# 15.1 The LP File Format¶

**MOSEK** supports the LP file format with some extensions. The LP format is not a completely well-defined standard and hence different optimization packages may interpret the same LP file in slightly different ways. **MOSEK** tries to emulate as closely as possible CPLEX’s behavior, but tries to stay backward compatible.

The LP file format can specify problems on the form

where

- \(x \in \real^n\) is the vector of decision variables.
- \(c \in \real^n\) is the linear term in the objective.
- \(q^o: \in \real^n \rightarrow \real\) is the quadratic term in the objective where

and it is assumed that

- \(A \in \real^{m\times n}\) is the constraint matrix.
- \(l^c \in \real^m\) is the lower limit on the activity for the constraints.
- \(u^c \in \real^m\) is the upper limit on the activity for the constraints.
- \(l^x \in \real^n\) is the lower limit on the activity for the variables.
- \(u^x \in \real^n\) is the upper limit on the activity for the variables.
- \(q: \real^n \rightarrow \real\) is a vector of quadratic functions. Hence,

where it is assumed that

- \(\mathcal{J} \subseteq \{1,2,\ldots ,n\}\) is an index set of the integer constrained variables.

## 15.1.1 File Sections¶

An LP formatted file contains a number of sections specifying the objective, constraints, variable bounds, and variable types. The section keywords may be any mix of upper and lower case letters.

### Objective Function¶

The first section beginning with one of the keywords

```
max
maximum
maximize
min
minimum
minimize
```

defines the objective sense and the objective function, i.e.

The objective may be given a name by writing

```
myname:
```

before the expressions. If no name is given, then the objective is named `obj`

.

The objective function contains linear and quadratic terms. The linear terms are written as:

```
4 x1 + x2 - 0.1 x3
```

and so forth. The quadratic terms are written in square brackets (`[`

`]`

) and are either squared or multiplied as in the examples

```
x1^2
```

and

```
x1 * x2
```

There may be zero or more pairs of brackets containing quadratic expressions.

An example of an objective section is

```
minimize
myobj: 4 x1 + x2 - 0.1 x3 + [ x1^2 + 2.1 x1 * x2 ]/2
```

Please note that the quadratic expressions are multiplied with \(\half\) , so that the above expression means

If the same variable occurs more than once in the linear part, the coefficients are added, so that `4 x1 + 2 x1`

is equivalent to `6 x1`

. In the quadratic expressions `x1 * x2`

is equivalent to `x2 * x1`

and, as in the linear part, if the same variables multiplied or squared occur several times their coefficients are added.

### Constraints¶

The second section beginning with one of the keywords

```
subj to
subject to
s.t.
st
```

defines the linear constraint matrix \(A\) and the quadratic matrices \(Q^i\).

A constraint contains a name (optional), expressions adhering to the same rules as in the objective and a bound:

```
subject to
con1: x1 + x2 + [ x3^2 ]/2 <= 5.1
```

The bound type (here `<=`

) may be any of `<`

, `<=`

, `=`

, `>`

, `>=`

(`<`

and `<=`

mean the same), and the bound may be any number.

In the standard LP format it is not possible to define more than one bound, but **MOSEK** supports defining ranged constraints by using double-colon (`::`

) instead of a single-colon (`:`

) after the constraint name, i.e.

may be written as

```
con:: -5 < x_1 + x_2 < 5
```

By default **MOSEK** writes ranged constraints this way.

If the files must adhere to the LP standard, ranged constraints must either be split into upper bounded and lower bounded constraints or be written as an equality with a slack variable. For example the expression (1) may be written as

### Bounds¶

Bounds on the variables can be specified in the bound section beginning with one of the keywords

```
bound
bounds
```

The bounds section is optional but should, if present, follow the `subject to`

section. All variables listed in the bounds section must occur in either the objective or a constraint.

