Counting number of instances of a condition per row R [duplicate]

by

You can use rowSums. 

df$no_calls <- rowSums(df == "nc")
df
#  rsID sample1 sample2 sample3 sample1304 no_calls
#1 abcd      aa      bb      nc         nc        2
#2 efgh      nc      nc      nc         nc        4
#3 ijkl      aa      ab      aa         nc        1

Or, as pointed out by MrFlick, to exclude the first column from the row sums, you can slightly modify the approach to

df$no_calls <- rowSums(df[-1] == "nc")

Regarding the row names: They are not counted in rowSums and you can make a simple test to demonstrate it:

rownames(df)[1] <- "nc"  # name first row "nc"
rowSums(df == "nc")      # compute the row sums
#nc  2  3             
# 2  4  1        # still the same in first row

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