# MPS format

##

MPS format

MPS format is the oldest and most widely used format for storing math programming models. There are actually two variants of this format in wide use. In fixed format, the various fields must always start at fixed columns in the file. Free format is very similar, but the fields are separated by white space instead of appearing in specific columns. One important practical difference between the two formats is in name length. In fixed format, row and column names are exactly 8 characters, and spaces are part of the name. In free format, names can be arbitrarily long (although the Gurobi reader places a 255 character limit on name length), and names may not contain spaces. The Gurobi MPS reader reads both MPS types, and recognizes the format automatically.

Note that any line that begins with the `*`

character is a
comment. The contents of that line are ignored.

NAME section

The first section in an MPS format file is the `NAME`

section.
It gives the name of the model:

NAME AFIROIn fixed format, the model name starts in column 15.

ROWS section

The next section is the `ROWS`

section. It begins with the word
`ROWS`

on its own line, and continues with one line for each row
in the model. These lines indicate the constraint type (`E`

for
equality, `L`

for less-than-or-equal, or `G`

for
greater-than-or-equal), and the constraint name. In fixed format, the
type appears in column 2 and the row name starts in column 5. Here's
a simple example:

ROWS E R09 E R10 L X05 N COSTNote that an

`N`

in the type field indicates that the row is a
free row. The first free row is used as the objective
function.
If the file includes multiple `N`

rows, each including a
priority, weight, relative, and absolute tolerance field, then each
such row is treated as an objective in a multi-objective model. The
additional fields must appear after the name, separated by spaces.
For example, the following would capture a pair of objectives, where
the first has priority 2 and the second has priority 1 (and both have
identical weights, and relative and absolute tolerances):

N OBJ0 2 1 0 0 N OBJ1 1 1 0 0Please refer to the multi-objective, ObjNPriority, ObjNWeight, ObjNAbsTol, and ObjNRelTol sections for information on the meanings of these fields. Note that all objectives of a multi-objective optimization problem have to be linear.

LAZYCONS section

The next section is the `LAZY CONSTRAINT`

section. It begins with the
line `LAZYCONS`

, optional followed by a space and a laziness level 1-3
(if no laziness level is specified 1 is assumed), and continues with one
line for each lazy constraint. The format is the same as that of the
`ROWS`

section: each line indicates the constraint type (`E`

for
equality, `L`

for less-than-or-equal, or `G`

for
greater-than-or-equal), and the constraint name. In fixed format, the type
appears in column 2 and the row name starts in column 5. For example:

LAZYCONS E R01 G R07 L S01 LAZYCONS 2 E R02 G R03 L S11

Lazy constraints are linear constraints, and they are semantically
equivalent to standard linear constraints (i.e., entries in the `ROWS`

section). Depending on their laziness level they are enforced differently
by the MIP solver. Please refer to the description of the
Lazy attribute for details.

This section is optional.

COLUMNS section

The next and typically largest section of an MPS file is the
`COLUMNS`

section, which lists the columns in the model and the
non-zero coefficients associated with each. Each line in the columns
section provides a column name, followed by either zero, one, or two
non-zero coefficients from that column. Coefficients are specified
using a row name first, followed by a floating-point value. Consider
the following example:

COLUMNS X01 X48 .301 R09 -1. X01 R10 -1.06 X05 1. X02 X21 -1. R09 1. X02 COST -4.The first line indicates that column

`X01`

has a non-zero in row
`X48`

with coefficient `.301`

, and a non-zero in row
`R09`

with coefficient `-1.0`

. Note that multiple lines
associated with the same column must be contiguous in the file.
In fixed format, the column name starts in column 5, the row name for the first non-zero starts in column 15, and the value for the first non-zero starts in column 25. If a second non-zero is present, the row name starts in column 40 and the value starts in column 50.

Integrality markers

The `COLUMNS`

section can optionally include integrality markers. The
variables introduced between a pair of markers must take integer values.
All variables within markers will have a default lower bound of 0 and a
default upper bound of 1 (other bounds can be specified in the
`BOUNDS`

section). The beginning of an integer section is marked by
an `INTORG`

marker:

MARK0000 'MARKER' 'INTORG'The end of the section is marked by an

`INTEND`

marker:
MARK0000 'MARKER' 'INTEND'The first field (beginning in column 5 in fixed format) is the name of the marker (which is ignored). The second field (in column 15 in fixed format) must be equal to the string

`'MARKER'`

(including the single quotes).
The third field (in column 40 in fixed format) is `'INTORG'`

at the
start and `'INTEND'`

at the end of the integer section.
The `COLUMNS`

section can contain an arbitrary number of such
marker pairs.

RHS section

The next section of an MPS file is the `RHS`

section, which
specifies right-hand side values. Each line in this section may
contain one or two right-hand side values.

RHS B X50 310. X51 300. B X05 80. X17 80.The first line above indicates that row

`X50`

has a right-hand
side value of `310`

, and `X51`

has a right-hand side value
of `300`

. In fixed format, the variable name for the first
bound starts in column 15, and the first bound value starts in column
25. For the second bound, the variable name starts in column 40 and
the value starts in column 50. The name of the RHS is specified in
the first field (column 5 in fixed format), but this name is ignored
by the Gurobi reader. If a row is not mentioned anywhere in the
`RHS`

section, that row takes a right-hand side value of
`0`

.
BOUNDS section

The next section in an MPS file is the optional `BOUNDS`

section. By default, each variable takes a lower bound of 0 and an
infinite upper bound. Each line in this section can modify the lower
bound of a variable, the upper bound, or both. Each line indicates a
bound type (in column 2 in fixed format), a bound name (ignored), a
variable name (in column 15 in fixed format), and a bound value (in
columns 25 in fixed format). The different bound types, and the
meaning of the associate bound value, are as follows:

