compare two integer column values in R and fill a new column

问题

I am trying to fill new column values in R data frame based on a condition that compares values from two columns. Using for loop and if-else control statement.

Here's my sample dataset

Year1 | Year2 
----- | -----
1990  | 1990
1992  | 1992
1995  | 1998
1997  | 2000

I would like to do something like this:

for (i in 1:length(year1)
{
if (year1[i] == year2[i]) 
   flag = 1 
   next
else 
   flag = 2
}

This doesn't seem to be working. For some reason, all the conditions are evaluated as TRUE and flag is always 1.

I wonder why all conditions are evaluated as TRUE? I verified that the loop iterates over all values of year1.

Any suggestions would be much appreciated!

回答1:

Perhaps:

df$flag <- 2
df[df$Year1==df$Year2,]$flag <- 1

回答2:

How about

df$flag <- with(df, abs((Year1 == Year2) - 2L))
df
#   Year1 Year2 flag
# 1  1990  1990    1
# 2  1992  1992    1
# 3  1995  1998    2
# 4  1997  2000    2

回答3:

Something along the lines of:

df$flag=ifelse(df$year1==df$year2,1,2)

回答4:

Another solution using apply function

df = data.frame(Year1 = c(1990, 1992, 1995, 1997), Year2 = c(1990, 1992, 1998, 2000))

apply function:

df$flag = apply(df, 1, function(x){ ifelse(x[1] == x[2], 1, 2) })

or using variable names

df$flag apply(df, 1, function(x){ ifelse(x["Year1"] == x["Year2"], 1, 2) })

回答5:

Above all the solutions compare the data with respect to same row and output the flag.

Rather, I would like you to consider this query. Which, not only covers above condition but also checks a column value (here year1) with the rest of the records present in another column (year2).

df = data.frame(Year1 = c(1990, 1998, 1992, 1997), Year2 = c(1990, 1992, 1998, 2000))

df$flag <- ifelse(df$Year1 %in% df$Year2, 1, 2)
df

# Result:
#   Year1 Year2 flag
#1  1990  1990    1
#2  1998  1992    1
#3  1992  1998    1
#4  1997  2000    2

Hope it helped!

来源：https://stackoverflow.com/questions/42497851/compare-two-integer-column-values-in-r-and-fill-a-new-column