Factorial in Pascal - iteratively

 <<algorithm

program fact_iter;
var n,s,i: integer;
begin
  write('n = ');
  readln(n);
  i:=1;
  s:=1;
  while i<=n do begin
    s:=s*i;
    i:=i+1;
  end;
  writeln(n,'! = ',s);
  readln;
end. 

The first line of the program can be treated as an embellishment, yet it is required by the Pascal language syntax. Every Pascal program must start with the keyword program preceding the program's name, and it contributes nothing to the algorithm's execution. The second line is auxiliary in nature. We know that our algorithm uses three variables storing integers. In Pascal, all variables must be declared before the start of the code block. When declaring a variable, we must specify the type of data it will store.

The third line, containing the keyword begin, corresponds to the START block (1) of our algorithm.

Lines 4 through 5 implement the input/output operation (2), where the input data is retrieved—the number for which we want to calculate the factorial function.

In line 6, we assign the initial value to the auxiliary variable i, which numbers the subsequent steps of the iterative loop of our algorithm, and in line 7, we assign the initial value of 1 to the variable s. Lines 6-7 execute step (3) of the algorithm.

The while statement is the equivalent of the selection block (4) from our algorithm. The instructions contained in lines 9-10 (block 5) will be executed as long as the condition in the while statement is met, meaning as long as i is less than or equal to n. When the constantly incremented i becomes greater than n, we proceed to the statements after the while loop, starting at line 12, where the result is printed (6).

The readln instruction on line 13 has no relation to the execution of the division algorithm (Note: the original Polish text incorrectly mentions "division algorithm" here, but the context is still about pausing the program). It serves to pause the program until the Enter key is pressed, which prevents the program window from closing immediately.

The instruction end. on line 14 concludes the program's execution (STOP block (7) of the algorithm).