This function counts the occurrence of each of the 20 amino acids
at each of the first 'n' positions across a vector of peptide sequences.
Usage
count_aa(peptides, n = 4)
Value
A data frame with 'n' rows and 20 columns where each row represents a position
in the peptide sequence and each column represents an amino acid. Each cell
in the data frame contains the count of a particular amino acid at a particular position.
Arguments
peptides
A character vector of peptide sequences.
n
The number of initial positions to consider in each peptide sequence.