The default lower and upper bounds are \(0\) and \(+\infty\) . A variable may be declared free with the keyword `free`

, which means that the lower bound is \(-\infty\) and the upper bound is \(+\infty\) . Furthermore it may be assigned a finite lower and upper bound. The bound definitions for a given variable may be written in one or two lines, and bounds can be any number or \(\pm \infty\) (written as `+inf`

/`-inf`

/`+infinity`

/`-infinity`

) as in the example

```
bounds
x1 free
x2 <= 5
0.1 <= x2
x3 = 42
2 <= x4 < +inf
```

### Variable Types¶

The final two sections are optional and must begin with one of the keywords

```
bin
binaries
binary
```

and

```
gen
general
```

Under `general`

all integer variables are listed, and under `binary`

all binary (integer variables with bounds 0 and 1) are listed:

```
general
x1 x2
binary
x3 x4
```

Again, all variables listed in the binary or general sections must occur in either the objective or a constraint.

## 15.1.2 LP File Examples¶

Linear example `lo1.lp`

```
\ File: lo1.lp
maximize
obj: 3 x1 + x2 + 5 x3 + x4
subject to
c1: 3 x1 + x2 + 2 x3 = 30
c2: 2 x1 + x2 + 3 x3 + x4 >= 15
c3: 2 x2 + 3 x4 <= 25
bounds
0 <= x1 <= +infinity
0 <= x2 <= 10
0 <= x3 <= +infinity
0 <= x4 <= +infinity
end
```

Mixed integer example `milo1.lp`

```
maximize
obj: x1 + 6.4e-01 x2
subject to
c1: 5e+01 x1 + 3.1e+01 x2 <= 2.5e+02
c2: 3e+00 x1 - 2e+00 x2 >= -4e+00
bounds
0 <= x1 <= +infinity
0 <= x2 <= +infinity
general
x1 x2
end
```

## 15.1.3 LP Format peculiarities¶

### Comments¶

Anything on a line after a `\`

is ignored and is treated as a comment.

### Names¶

A name for an objective, a constraint or a variable may contain the letters a-z, A-Z, the digits 0-9 and the characters

```
!"#$%&()/,.;?@_'`|~
```

The first character in a name must not be a number, a period or the letter `e`

or `E`

. Keywords must not be used as names.

**MOSEK** accepts any character as valid for names, except `\0`

. A name that is not allowed in LP file will be changed and a warning will be issued.

The algorithm for making names LP valid works as follows: The name is interpreted as an `utf-8`

string. For a unicode character `c`

:

- If
`c==_`

(underscore), the output is`__`

(two underscores). - If
`c`

is a valid LP name character, the output is just`c`

. - If
`c`

is another character in the ASCII range, the output is`_XX`

, where`XX`

is the hexadecimal code for the character. - If
`c`

is a character in the range 127-65535, the output is`_uXXXX`

, where`XXXX`

is the hexadecimal code for the character. - If
`c`

is a character above 65535, the output is`_UXXXXXXXX`

, where`XXXXXXXX`

is the hexadecimal code for the character.

Invalid `utf-8`

substrings are escaped as `_XX'`

, and if a name starts with a period, e or E, that character is escaped as `_XX`

.

### Variable Bounds¶

Specifying several upper or lower bounds on one variable is possible but **MOSEK** uses only the tightest bounds. If a variable is fixed (with `=`

), then it is considered the tightest bound.

**MOSEK** Extensions to the LP Format¶

Some optimization software packages employ a more strict definition of the LP format than the one used by **MOSEK**. The limitations imposed by the strict LP format are the following:

- Quadratic terms in the constraints are not allowed.
- Names can be only 16 characters long.
- Lines must not exceed 255 characters in length.

To get around some of the inconveniences converting from other problem formats, **MOSEK** allows lines to contain 1024 characters and names may have any length (shorter than the 1024 characters).

## 15.1.4 Formatting of an LP File¶

A few parameters control the visual formatting of LP files written by **MOSEK** in order to make it easier to read the files. These parameters are

The first parameter sets the maximum number of characters on a single line. The default value is 80 corresponding roughly to the width of a standard text document.

The second parameter sets the maximum number of terms per line; a term means a sign, a coefficient, and a name (for example `+ 42 elephants`

). The default value is 0, meaning that there is no maximum.

### Unnamed Constraints¶

Reading and writing an LP file with **MOSEK** may change it superficially. If an LP file contains unnamed constraints or objective these are given their generic names when the file is read (however unnamed constraints in **MOSEK** are written without names).