ROIoptimize Development - Add ROI_write function

[sbmack]

Great start for the project. First suggestion: Integrate the ROI_write function so that a model structure can be visually inspected in standard form.

[dirkschumacher]

Sounds like a good idea

[dirkschumacher]

Added an issue r-opt/ROIoptimizer#5

[dirkschumacher]

The most recent version of ROIoptimizer now supports ROI's as.OP function:

library(rmpk)
library(ROI.plugin.glpk)
library(ROI)
#> ROI: R Optimization Infrastructure
#> Registered solver plugins: nlminb, glpk, alabama, quadprog.
#> Default solver: auto.
solver <- ROI_optimizer("glpk")
v <- 1:10
w <- 1:10
model <- optimization_model(solver)
x <- model$add_variable("x", type = "binary", i = 1:10)
model$set_objective(sum_expr(v[i] * x[i], i = 1:10), sense = "max")
model$add_constraint(sum_expr(w[i] * x[i], i = 1:10) <= 10)

roi_OP <- as.OP(solver)
f <- tempfile()
ROI::ROI_write(roi_OP, f, "mps_fixed", "glpk")
print(readLines(f))
#>  [1] "* Problem:"                                                   
#>  [2] "* Class:      MIP"                                            
#>  [3] "* Rows:       1"                                              
#>  [4] "* Columns:    10 (10 integer, 10 binary)"                     
#>  [5] "* Non-zeros:  10"                                             
#>  [6] "* Format:     Fixed MPS"                                      
#>  [7] "*"                                                            
#>  [8] "NAME"                                                         
#>  [9] "ROWS"                                                         
#> [10] " N  R0000000"                                                 
#> [11] " L  R0000001"                                                 
#> [12] "COLUMNS"                                                      
#> [13] "    M0000001  'MARKER'                 'INTORG'"              
#> [14] "    C0000001  R0000000            -1   R0000001             1"
#> [15] "    C0000002  R0000000            -2   R0000001             2"
#> [16] "    C0000003  R0000000            -3   R0000001             3"
#> [17] "    C0000004  R0000000            -4   R0000001             4"
#> [18] "    C0000005  R0000000            -5   R0000001             5"
#> [19] "    C0000006  R0000000            -6   R0000001             6"
#> [20] "    C0000007  R0000000            -7   R0000001             7"
#> [21] "    C0000008  R0000000            -8   R0000001             8"
#> [22] "    C0000009  R0000000            -9   R0000001             9"
#> [23] "    C0000010  R0000000           -10   R0000001            10"
#> [24] "    M0000002  'MARKER'                 'INTEND'"              
#> [25] "RHS"                                                          
#> [26] "    RHS1      R0000001            10"                         
#> [27] "BOUNDS"                                                       
#> [28] " UP BND1      C0000001             1"                         
#> [29] " UP BND1      C0000002             1"                         
#> [30] " UP BND1      C0000003             1"                         
#> [31] " UP BND1      C0000004             1"                         
#> [32] " UP BND1      C0000005             1"                         
#> [33] " UP BND1      C0000006             1"                         
#> [34] " UP BND1      C0000007             1"                         
#> [35] " UP BND1      C0000008             1"                         
#> [36] " UP BND1      C0000009             1"                         
#> [37] " UP BND1      C0000010             1"                         
#> [38] "ENDATA"