<-- Ready to Review -->

TRANSPORTATION SOLUTION -- Non-zero shipments
TotalCost = 8600

Displaying Information

src="display.jpg"

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 omit_zero_rows option:

src="omit.jpg"

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

src="display_1col.jpg"

Printing Information

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

Printing to a File

All the output commands can be directed to a file. Adding > <filename&gt 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 >> <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 printf), for example

• display.jpg:
  

• omit.jpg:
  

• display_1col.jpg:
  

• print.jpg:
  

• printf.jpg:
  

• printf_set.jpg:
  

• brewery.run: brewery.run

How many decimal places?

??? Up to here ???

• four_dp.jpg:
  

• five_dp.jpg:
  

<-- Under Construction -->

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?

??? Up to here ??? 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 {\tt display_precision} option. For example, if your inputs are specified to 4 significant digits, e.g., {\tt 0.013}, you can use

\begin{verbatim} option display_precision 4; \end{verbatim}

to get your solution values to the same number of significant digits.

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

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

Displaying Information

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 {\tt brewery.out}.

brewery.out

\begin{verbatim} TRANSPORTATION SOLUTION -- Non-zero shipments TotalCost = 8600

Ship 300 crates of beer from warehouse A to pub 1 Ship 700 crates of beer from warehouse A to pub 5 Ship 200 crates of beer from warehouse B to pub 1 Ship 900 crates of beer from warehouse B to pub 2 Ship 1800 crates of beer from warehouse B to pub 3 Ship 200 crates of beer from warehouse B to pub 4 \end{verbatim}

Displaying and Printing in AMPL

-- MichaelOSullivan - 02 Mar 2008