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?

??? 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
<p>\begin{verbatim}
option display_precision 4;
\end{verbatim}<p>
to get your solution values to the same number of significant digits.
<p><img src="four_dp.jpg" /><p>
 
Note that AMPL still keeps its numbers at full precision, so you need to be careful with your objective function values.
<p><img src="five_dp.jpg" /><p>
 
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.

<a name="display"><h3>Displaying Information</h3></a>
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</a> the same way, e.g., we might want to see how supply we are using in a transportation problem.
<p><img src="display.jpg" /><p>
 
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:
<p>\begin{verbatim}
option omit_zero_rows 1;
\end{verbatim}<p>
To stop any columns of zeros being displayed you can set the {\tt omit_zero_cols} option:
<p>\begin{verbatim}
option omit_zero_cols 1;
\end{verbatim}<p>
<p><img src="omit.jpg" /><p>
 
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.
<p><img src="display_1col.jpg" /><p>

<a name="print"><h3>Printing Information</h3></a>
 
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<img src="print.jpg" /><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 <a href="https://www.cppreference.com/stdio/printf.html">{\tt printf} format</a> as C and Matlab.
<p><img src="printf.jpg" /> <p>
 
You can print over <a href="Sets in AMPL">sets</a> or <a href="Sets in AMPL">set expressions</a> as well
<p><img src="printf_set.jpg" /><p>

<a name="file"><h3>Printing to a File</h3></a>

All the output commands can be directed to a file. Adding {\tt &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 {\tt &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 {\tt printf}), for example
<h4>brewery.run</h4>
<p>\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}<p>

Running {\tt brewery.run} in AMPL creates a file {\tt brewery.out}.
<h4>brewery.out</h4>
<p>\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}<p>

-- Main.MichaelOSullivan - 02 Mar 2008
This topic: OpsRes > WebHome > AMPLGuide > AMPLSyntax > PrintingInAMPL
Topic revision: r2 - 2008-03-02 - MichaelOSullivan