PrintingInAMPL < OpsRes

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---+<a name="top"></a> Displaying and Printing in AMPL 

   1 [[#dp][How many decimal places?]]
   1 [[#display][Displaying Information]]
   1 [[#print][Printing Information]]
   1 [[#file][Printing to a File]]

---++<a name="dp"></a>How many decimal places?

When solving mathematical programming problems, the accuracy of your solutions should *%RED%never%ENDCOLOR%* 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
<pre>
option display_precision 4;
</pre>
to get your solution values to the same number of significant digits.

<img src="%ATTACHURLPATH%/four_dp.jpg" alt="four_dp.jpg" width='669' height='290' />
 
Note that AMPL still keeps its numbers at full precision, so you need to be careful with your objective function values.

<img src="%ATTACHURLPATH%/five_dp.jpg" alt="five_dp.jpg" width='669' height='290' />
 
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 *%RED%check the objective value%ENDCOLOR%* before quoting your solution.

---++<a name="display"></a> Displaying Information

You have already seen how to display a variable using the =display= command. We can also display [[ExpressionsInAMPL][AMPL expressions]] the same way, e.g., we might want to see how supply we are using in a transportation problem.

<img src="%ATTACHURLPATH%/display.jpg" alt="display.jpg" width='669' height='386' />
 
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:
<pre>
option omit_zero_rows 1;
</pre>
To stop any columns of zeros being displayed you can set the =omit_zero_cols= option:
<pre>
option omit_zero_cols 1;
</pre>

<img src="%ATTACHURLPATH%/omit.jpg" alt="omit.jpg" width='669' height='242' />

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.

<img src="%ATTACHURLPATH%/display_1col.jpg" alt="display_1col.jpg" width='669' height='302' />

---++<a name="print"></a> 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=.

<img src="%ATTACHURLPATH%/print.jpg" alt="print.jpg" width='669' height='122' />

The =print= command only writes strings to the output.
 
The =printf= command allows you to print text and values together in a format you can control. It uses the same [[https://www.cppreference.com/stdio/printf.html][<tt>printf</tt> format]] as C and Matlab.

<img src="%ATTACHURLPATH%/printf.jpg" alt="printf.jpg" width='669' height='218' />
 
You can print over [[SetsInAMPL][sets]] or [[SetsInAMPL#expressions][set expressions]] as well

<img src="%ATTACHURLPATH%/printf_set.jpg" alt="printf_set.jpg" width='669' height='218' />

---++<a name="file"></a> Printing to a File

All the output commands can be directed to a file. Adding =&gt; &lt;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 =&gt;&gt; &lt;filename&gt;= 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
<pre>
# 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;
</pre>

Running =brewery.run= in AMPL creates a file =brewery.out=:
<pre>
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
</pre>

-- Main.MichaelOSullivan - 02 Mar 2008
This topic: OpsRes > WebHome > AMPLGuide > AMPLSyntax > PrintingInAMPL
Topic revision: r6 - 2008-03-04 - MichaelOSullivan