#create this matrix
mymat<-matrix(1:20,nrow=5,byrow=T)
#
# it should look like this
#     [,1] [,2] [,3] [,4]
#[1,]    1    2    3    4
#[2,]    5    6    7    8
#[3,]    9   10   11   12
#[4,]   13   14   15   16
#[5,]   17   18   19   20


# compute the column sums 3 different ways:

#a) with 2 nested loops but without any functions like sum() or apply()

#b) using one loop and you are allowed to use a function such as sum()

#c) you are not allowed to use any loops but you will likely need to use apply()

for(i in 1:4){
	mysum<-0
	for(j in 1:5){
		mysum<-mysum+mymat[j,i]
	}
	cat("sum1: ",mysum,"\n")
}

for(i in 1:4){
	mysum<-sum(mymat[,i])
	cat("sum2: ",mysum,"\n")
}

mysum<-apply(mymat,2,sum)
cat("sum3: ",mysum,"\n")