ILOG CPLEX 11.0 User's Manual

Understanding File FormatsThe reference manual ILOG CPLEX File Formats documents the file formats thatILOG CPLEX supports. The following sections cover programming considerations aboutwidely used file formats.◆ Working with LP Files on page 144;◆ Working with MPS Files on page 145;◆ Converting File Formats on page 146.Working with LP FilesLP files are row-oriented so you can look at a problem as you enter it in a naturally andintuitively algebraic way. However, ILOG CPLEX represents a problem internally in acolumn-ordered format. This difference between the way ILOG CPLEX accepts a problemin LP format and the way it stores the problem internally may have an impact on memory useand on the order in which variables are displayed on screen or in files.Variable Order and LP FilesAs ILOG CPLEX reads an LP format file by rows, it adds columns as it encounters them ina row. This convention will have an impact on the order in which variables are named anddisplayed. For example, consider this problem:Maximize 2x 2 + 3x 3subject to-x 1 + x 2 + x 3 ≤ 20x 1 - 3x 2 + x 3 ≤ 30with these bounds0 ≤ x 1 ≤ 400 ≤ x 2 ≤ +∞0 ≤ x 3 ≤ +∞Since ILOG CPLEX reads the objective function as the first row, the two columns appearingthere will become the first two variables. When the problem is displayed or rewritten into144 ILOG CPLEX 11.0 — USER’ S MANUAL