How to remove coefficients form R formula string

Question

I have strings like the following:

b="+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-sl_1__1_1_2"
b1 ="+sh_8_6_1_1_1-1000*sdp_8_6_1_1_1"

I am trying to split it into the following:

b:  Variables             Coefficient   
     sl_1__1_1_1                 1
    from_st_1_4_1_1_1            5
     into_st_4_1_1_1_1          -70
     sl_1__1_1_2                -1
b1: Variables                   Coefficient       
    sh_8_6_1_1_1                 1
    sdp_8_6_1_1_1              -1000

The strsplit function I am using at the moment is not able to pick up coefficients with more than one digit (ie the 1000 coefficient).

Any help would be much appreciated.

Answer 1

Here is one approach with strpslit :

b= "+fg_1+5*ug1_1-7*tg_4" # original example string

Coefficient <- as.numeric(sub("\\+$", "1", strsplit(b, "\\**[a-z0-9]+_\\d", perl=TRUE)[[1]]))
Variable <- sub("\\+", "", strsplit(b, "[+-]\\d\\**", perl=TRUE)[[1]])
data.frame(Variable, Coefficient)
#  Variable Coefficient
#1     fg_1           1
#2    ug1_1           5
#3     tg_4          -7

Explanation of \\\\**[a-z0-9]+_\\\\d :

• Matches an optional asterisk: \\\\**
  • The end * means repeated 0 or more times. Could easily use ? for optional instead: \\\\*?
• Followed by any lowercase letter, followed by any digit 0 to 9: [a-z0-9]
  • + one or more times: [a-z0-9]+
• Followed by an underscore: _
• Followed by a digit character: \\\\d

Edit: Update for new sample string with slightly different pattern and a possible constant at the end (eg +50 in the below example):

# new sample strings
b="+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-sl_1__1_1_2"
#b="+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-sl_1__1_1_2+50"
#b ="+sh_8_6_1_1_1-1000*sdp_8_6_1_1_1"
#b= "+fg_1+5*ug1_1-7*tg_4"              # 1st sample string from original question

Variable <- strsplit(b, "[+-][0-9]*\\**", perl=TRUE)[[1]]
Variable <- Variable[!Variable == ""]

Coefficient <- as.numeric(sub("([+-]$)", "\\11", 
                 strsplit(b, "(?<=[+-])\\D+.*?(?=[+-]|$)|(?<=\\d)\\*.*?(?=[+-]|$)",
                 perl=TRUE)[[1]]))

# handle possible constant at end of string:
ifelse(length(Coefficient) == (length(Variable)+1L), 
    df <- data.frame(Variable=c(Variable, "constant"), Coefficient), 
    df <- data.frame(Variable, Coefficient))
df
#           Variable Coefficient
#1       sl_1__1_1_1           1
#2 from_st_1_4_1_1_1           5
#3 into_st_4_1_1_1_1         -70
#4       sl_1__1_1_2          -1

Answer 2

Here's another

b <- "+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-sl_1__1_1_2"
b1 <- "+sh_8_6_1_1_1-1000*sdp_8_6_1_1_1"
b2 <- "+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-sl_1__1_1_2+50" 

First step, to make life easier, is to add in 1s when no coefficient is given, and then use the regex I mentioned in the comments.

(b <- gsub('([+-])(\\D)', '\\11+\\2', b2))
# [1] "+1+sl_1__1_1_1+5*from_st_1_4_1_1_1-70*into_st_4_1_1_1_1-1+sl_1__1_1_2"

(bb <- regmatches(b, gregexpr('[+-]?\\w+', text = b))[[1]])
# [1] "+1"                "+sl_1__1_1_1"      "+5"               
# [4] "from_st_1_4_1_1_1" "-70"               "into_st_4_1_1_1_1"
# [7] "-1"                "+sl_1__1_1_2"  

then do some final re-arranging and formatting

(bb <- data.frame(matrix(bb, ncol = 2, byrow = TRUE)[, 2:1]))
#                  X1  X2
# 1      +sl_1__1_1_1  +1
# 2 from_st_1_4_1_1_1  +5
# 3 into_st_4_1_1_1_1 -70
# 4      +sl_1__1_1_2  -1

within(bb, {
  X1 <- gsub('\\-|\\+', '', as.character(X1))
  X2 <- as.numeric(as.character(X2))
})

#                  X1  X2
# 1       sl_1__1_1_1   1
# 2 from_st_1_4_1_1_1   5
# 3 into_st_4_1_1_1_1 -70
# 4       sl_1__1_1_2  -1

This would be easier to use if you put it into a function. I also added a check for trailing constant terms

f <- function(x) {
  ## check constant
  x <- gsub('([+-]\\d+)$', '\\1*constant', x)
  x <- gsub('([+-])(\\D)', '\\11+\\2', x)
  x <- regmatches(x, gregexpr('[+-]?\\w+', text = x))[[1]]
  x <- data.frame(matrix(x, ncol = 2, byrow = TRUE)[, 2:1],
                  stringsAsFactors = FALSE)
  x[, 2] <- as.numeric(x[, 2])
  x[, 1] <- gsub('\\-|\\+', '' , x[, 1])
  setNames(x, c('Variable','Coefficient'))
}

f(b)

#            Variable Coefficient
# 1       sl_1__1_1_1           1
# 2 from_st_1_4_1_1_1           5
# 3 into_st_4_1_1_1_1         -70
# 4       sl_1__1_1_2          -1

f(b1)

#        Variable Coefficient
# 1  sh_8_6_1_1_1           1
# 2 sdp_8_6_1_1_1       -1000

f(b2)

#            Variable Coefficient
# 1       sl_1__1_1_1           1
# 2 from_st_1_4_1_1_1           5
# 3 into_st_4_1_1_1_1         -70
# 4       sl_1__1_1_2          -1
# 5          constant          50