematical programming problems, the accuracy of your solutions should never be more than the accuracy of your data. You can set the precision that AMPL displays values via the display_precision option. For example, if your inputs are specified to 4 significant digits, e.g., 0.013, you can use

option display_precision 4;
to get your solution values to the same number of significant digits.

four_dp.jpg

Note that AMPL still keeps its numbers at full precision, so you need to be careful with your objective function values.

five_dp.jpg

When we change the display_precision to 5 significant digits, the displayed TotalCost and the cost calculated by using the displayed Amount values differ. If you are using rounded solution values, make sure to check the objective value before quoting your solution.

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Displaying Information

You have already seen how to display a variable using the {\tt display} command. We can also display {a href="Expressioins in AMPL">AMPL expressions the same way, e.g., we might want to see how supply we are using in a transportation problem.

Often when we display something (like variable values) many of the resulting numbers are 0 and we are only interested in the non-zero numbers. To stop any rows of zeros being displayed you can set the {\tt omit_zero_rows} option:

\begin{verbatim} option omit_zero_rows 1; \end{verbatim}

To stop any columns of zeros being displayed you can set the {\tt omit_zero_cols} option:

\begin{verbatim} option omit_zero_cols 1; \end{verbatim}

You can also force {\tt display} to use either tables or a single column by using the {\tt display_1col} option. This option will use one column if the number of values to display is less than {\tt display_1col}. The initial value of {\tt display_1col} is 20, so any {\tt display} command that shows less than 20 values will be displayed as a column. Setting {\tt display_1col} to 0 forces {\tt display} to use tables whenever possible.

Printing Information

By playing with the {\tt display} options we can get the {\tt display} command to format output in a nice way. However, we can also decide exactly what is displayed by using {\tt print} and {\tt printf}. <p

The {\tt print} command only writes strings to the output.

The {\tt printf} command allows you to print text and values together in a format you can control. It uses the same {\tt printf} format as C and Matlab.

You can print over sets or set expressions as well

Printing to a File

All the output commands can be directed to a file. Adding {\tt > <filename>} to the end of an output command creates the file with the given name and writes to it. Subsequent output commands append output to the file by adding {\tt >> <filename>} to the commands. You should close your file when done so you can open it with another program. This is very useful for saving your solutions (in a useful format with {\tt printf}), for example

brewery.run

\begin{verbatim} # brewery.run reset;

model transportation.mod; data brewery.dat;

option solver cplex;

solve;

print 'TRANSPORTATION SOLUTION -- Non-zero shipments' > brewery.out;

display TotalCost >> brewery.out;

printf {s in SUPPLY_NODES, d in DEMAND_NODES : Flow[s, d] > 0} 'Ship %d crates of beer from warehouse %s to pub %s\n', Flow[s, d], s, d >> brewery.out;

close brewery.out; \end{verbatim}

Running {\tt brewery.run} in AMPL creates a file brewery.out.

-- MichaelOSullivan - 02 Mar 2008

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