Probleme rezolvate.Tipul string

program numar_aparitii_MA;
var s:string; i,na:integer;
begin writeln('Dati sirul');
readln(s); for i:=2 to length(s) do
if s[i-1]+s[i]='MA' then na:=na+1;
writeln('numarul de aparitii a silabei ''MA'' este=',na);
readln
end.

program substituire_A;
var s:string; i:integer;
begin writeln('Dati sirul');
readln(s); for i:=1 to length(s) do
if s[i]='A' then s[i]:='*';
writeln('sirul obtinut este=',s);
readln
end.

program cifrare_sir;var s:string;
i:integer;begin writeln('Dati sirul');
readln(s); for i:=1 to length(s) do case s[i] of 'A'..'Y':s[i]:=succ(s[i]);
'Z':s[i]:='A'; ' ':S[I]:='_'; end; writeln('sirul cifrat este=',s);
readln
end.
program substituire_MA_cu_TA;
var s:string;
i:integer;begin writeln('Dati sirul'); readln(s); i:=2;
while i<=length(s) do if s[i-1]+s[i]='MA' then s[i-1]:='T'
else i:=i+1;
writeln('sirul obtinut este=',s); readln
end.

program numar_de_aparitii_A;
var s:string; i,na:integer;
begin writeln('Dati sirul');
readln(s); na:=0; for i:=1 to length(s) do
if s[i]='A' then na:=na+1;
writeln('Numarul de aparitii ale caracterului ''A'' este=',na);
readln
end.

program radiere_B;
var s:string; i:integer;
begin writeln('Dati sirul');
readln(s); for i:=1 to length(s) do
if s[i]='B' then delete(s,i,1);
writeln('sirul obtinut este=',s);
readln
end.