R function keeps returning NULL - r

Question

I am having a very odd problem in R. The question was to make a function for global and semi global allignment. Appropriate algorithms were made which are able to "print out" the correct allignment. However "returning" the alginment seems to be a problem for the semi global algorithm.
Below are the functions for both alignments which both contain two functions: one computing the score matrix and the other outputs the alignment. As you can see, the output function for semi global was inspired by the global one but although it is able to print out values A and B, when returning A and B a value NULL is returned.
It came to my attention that when making defining A and B, they also contain a NULL part which seen by printing the structures of A and B at the end. This is also the case in the global alignment but does not seem to be a problem here.
Global Alignment Algorithm
########### GLOBAL ALLIGNMENT ALGORITHM ############
GA_score = function(v,w,score.gap=-3,score.match=8,score.mismatch=-5){
v = strsplit(v,split="")[[1]]
w = strsplit(w,split="")[[1]]
S = matrix(0,nrow=(length(v)+1),ncol = (length(w)+1) )
S[1,1] = 0
for(j in 2:dim(S)[2]){
S[1,j] = score.gap*(j-1)
}
for(i in 2:dim(S)[1]){
S[i,1] = score.gap*(i-1)
for(j in 2:dim(S)[2]){
if(v[i-1]==w[j-1]){diag = S[i-1,j-1] + score.match} else {diag = S[i-1,j-1] + score.mismatch}
down = S[i-1,j] + score.gap
right = S[i,j-1] + score.gap
S[i,j] = max(diag,down,right)
}
}
return(S)
}
GA_output = function(v,w,S,score.gap=-3,score.match=8,score.mismatch=-5){
v = strsplit(v,split="")[[1]]
w = strsplit(w,split="")[[1]]
A=c()
B=c()
GA_rec = function(A,B,S,i,j,v,w,score.gap,score.match,score.mismatch){
if (i==1 | j==1){
if(i>1){
for(i1 in seq(i-1,1,-1)){
A = c(v[i1],A)
B = c("-",B)
}
}
if(j>1){
for(j1 in seq(j-1,1,-1)){
A = c("-",A)
B = c(w[j1],B)
}
}
return(list(v=A,w=B))
}
if(v[i-1]==w[j-1] ){diag = score.match} else {diag=score.mismatch}
if (S[i,j] == (S[i-1,j-1] + diag)){
A.temp = c(v[i-1],A)
B.temp = c(w[j-1],B)
GA_rec(A.temp,B.temp,S,i-1,j-1,v,w,score.gap,score.match,score.mismatch)
}
else if (S[i,j] == (S[i-1,j] + score.gap)){
A.temp <- c(v[i-1],A)
B.temp <- c("-",B)
GA_rec(A.temp,B.temp,S,i-1,j,v,w,score.gap,score.match,score.mismatch)
}
else {
A.temp = c("-",A)
B.temp = c(w[j-1],B)
GA_rec(A.temp,B.temp,S,i,j-1,v,w,score.gap,score.match,score.mismatch)
}
}
return( GA_rec(A,B,S,length(v)+1,length(w)+1,v,w,score.gap,score.match,score.mismatch))
}
Semi-Global Alignment Algorithm
########### SEMI GLOBAL ALLIGNMENT ALGORITHM ############
SGA_score = function(sequence1,sequence2,score.gap=-1,score.match=1,score.mismatch=-1){
v=sequence2
w=sequence1
v = strsplit(v,split="")[[1]]
w = strsplit(w,split="")[[1]]
S = matrix(0,nrow=length(v)+1,ncol=length(w)+1)
for(i in 1:(length(w)+1)){
for( j in 1:(length(v)+1)){
if (i==1|j==1){S[i,j]=0}
else{
if((i==length(w)+1) | (j==length(v)+1)){
from.top = S[i,j-1]
from.left = S[i-1,j]
}
else{
from.top = max(S[i,j-1]+score.gap) # Max is artifact from max(0,... )
from.left = max(S[i-1,j]+score.gap)
}
if(w[i-1] == v[j-1]){
from.diag = S[i-1,j-1]+score.match
}
else{
from.diag = S[i-1,j-1]+score.mismatch
}
S[i,j] = max(from.top,from.left,from.diag)
}
}
}
return(S)
}
SGA_output = function(v,w,S,score.gap=-1,score.match=1,score.mismatch=-1){
v = strsplit(v,split="")[[1]]
w = strsplit(w,split="")[[1]]
A=c()
B=c()
print(str(A))
print(str(B))
SGA_rec = function(A,B,S,i,j,v,w,score.gap,score.match,score.mismatch){
if (i==1 | j==1){
if(i>1){
for(i1 in seq(i-1,1,-1)){
A = c(v[i1],A)
B = c("-",B)
}
}
if(j>1){
for(j1 in seq(j-1,1,-1)){
A = c("-",A)
B = c(w[j1],B)
}
}
print(A)
print(B)
out = list(v=A,w=B)
#print(out)
print(str(A))
print(str(B))
print(str(out))
return(out)
}
if(v[i-1]==w[j-1] ){diag = score.match} else {diag=score.mismatch}
if (S[i,j] == (S[i-1,j-1] + diag)){
A.temp = c(v[i-1],A)
B.temp = c(w[j-1],B)
SGA_rec(A.temp,B.temp,S,i-1,j-1,v,w,score.gap,score.match,score.mismatch)
}
#####
if ( j==length(w)+1) { # Are we in last row?
score.temp = score.gap
score.gap=0
}
else{score.temp=score.gap}
if(S[i,j] == (S[i-1,j] + score.gap)){
A.temp <- c(v[i-1],A)
B.temp <- c("-",B)
score.gap = score.temp
SGA_rec(A.temp,B.temp,S,i-1,j,v,w,score.gap,score.match,score.mismatch)
}
score.gap=score.temp
####
if(i==length(v)+1){
score.temp=score.gap
score.gap=0
}
else{score.temp=score.gap}
if(S[i,j] == (S[i,j-1] + score.gap)){
A.temp = c("-",A)
B.temp = c(w[j-1],B)
score.gap=score.temp
SGA_rec(A.temp,B.temp,S,i,j-1,v,w,score.gap,score.match,score.mismatch)
}
}
return(SGA_rec(A,B,S,length(v)+1,length(w)+1,v,w,score.gap,score.match,score.mismatch))
}
S1 = SGA_score("ACGTCAT","TCATGCA")
S1
align = SGA_output("ACGTCAT","TCATGCA",S1)
align
I am surpised that the global alignment works but the semi global one doesn't, even tough they both have this NULL part (can someone maybe explain what this is? Has it something to do with internal objects in a function?) and the semi global knows what A and B is.
Any help is greatly appreciated!

Answer 1

SGA_rec seems to be missing a return value. You need an else {return(<something>)) after the last if.
Illustration:
fun <- function() if (FALSE) 1
x <- fun()
x
#NULL
Read help("NULL") to learn what it means.