The AMPL Process

Starting AMPL

AMPL is a command line environment, i.e., you enter commands at the AMPL prompt and AMPL will decipher these commands and perform the appropriate actions.

To start AMPL in Windows you simply start a command line application and type ampl.

ampl.jpg

You will then be able to enter AMPL commands at the AMPL prompt.

ampl_examples.jpg

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AMPL Commands

AMPL commands consist of statements in the AMPL syntax terminated with a ;.

Some examples include:

set INGREDIENTS; 

data; 

display {i in INGREDIENTS} Percentage[i]; 

If you forget the ;, then the ampl: prompt will be replaced by the ampl? prompt, indicating that your AMPL command has not been finished. You can finish your command or, if you are not sure how to finish it, type ;. AMPL will then interpret your AMPL command and display any problems.

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The Model Environment

AMPL encourages a clean modelling style by explicitly separating the model formulation from the particular problem data. The model formulation, as written in the AMPL syntax, can be saved as a .mod file. This file contains all set, variable, and parameter definitions, as well as all constraints and the objective function.

It should not contain any problem specific data or values relating to the actual real world problem you are solving.

The AMPL model is input after switching to the (default) model environment, by typing model; and then using the include command to import the model file, or else by typing in the commands for the AMPL model directly at the ampl: prompt.

model_include.jpg

Alternatively, you can type model .mod;.

model_file.jpg

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The Data Environment

All data and values specific to the problem instance you are solving are separated from the model formulation when using AMPL. This is enforced by the method used to input the problem details into AMPL - the model formulation is entered in the model environment, the problem specific data is entered in the data environment. To switch from the model environment to the data environment one types data; at the AMPL prompt ampl: (and switches back to the model environment by typing model;).

model_data.jpg

As with entering the model formulation, it is common to store the data for a specific problem in a data file (.dat). This data is then imported while in data mode by using the include command, followed by the filename and a semicolon (or by typing in the AMPL commands in directly).

data_include.jpg

Again, one can use the alternative method of typing data .dat (which does not require a switch from the model environment).

data_file.jpg

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Scripting in AMPL

A script is a text file used to run a number of AMPL commands. It is useful when you may wish to run the same commands multiple times, possibly with slight editing changes. It is also useful for keeping track of what you have done, or if you may wish to rerun your models at a later date. Script files run in the model environment, and are usually named with a .run extension. It is good practice to get into the habit of using script files for your AMPL sessions. A typical script file should begin:

reset; model .mod; data .dat; . . . 
and you run this file using the include command.

script_file.jpg

Setting Data Dynamically

AMPL allows you to set data "on the fly" using the let keyword. If you have a set, parameter or variable as part of your model and have defined it using a data file (i.e., you did not use an expression to set a value for it), then you can change that structure on the fly with let. For example,

reset;  
model;  
param a; 
param b; 
param maxab := max(a, b);  
data;  
param a := 1; 
param b := 2;  # Sets maxab = 2  
display a, b, maxab; # Shifts AMPL back to the model environment  
let a := 3;    # Sets a and maxab = 3  
display a, b, maxab;  
let maxab := 4; # Syntax error, parameter defined by an expression  display a, b, maxab; 

dynamic_error.jpg

The previous example also showed how to use both the model and data environments in the same script file. Furthermore, rather than using model; to enter the model environment (and leave the data environment), it jumps from the data environment to the model environment by using a command that is not valid in the data environment (the display command).

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Notes on AMPL Files

Editing AMPL files in Windows

The easiest way to edit AMPL files is using a text editor such as Wordpad. To open Wordpad go to the start menu, select Programs, then Accessories and finally Wordpad.

run_wordpad.jpg

When you have finished editing, save the file in the appropriate directory. Be sure to save your file as a basic text file, NOT in Rich Text Format.

save_ampl.jpg

Running AMPL files

Once you have finished editing your AMPL files, you can run them by:

  1. Either starting a command line application or double-clicking on setup_ampl.bat (if you are at home);
  2. Navigating to the directory where you saved the files;
  3. Starting AMPL;
  4. Typing AMPL commands, e.g., model .mod;, data .dat;, include .run.

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-- MichaelOSullivan - 25 Feb 2008

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Topic revision: r10 - 2009-07-23 - CameronWalker
 
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