r - Create linestring from two points in same row in dataframe

We can loop through the rows, with pmap and apply the st_linestring on a matrix created

library(tidyverse)
library(sf)
out <- pmap(df[-1], ~
               c(...) %>%
                matrix(., , ncol=2, byrow = TRUE) %>% 
                st_linestring) %>%
          reduce(st_sfc) %>%
          mutate(df, geometry = .)

out$geometry
#Geometry set for 2 features 
#geometry type:  LINESTRING
#dimension:      XY
#bbox:           xmin: 1 ymin: 3 xmax: 6 ymax: 8
#epsg (SRID):    NA
#proj4string:    NA
#LINESTRING (1 3, 5 7)
#LINESTRING (2 4, 6 8)

Update - 30th Jan 2021

The issue with my original answer is it doesn't correctly set the bounding box.

Today I would use this approach using sfheaders and data.table

library(data.table)
library(sfheaders)

dt <- as.data.table(df)

## To use `sfheaders` the data needs to be in long form

dt1 <- dt[, .(id, lon = lon1, lat = lat1)]
dt2 <- dt[, .(id, lon = lon2, lat = lat2)]

## Add on a 'sequence' variable so we know which one comes first
dt1[, seq := 1L ]
dt2[, seq := 2L ]

## put back together
dt <- rbindlist(list(dt1, dt2), use.names = TRUE)
setorder(dt, id, seq)

sf <- sfheaders::sf_linestring(
  obj = dt
  , x = "lon"
  , y = "lat"
  , linestring_id = "id"
)

sf

# Simple feature collection with 2 features and 1 field
# geometry type:  LINESTRING
# dimension:      XY
# bbox:           xmin: 1 ymin: 3 xmax: 6 ymax: 8
# CRS:            NA
#   id              geometry
# 1  a LINESTRING (1 3, 5 7)
# 2  b LINESTRING (2 4, 6 8)

---

Original Answer

An alternative approach using data.table

require(data.table)

dt <- as.data.table(df)

sf <- dt[
    , {
        geometry <- sf::st_linestring(x = matrix(c(lon1, lon2, lat1, lat2), nrow = 2, ncol = 2))
        geometry <- sf::st_sfc(geometry)
        geometry <- sf::st_sf(geometry = geometry)
    }
    , by = id
]

sf::st_as_sf(sf)
# Simple feature collection with 2 features and 1 field
# geometry type:  LINESTRING
# dimension:      XY
# bbox:           xmin: 1 ymin: 3 xmax: 5 ymax: 7
# epsg (SRID):    NA
# proj4string:    NA
# id              geometry
# 1  a LINESTRING (1 3, 5 7)
# 2  b LINESTRING (2 4, 6 8)