Delete rows with blank values in one particular column

An elegant solution with dplyr would be:

df %>%
  # recode empty strings "" by NAs
  na_if("") %>%
  # remove NAs
  na.omit

An easy approach would be making all the blank cells NA and only keeping complete cases. You might also look for na.omit examples. It is a widely discussed topic.

df[df==""]<-NA
df<-df[complete.cases(df),]

It is the same construct - simply test for empty strings rather than NA:

Try this:

df <- df[-which(df$start_pc == ""), ]

In fact, looking at your code, you don't need the which, but use the negation instead, so you can simplify it to:

df <- df[!(df$start_pc == ""), ]
df <- df[!is.na(df$start_pc), ]

And, of course, you can combine these two statements as follows:

df <- df[!(df$start_pc == "" | is.na(df$start_pc)), ]

And simplify it even further with with:

df <- with(df, df[!(start_pc == "" | is.na(start_pc)), ])

You can also test for non-zero string length using nzchar.

df <- with(df, df[!(nzchar(start_pc) | is.na(start_pc)), ])

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Disclaimer: I didn't test any of this code. Please let me know if there are syntax errors anywhere

 df[!(is.na(df$start_pc) | df$start_pc==""), ]