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S.3 Pascal Notes4
Shatin Tsung Tsin Secondary School
S.3 Computer and Technology
Pascal Programming
Selection Statement in Pascal (If…then…else…)
Consider solving quadratic equation.
x2 + 3x + 2 = 0
(x + 1)(x +2) = 0
x = -1 or x = -2
We may solve the equation by “Quadratic Equation Formula”
x1 =
2
− b + b 2 − 4ac − 3 + 3 − 4(1)(2) − 3 + 1 − 3 + 1
=
=
=
= −1
2a
2(1)
2
2
− 3 − 3 2 − 4(1)(2) − 3 − 1 − 3 − 1
− b − b 2 − 4ac
x2 =
==
=
=
= −2
2a
2(1)
2
2
Ex.1
program quadratic;
var
a, b, c : integer;
x1, x2 : real;
begin
writeln(‘Input the coefficient of quadratic equation’);
write(‘a = ’);
readln(a);
write(‘b = ’);
readln(b);
write(‘c = ’);
readln(c);
x1 := (-b + sqrt(b*b – 4*a*c))/2/a;
x2 := (-b – sqrt(b*b – 4*a*c))/2/a;
writeln;
writeln(‘the roots of equation’);
writeln(‘x1 = ’,x1:0:1);
writeln(‘x2 = ’,x2:0:1);
end.
Save the file as “quad1.pas” and press [Ctrl] + [F9] to execute the program
But in some cases, we cannot solve the equations. e.g.
x2 + x + 1 =0
There is no solution for x. It is because:
− 1 + 12 − 4(1)(1) − 3 + − 3
− b + b 2 − 4ac
==
=
x=
2a
2(1)
2
x=
− 1 − 12 − 4(1)(1) − 3 − − 3
− b − b 2 − 4ac
==
=
2a
2(1)
2
We cannot find
−3
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S.3 Pascal Notes4
That is when b2 – 4ac < 0, we cannot solve the equation and there will be no solution for x.
Logical decision:
If b2 – 4ac < 0 then writeln(‘no solution’);
else calculate the roots
true
b2-4ac<0
false
Ex.2
program quadratic;
writeln(‘no
var
solution’);
a, b, c : integer;
x1, x2 : real;
begin
writeln(‘Input the coefficient of quadratic equation’);
write(‘a = ’);
readln(a);
write(‘b = ’);
readln(b);
write(‘c = ’);
readln(c);
if (b*b – 4*a*c) < 0
then begin
writeln;
writeln(‘no solution’);
end
else begin
x1 := (-b + sqrt(b*b – 4*a*c))/2/a;
x2 := (-b – sqrt(b*b – 4*a*c))/2/a;
writeln;
writeln(‘the roots of equation’);
writeln(‘x1 = ’,x1:0:1);
writeln(‘x2 = ’,x2:0:1);
end;
end.
Save the file as “quad2.pas” and press [Ctrl] + [F9] to execute the program
Ex.3
program pos_or_neg;
var
a : integer;
begin
writeln(‘Input a number’);
readln(a);
if a >= 0 then writeln (‘It is a positive number’);
else writeln (‘It is a negative number’);
end.
Save the file as “test1.pas” and press [Ctrl] + [F9] to execute the program
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Calculate the
roots
S.3 Pascal Notes4
Homework:
Write a program to decide an input mark of student is ‘pass’ or ‘fail’
i) Define the variable “mark” as integer
ii) Input the value of mark
iii) If the mark is less than 50, it is fail. Otherwise, it is pass.
iv) The output should be as follows:
Input a mark
Input a mark
37
70
fail!
pass!
Save the file as “assig04.pas” and press [Ctrl] + [F9] to execute and test the program
Ex.4 program CalculateWages;
const
bonus = 500;
var
day_of_abs, wages : integer;
begin
wages := 4000;
writeln(‘Enter the number of days absent’);
readln(day_of_abs);
if day_of_abs = 0
then wages := wages + bonus;
writeln(‘The wages are $’, wages,);
end.
Save the file as “wage.pas” and execute it.
day_of_abs
=0
wages:=wages
+ bonus;
false
writeln(‘The
wages are …’);
Nested if…then…else statement
Ex. 5
program pos_zero_neg;
var a : integer;
begin
writeln(‘Input a number’);
readln(a);
if a = 0 then writeln (‘It is zero)
else if a < 0 then writeln (‘It is negative’)
else writeln (‘It is positive);
end.
Save the file as “test2.pas” and press [Ctrl] + [F9] to execute the program
Relational operators
Relational operator Meaning
=
equal
<>
not equal to
>
greater than
>=
greater than or equal to
<
less than
<=
less than or equal to
Compound Boolean expression
p
q
true
true
true
false
false
true
false
false
Boolean expression
‘A’ = ‘a’
‘A’ <> ‘a’
3>1
3 >= 1
3<1
3 <= 1
p and q
true
false
false
false
p.3/4
true
Truth value
false
true
true
true
false
false
p or q
true
true
true
false
S.3 Pascal Notes4
p
true
false
not p
false
true
Order of precedence
Operator
not, *, /, div, mod, and
+, -, or
=, <>, >, >=, <, <=
Priority
1 (highest)
2
3
4 (lowest)
e.g. 25 + 3 * 10
25 + 30
55
true
>= 50
>= 50
>= 50
e.g. not true or true and false
false
or true and false
false
or false
false
e.g. not (4 <= 5)
not true
false
false
and (3 <> 2)
and true
and true
e.g. ((‘c’ < ‘a’)
(false
false
false
or (‘c’ > ‘f’)) and (‘c’ <> ‘u’)
or false)
and true
and true
Ex. 6
program mark_grade;
var mark : integer;
begin
write(‘Enter exam mark: ‘);
readln(mark);
if mark >= 90
then writeln (‘A’);
if (mark >= 80) and (mark < 90)
then writeln (‘B’);
if (mark >= 70) and (mark < 80)
then writeln (‘C’);
if (mark >= 60) and (mark < 70)
then writeln (‘D’);
if (mark >= 50) and (mark < 60)
then writeln (‘E’);
if mark < 50
then writeln (‘F’);
end.
Save the file as “grade.pas” and press [Ctrl] + [F9] to execute the program
Homework:
Write a program to decide the biggest number from 3 input numbers
i)
Define the variable a, b and c as integer
ii)
Input the value of a, b and c
iii)
Decide which is the biggest one
iv)
The output should be as follows:
Input three numbers
Input three numbers
Input three numbers
147
492
826
The biggest number is 7
The biggest number is 9
The biggest number is 8
Save the file as “assig05.pas” and press [Ctrl] + [F9] to execute and test the program
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