How to find the sum of variables by row in an R data frame?

To find the sum of variables by row we mean the sum of row values in the data frame. This can be easily done with the help of rowSums function. For example, if we have a data frame called df then the sum of variables by row can be found by using the command −
rowSums(df)
Consider the below data frame −
Live Demo
> x1<-rpois(20,2)
> x2<-rpois(20,2)
> x3<-rpois(20,2)
> df1<-data.frame(x1,x2,x3)
> df1
   x1 x2 x3
1   0  2  3
2   1  0  1
3   1  0  2
4   3  3  2
5   4  2  2
6   3  1  5
7   2  2  1
8   4  2  2
9   2  4  0
10  3  1  5
11  3  1  2
12  0  3  1
13  2  3  0
14  1  1  0
15  1  1  1
16  3  1  0
17  1  2  1
18  1  0  2
19  4  2  1
20  0  2  3
Finding row sums for data in df1 −
> df1$x_total<-rowSums(df1)
> df1
   x1 x2 x3 x_total
1   0  2  3       5
2   1  0  1       2
3   1  0  2       3
4   3  3  2       8
5   4  2  2       8
6   3  1  5       9
7   2  2  1       5
8   4  2  2       8
9   2  4  0       6
10  3  1  5       9
11  3  1  2       6
12  0  3  1       4
13  2  3  0       5
14  1  1  0       2
15  1  1  1       3
16  3  1  0       4
17  1  2  1       4
18  1  0  2       3
19  4  2  1       7
20  0  2  3       5
   x1 x2 x3 x_total
1   0  2  3       5
2   1  0  1       2
3   1  0  2       3
4   3  3  2       8
5   4  2  2       8
6   3  1  5       9
7   2  2  1       5
8   4  2  2       8
9   2  4  0       6
10  3  1  5       9
11  3  1  2       6
12  0  3  1       4
13  2  3  0       5
14  1  1  0       2
15  1  1  1       3
16  3  1  0       4
17  1  2  1       4
18  1  0  2       3
19  4  2  1       7
20  0  2  3       5
Live Demo
> y1<-rnorm(20)
> y2<-rnorm(20)
> y3<-rnorm(20)
> df2<-data.frame(y1,y2,y3)
> df2
            y1         y2           y3
1   0.59017486 -0.1196278  0.822573265
2  -1.12831016  0.7454030  0.173110462
3   0.65838766 -0.4798106  0.700891619
4  -3.09790550 -0.4401321  0.343264756
5  -0.27170928 -0.8106993 -0.658724220
6   1.51823786  0.1162130 -0.145487501
7   1.66852069 -0.4254530 -0.372181307
8   0.39011721  1.7260926 -0.253079767
9   0.12034466  0.4142485 -0.547526473
10  0.73264687  0.3537576 -0.248502362
11 -1.19683499  1.0923578  0.139092087
12 -0.02475713 -0.7436116  1.154820901
13 -1.68760189 -1.1306767 -0.002655384
14 -0.56164347  0.6580410  1.831938297
15  0.26013745 -0.6617675 -1.398981829
16  0.68685167 -1.2256218  0.435335557
17 -0.01735620  1.2882963 -0.378027363
18 -0.86458207  1.6013430  0.567320925
19  0.15593632  0.7086373 -0.231115639
20  0.54771681  1.8800467  0.235589089
Finding row sums for data in df2 −
> df2$y_total<-rowSums(df2)
> df2
            y1         y2           y3     y_total
1   0.59017486 -0.1196278  0.822573265  1.29312029
2  -1.12831016  0.7454030  0.173110462 -0.20979674
3   0.65838766 -0.4798106  0.700891619  0.87946873
4  -3.09790550 -0.4401321  0.343264756 -3.19477280
5  -0.27170928 -0.8106993 -0.658724220 -1.74113281
6   1.51823786  0.1162130 -0.145487501  1.48896334
7   1.66852069 -0.4254530 -0.372181307  0.87088643
8   0.39011721  1.7260926 -0.253079767  1.86313004
9   0.12034466  0.4142485 -0.547526473 -0.01293328
10  0.73264687  0.3537576 -0.248502362  0.83790209
11 -1.19683499  1.0923578  0.139092087  0.03461487
12 -0.02475713 -0.7436116  1.154820901  0.38645212
13 -1.68760189 -1.1306767 -0.002655384 -2.82093399
14 -0.56164347  0.6580410  1.831938297  1.92833585
15  0.26013745 -0.6617675 -1.398981829 -1.80061190
16  0.68685167 -1.2256218  0.435335557 -0.10343459
17 -0.01735620  1.2882963 -0.378027363  0.89291278
18 -0.86458207  1.6013430  0.567320925  1.30408187
19  0.15593632  0.7086373 -0.231115639  0.63345799
20  0.54771681  1.8800467  0.235589089  2.66335255