Implementation of the algorithm solving a quadratic equation in Pascal

 <<algorithm

alg square en

program Quadratic_equation;
procedure linear(a: double; b: double);
var x: double;
begin
  if a=0 then
    if b=0 then
      writeln('identity equation')
    else
      writeln('contrary equation')
  else begin
    x:=-b/a;
    writeln('Linear equation, x = ',x);
  end;
end;
var a,b,c,delta,x,x1,x2: double;
begin
  write('a = ');
  readln(a);
  write('b = ');
  readln(b);
  write('c = ');
  readln(c);
  if a=0 then 
    linear(b,c)
  else begin
    delta:=b*b-4*a*c;
    if delta<0 then
      writeln('lack of solutions')
    else
    if delta=0 then begin
      x:=-b/(2*a);
      writeln('one solution x = ',x);
    end else begin
      x1:=(-b+sqrt(delta))/(2*a);
      x2:=(-b-sqrt(delta))/(2*a);
      writeln('x1 = ',x1);
      writeln('x2 = ',x2);
    end;
  end;
end.

Program line Description Block algorithm
 1 Every program in the pascal must start with the keyword program preceding the name of the program, it adds nothing to the implementation of the algorithm. -
3 - 15 We declare and define a linear procedure that implements the algorithm of solving a linear equation. 4
17 We declare all variables used in the main program. In a pascal, we must do this before the keyword begin, which starts the operating part of the code. -
18 The beginning of the program code. 1
19 - 24 On the screen, we print the prompts for the user and load the values of the coefficients a, b and c entered into the corresponding, previously declared, variables. 2
25 We check if the coefficient a is equal to 0. If this is the case with the linear equation. 3
26 In this line we will only find ourselves if a = 0, ie when the equation is a linear equation. We invoke procedure linear(b, c) that realizes the algorithm of solving a linear equation. It is worth noting that the coefficient at the first power of the variable x is marked with the symbol b, and the free expression c (traditionally in the linear equation, a and b are used respectively). 4
27 else, i.e. otherwise. At this point, the solution of the equation begins, when a ≠ 0, in other words when we actually deal with a quadratic equation. -
28 We calculate the value of the delta variable. 5
29 We check if delta < 0. 6
30 When delta < 0, we print an appropriate message about the lack of solutions. 7
 31 Here the program block begins when the delta is not less than zero, ie when the equation has solutions (one or two). -
32 We check if delta = 0. 8
33 We calculate and store in the x variable the solution of the equation (single, double - delta=0). 9
34 We display the calculated result. 10
35 We start the program block executed in the situation when the condition delta = 0 (and earlier delta <0 and a ≠ 0) was not met. -
36 - 37 We calculate solutions and their values we assign to variables x1 and x2. 11
38 - 39 We display the calculated result - two solutions of the quadratic equation. 12
40 - 41 We close the blocks of compound instructions. -
42 The end of the program implementation. 13