Algol Prime Finder
begin
comment Algol program print the primes less than 1000 using the
sieve method.;
Boolean array sieve[2:1000];
integer p, count;

comment Eliminate the multiples of the argument prime number;
procedure eliminate(p);
integer p;
begin
integer i;
for i := 2*p step p until 1000 do
sieve[i] := false
end;

comment Clear the sieve.;
integer i;
for i := 2 step 1 until 1000 do
sieve[i] := true;

comment Find the primes in range.;
for i := 2 step 1 until 1000 do
if sieve[i] then
eliminate(i);

comment Print the results, 10 per line.;
p := 2;
for count := 0 while p <= 1000 do begin
comment This construct is similar to a do .. while built with a goto.;
makeline:
if sieve[p] then begin
outinteger(1,p);
outstring(1," ");
count := count + 1
end;
p := p + 1;
if p <= 1000 & count < 10 then goto makeline;
outstring(1,"\n")
end

end

Notice that the semicolon is used to separate things, not as a terminator.

The keywords begin and end are similar to the curly braces in C and its descendants.

Notice how the type of the parameter to eliminate is specified: between the header and the start of the body. The C language originally used a notation something like this:

int f(i)
int i;
{
...
}
A standard plain-C compiler (not C++) should still accept this notation.