Fill empty matrix with a function and a for loop

#Hi guys, i've an issue about a personal exercise in R, i've tried to fill an empty matrix with for #loop but i didn't manage to

#I've this function

classification_perf = function(pred, obs){
overallrate = 1-mean(pred == obs)
cf = table(pred, obs)
sensitivity = cf[2,2]/sum(cf[,2])
specificity = cf[1,1]/sum(cf[,1])
return(c(overall = overallrate,
sens = sensitivity,
spec = specificity))
}

#And this for loop:

vec = c(1,2,3)
errorvec = c()
for(i in 1:length(vec)){
out = knn(train = train[,c(1,2)],
test = test[,c(1,2)],
cl = train[,3],
k = vec[i],
prob = TRUE,
use.all = TRUE)
errorvec[i] = 1-mean(out, test[,3])
}

#Obviously the second codes lead me to fill che errorvec = c(), but i want to as output a #matrix given by the function above "classification_perf" leading me to obtain a 3*3 matrix #with "vec=c(1,2,3)" on rows and "c(overall=overallrate, sens = sensitivity,spec = specificity))" #on cols.

#I've tried in this way bu it gives me an error:

vec = c(1,2,3)
classification = matrix(nrow = 3, ncol =3)
for(j in 1:ncol(classification)){
for(i in 1:length(vec)){
out = knn(train = train[,c(1,2)],
test = test[,c(1,2)],
cl = train[,3],
k = vec[i],
prob = TRUE,
use.all = TRUE)
classification[i,j] = classification_perf(out, test$y)
}
}

#Please help me LOL