Counting frequency of amino acids at each position in multiple-sequence alignments - r

Question

I'm wondering if anyone knows any tools which allow me to count the frequency of amino acids at any specific position in a multiple-sequence alignment.
For example if I had three sequences:
Species 1 - MMRSA
Species 2 - MMLSA
Species 3 - MMRTA
I'd like for a way to search by position for the following output:
Position 1 - M = 3;
Position 2 - M = 3;
Position 3 - R = 2, L = 1;
Position 4 - S = 2, T = 1;
Position 5 - A = 3.
Thanks! I'm familiar with R and Linux, but if there's any other software that can do this I'm sure I can learn.

Answer 1

Using R:
x <- read.table(text = "Species 1 - MMRSA
Species 2 - MMLSA
Species 3 - MMRTA")
ixCol = 1
table(sapply(strsplit(x$V4, ""), "[", ixCol))
# M
# 3
ixCol = 4
table(sapply(strsplit(x$V4, ""), "[", ixCol))
# S T
# 2 1
Depending input file format, there are likely a purpose built bioconductor packages/functions.

Answer 2

That is really easy to parse, you can use any language of choice.
Here is an example in Python using a dict and Counter to assemble the data in a simple object.
from collections import defaultdict, Counter
msa = '''
Species 1 - MMRSA
Species 2 - MMLSA
Species 3 - MMRTA
'''
r = defaultdict(list) #dictionary having the sequences index as key and the list of aa found at that index as value
for line in msa.split('\n'):
line = line.strip()
if line:
sequence = line.split(' ')[-1]
for i, aa in enumerate(list(sequence)):
r[i].append(aa)
count = {k:Counter(v) for k,v in r.items()}
print(count)
#{0: Counter({'M': 3}), 1: Counter({'M': 3}), 2: Counter({'R': 2, 'L': 1}), 3: Counter({'S': 2, 'T': 1}), 4: Counter({'A': 3})}
To print the output as you specified:
for k, v in count.items():
print(f'Position {k+1} :', end=' ') #add 1 to start counting from 1 instead of 0
for aa, c in v.items():
print(f'{aa} = {c};', end=' ')
print()
It prints:
Position 1 : M = 3;
Position 2 : M = 3;
Position 3 : R = 2; L = 1;
Position 4 : S = 2; T = 1;
Position 5 : A = 3;