LO | lower bound |

UP | upper bound |

FX | variable is fixed at the specified value |

FR | free variable (no lower or upper bound) |

MI | infinite lower bound |

PL | infinite upper bound |

BV | variable is binary (equal 0 or 1) |

LI | lower bound for integer variable |

UI | upper bound for integer variable |

SC | upper bound for semi-continuous variable |

Consider the following example:

BOUNDS UP BND X50 80. LO BND X51 20. FX BND X52 30.In this

`BOUNDS`

section, variable `X50`

gets a upper
bound of `80`

(lower bound is unchanged at `0`

,
`X51`

gets a lower bound of `20`

(infinite upper bound is
unchanged), and `X52`

is fixed at `30`

.
QUADOBJ section

The next section in an MPS file is the optional `QUADOBJ`

section, which contains quadratic objective terms. Each line in this
section represents a single non-zero value in the lower triangle of
the Q matrix. The names of the two variable that participate in the
quadratic term are found first (starting in columns 5 and 15 in fixed
format), followed by the numerical value of the coefficient (in column
25 in fixed format). By convention, the Q matrix has an implicit
one-half multiplier associated with it. Here's an example containing
three quadratic terms:

QUADOBJ X01 X01 10.0 X01 X02 2.0 X02 X02 2.0These three terms would represent the quadratic function (recall that the single off-diagonal term actually represents a pair of non-zero values in the symmetric Q matrix).

QCMATRIX section

The next section in an MPS file contains zero or more `QCMATRIX`

blocks. These blocks contain the quadratic terms associated with the
quadratic constraints. There should be one block for each quadratic
constraint in the model.

Each `QCMATRIX`

block starts with a line that indicates the name
of the associated quadratic constraint (starting in column 12 in fixed
format). This is followed by one of more quadratic terms. Each term
is described on one line, which gives the names of the two
involved variables (starting in columns 5 and 15 in fixed format),
followed by the coefficient (in column 25 in fixed format).
For example:

QCMATRIX QC0 X01 X01 10.0 X01 X02 2.0 X02 X01 2.0 X02 X02 2.0These four lines describe three quadratic terms: quadratic constraint

`QC0`

contains terms , , and .
Note that a `QCMATRIX`

block must contain a symmetric matrix, so
for example an `X01*X02`

term must be accompanied by a matching
`X02*X01`

term.
Linear terms for quadratic constraint `QC0`

appear in the
`COLUMNS`

section. The sense and right-hand side value appear in
the `ROWS`

and `RHS`

sections, respectively.

PWLOBJ section

The next section in an MPS file is the optional `PWLOBJ`

section,
which contains piecewise-linear objective functions. Each line in
this section represents a single point in a piecewise-linear objective
function. The name of the associated variable appears first (starting
in column 4), followed by the x and y coordinates of the point
(starting in columns 14 and 17). Here's an example containing two
piecewise-linear expressions, for variables `X01`

and `X02`

,
each with three points:

X01 1 1 X01 2 2 X01 3 4 X02 1 1 X02 3 5 X02 7 10

SOS section

The next section in an MPS file is the optional `SOS`

section.
The representation for a single SOS constraint contains one line that
provides the type of the SOS set (`S1`

for SOS type 1 or
`S2`

for SOS type 2, found in column 2 in fixed format) and the
name of the SOS set (column 5 in fixed format) of the SOS set. This
is followed by one line for each SOS member. The member line gives
the name of the member (column 5 in fixed format) and the associated
weight (column 15 in fixed format). Here's an example containing two
SOS2 sets.

SOS S2 sos1 x1 1 x2 2 x3 3 S2 sos2 x3 1 x4 2 x5 3

Indicator Constraint section

The indicator constraint section is optional in the MPS format. It
starts with the keyword `INDICATORS`

. Each subsequent line of the
indicator section starts with the keyword `IF`

(placed at column
2 in fixed format) followed by a space and a row name (the row must
have already been defined in the `ROWS`

section). The line
continues with a binary variable (placed at column 15 in fixed format)
and finally a value `0`

or `1`

(placed at column 25 in fixed
format).

Here a simple example:

INDICATORS IF row1 x1 0 IF row2 y1 1

The first indicator constraint in this example states that `row1`

has to be fulfilled if the `x1`

takes a value of `0`

.

General Constraint section

An MPS file may contain an optional section that captures more general
constraints. The general constraint section starts with the
keyword `GENCONS`

.

Each general constraint in this section starts with a general constraint
type specifier, such as *MIN*, *MAX*, *OR*, *AND*,
or *ABS* (found in column 2 in fixed format). Optionally a space and
a name may follow.

What follows this line depends on the general constraint type. All
general constraints start with the so-called resultant variable,
placed on it's own line (starting at column 5 in fixed format). For
*MIN* or *MAX* constraints, a non empty list of variables
or values follows this (each on its own line). For *OR* and
*AND* constraints, a list of binary variables follows (each on
its own line). For *ABS* constraints, one additional variable
follows (on its own line). In fixed format all of these variables or
values are placed at column 5.

The other general constraint type, the *INDICATOR* constraint,
appears in a separate `Indicator`

section, which is described
above.

The following shows an example of a general constraint section:

GENCONS MAX gc0 r1 x1 x2 x10 0.7 MIN gencons1 r2 y0 10 y1 r1 AND and1 r b1 b2 OR or1 r b3 b4 ABS GC14 xabs x

For more information, consult the general constraint discussion.

ENDATA

The final line in an MPS file must be an `ENDATA`

statement.

Additional notes

Note that in the Gurobi optimizer, MPS models are always written in full precision. That means that if you write a model and then read it back, the data associated with the resulting model will be bit-for-bit identical to the original data.