What causes problems with this piece of code?

Question

I was just wondering why this certain problem happens to me. If you can help me, I will appreciate it.

Program factorial;
uses crt;
var
  f, i: Integer;
begin
  f:=1;
  for i:=1 to 5 do
    f:= f * i;
  write(f);
  readkey;
end.

Okay, That works fine and the result is 120. And it's true.

Now, here's the problem. If I asked the user to enter the value of the number, it won't work.

Program factorial;
uses crt;
var
  i,r: Integer;
begin
  write('Enter the number');
  read(r);

  for i:=1 to r do
    r:= r * i;

  write(r);
  readkey;
end.

If I wrote 5, the output will be 600.

Answer 1

You are using the value r as the stopping condition of the loop and modifying the value in the loop.

Program factorial;
uses crt;
var
    i,r, f: Integer;
begin
  write('Enter the number');
  read(r);
  f := 1;
  for i:=1 to r do
     f:= f * i;
  write(f);
  readkey;
end.

Answer 2

You reuse the r variable. If you input 5 for r your program will in effect one to many times. You should start with 1 as the first f.

Program factorial;
uses crt;
var
    i,r, f: Integer;
begin
    write('Enter the number');
    read(r);
    f:=1
    for i:=1 to r do
        f:= f * i;
    write(r);
    readkey;
end.

Answer 3

try:

Program factorial;
uses crt;
var
i,r,x: Integer;
begin
write('Enter the number');
read(x);
r:=1
for i:=1 to x do
r:= r * i;
write(r);
readkey;
end.