Analysing Character Strings for subsequent frequency analysis

Hi
I'm trying to get to grips with R and establish R as a teaching medium in my
secondary school. I would like to use R to analyse text so I can produce
frequency analysis of the text for subsequent examination of ciphers. I can
produce code in VBA but I am struggling when writing in R to examine each
character. There must be a clear method using the vectorised format of R.
Furthermore, how do you substr a text string and reference each letter? I
can use nchar to see how many letters per string but not to select each
letter. I would prefer to remain in R and not deviate to Python etc as
getting R onto the school mainframe has been a long journey...
Many thanks

Bob Stoner
Sleaford, Lincolnshire, UK