Changing the values in txt file using R r - r

Question

I want to write a softcode that will change the values of required parameters of my Hydrological model input txt file. some parametres are fixed and some I will change to the observed values. For exampple HYDRUS_Version=4 is fixed and WaterFlow=1 and SoluteTransport=0 etc are the parametres that I want to change their values. I want to assign the values of waterFlow=5 or WaterFlow=3.1 and SoluteTransport=2 or =2.2
I tried this code for water Flow but the values in my txt files are not changed. I am just new learner of R.
lines <- readLines("G:/Rlearning/HYDRUS1D.txt")
library(gsubfn)
i1 <- grepl("Vertical Conductivity", lines)
lines[i1] <- gsubfn("[0-9.]+", ~format(as.numeric(x)*2,
scientific = FALSE), lines[i1])
Below is the complete txt file.
;
[Main]
HYDRUS_Version=4
WaterFlow=1
SoluteTransport=0
Unsatchem=0
Unsatchem=0
HP1=0
HeatTransport=0
EquilibriumAdsorption=1
MobileImmobile=0
RootWaterUptake=1
RootGrowth=0
MaterialNumbers=1
SubregionNumbers=1
SpaceUnit=cm
TimeUnit=days
PrintTimes=160
NumberOfSolutes=0
InitialCondition=1
;
[Profile]
NumberOfNodes=101
ProfileDepth=1.2E+02
ObservationNodes=5
GridVisible=1
SnapToGrid=1
ProfileWidth=80
LeftMargin=40
GridOrgX=0
GridOrgY=0
GridDX=5.E+00
GridDY=5.E+00

Answer 1

Create a custom function to search matching string and replace value:
replaceFn <- function(phrase, value, file){
line <- file[grep(phrase, file)]
value.pos <- as.numeric(gregexpr("=", line))
file[grep(phrase, file)] <- paste0(substring(line, 1, value.pos), value)
return(file)
}
dat <- replaceFn("WaterFlow", 3.1, dat)
dat <- replaceFn("SoluteTransport", 2.2, dat)
[1] ";" "[Main]" "HYDRUS_Version=4" "WaterFlow=3.1" "SoluteTransport=2.2" "Unsatchem=0"
[7] "Unsatchem=0" "HP1=0" "HeatTransport=0" "EquilibriumAdsorption=1" "MobileImmobile=0" "RootWaterUptake=1"
[13] "RootGrowth=0" "MaterialNumbers=1" "SubregionNumbers=1" "SpaceUnit=cm" "TimeUnit=days" "PrintTimes=160"
[19] "NumberOfSolutes=0" "InitialCondition=1" ";" "[Profile]" "NumberOfNodes=101" "ProfileDepth=1.2E+02"
[25] "ObservationNodes=5" "GridVisible=1" "SnapToGrid=1" "ProfileWidth=80" "LeftMargin=40" "GridOrgX=0"
[31] "GridOrgY=0" "GridDX=5.E+00" "GridDY=5.E+00"
Input data:
dat <- c(";", "[Main]", "HYDRUS_Version=4", "WaterFlow=1", "SoluteTransport=0",
"Unsatchem=0", "Unsatchem=0", "HP1=0", "HeatTransport=0", "EquilibriumAdsorption=1",
"MobileImmobile=0", "RootWaterUptake=1", "RootGrowth=0", "MaterialNumbers=1",
"SubregionNumbers=1", "SpaceUnit=cm", "TimeUnit=days", "PrintTimes=160",
"NumberOfSolutes=0", "InitialCondition=1", ";", "[Profile]",
"NumberOfNodes=101", "ProfileDepth=1.2E+02", "ObservationNodes=5",
"GridVisible=1", "SnapToGrid=1", "ProfileWidth=80", "LeftMargin=40",
"GridOrgX=0", "GridOrgY=0", "GridDX=5.E+00", "GridDY=5.E+00")