Off topic:nrow,ncol name,matrix name

Question

[gizemtatar]

My input;

seq1="GCATGCU"
seq2="GATTACA"

n=length(s1string) #seq1
m=length(s2string) #seq2

This matrix

D <- matrix(0,nrow=m+1, ncol=n+1)

seq1 and seq2 length = 7 but I must create matrix 8x8:

       [,1]  [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    0   -1   -2   -3   -4   -5   -6   -7
[2,]   -1    1    0   -1   -2   -3   -4   -5
[3,]   -2    0    0    1    0   -1   -2   -3
[4,]   -3   -1   -1    0    2    1    0   -1
[5,]   -4   -2   -2   -1    1    1    0   -1
[6,]   -5   -3   -3   -1    0    0    0   -1
[7,]   -6   -4   -2   -2   -1   -1    1    0
[8,]   -7   -5   -3   -1   -2   -2    0    0

I want to define matrix ncol with s1string, nrow= s2string but start colname and rowname [2, ], [,2]. For example, [,2]="G" [,3]="C" [,4]="A" [,5]="T" [,6]="G" [,7]="C" [,8]="T". I don't want define in this [1,1]. Can you help me for this topic?