Program structure
Variables
Integer (-32768 to + 32767)
Real (e.g. 5.74e+03 = 5.74x103)
Char (e.g. 'A', '3', '!')
String (e.g. 'TL girl')
Boolean (e.g. TRUE, FALSE)
Basic statement
Assignment statement A := 1;
Assignment operator :=
Variable A is like a box in computer memory
Input statements
readln(X);
read(X);
Input value from keyboard and store in variable X
Output statements
write vs writeln
write('A');
write('B');
write('C');
writeln('A');
writeln('B');
writeln('C');
出字後，開新行
Output:
A
B
C
Output: ABC
Formatted output
writeln('ABC' :6); Output: ABC
wrtieln(1.2345 :6:2); Output: 1.23
Operators
Arithmetic
＋, 2 + 3 gives 5
–, 14 – 8 gives 6
*, 5 * 3 gives 15
/, 12 / 5 gives 2.4
div, 9 div 2 gives 4 (quotient)
mod, 9 mod 2 gives 1 (remainder)
Relational
= equal to
<> not equal to
< less than
> greater than
<= less than or equal to
>= greater than or equal to
Functions
Mathematical
Ordinal
Transfer
sqr(X), square, sqr(2.5) gives 6.25
sqrt(X), square root, sqrt(4) gives 2
abs(X), absolute value, abs(-3) gives 3
int(X), integer, int(5.67) gives 5.0
pi, 3.1415926...
odd(X), test for odd number, odd(3) gives TRUE
odd(4) gives FALSE
chr(X), return char with ASCII value X, chr(65) gives 'A'
ord('X'), return ASCII value of char 'X', ord('A') gives 65
round(X), round(4.56) gives 5
trunc(X), trunc(4.56) gives 4
Random number generator
random gives real number between 0 and 1
random(N) gives integer between 0 and N-1
random(B - A + 1) + A gives an integer between A and B inclusively
Selection statement
if statement
case statement (multiple selection)
case N of
2, 3, 5, 7: writeln('prime');
4, 6, 8..10 : writeln('composite')
else writeln('1 is special')
end;
if mark >= 50
then writeln('Pass');
if mark >= 50
then writeln('Pass')
else writeln('Fail);
if mark >= 50
then begin
writeln('Pass');
writeln('Good')
end
else writeln('Fail);
Compound statement in if statement
Nested if statement
if <condition 1>
then if <condition 2>
then <statement 1>
else <statement 2>
else if <condition 3>
then <statement 3>
else <statement 4>;
case N of
'a', 'e', 'i', 'o', 'u' : writeln('vowels');
end;
Using assignment stmt in case stmt
case mark of
0..49 : mark := mark + 20;
50..90 : mark := mark + 10
end;
Iteration statement
Definite loop
(fixed number of iterations)
Indefinite loop
(number of iterations not fixed)
while ... do loop 先問後做
useful for validation, e.g. while input number is not acceptable, do ask user to input again
For ... do loop
repeat ... until loop 先做後問
useful for validation, e.g. repeat input until a positive value is entered
Count up
for count := 1 to 5 do
write(count);
output: 12345
Count down
for count := 5 downto 1 do
write(count);
output: 54321
Compound stmt in for loop
for count := 1 to 3 do
begin
write('A');
write('B')
end;
output: ABABAB
Count characters
for ch := 'A' to 'E' do
write(ch);
output: ABCDE
click to edit
Summing up
sum := 0;
for count := 1 to 3 do
sum := sum + count;
writeln(sum);
output: 6
click to edit
Nested for loop
for i := 1 to 2 do
for j := 1 to 3 do
write(i, j);
output: 111213212223
Table format output
for row := 1 to 3 do
begin
for col := 1 to 4 do
write(row, col);
writeln;
end;
repeat
readln(X)
until X > 0;
while X <= 0 do
readln(X);
count := 0;
repeat
count := count + 1;
write(count)
until count >= 5;
count := 0;
while count < 5 do
begin
count := count + 1;
write(count)
end;
output: 12345
click to edit
click to edit
program myfirst;
const age = 15;
var i : integer;
begin
{ comment }
writeln('Hello');
writeln(age)
end.
output：
11121314
21222324
31323334
Strings
Declaration
var
x : string[10]; {store at most 10 chars}
y : string; { store at most 255 chars }
click to edit
Comparison
'a' > 'A'
'abc' > 'abd'
'XYZ' = 'XYZ'
'PQR' > 'PQ'
'X' <> 'Y'
click to edit
Functions
length
concat, +
copy
pos
length('abc') gives 3
length('1234') gives 4
'a' + 'b' + 'c' gives 'abc'
'X' + 'YZ' gives 'XYZ'
concat('X', 'YZ') gives 'XYZ'
Format: copy(string, start position, number of copy chars)
copy('TRUE LIGHT', 1, 4) gives 'TRUE'
copy('TRUE LIGHT', 6, 5) gives 'LIGHT'
Search for position of a char in a string
pos('E', 'TRUE LIGHT) gives 4
pos('A', 'TRUE LIGHT') gives 0
Procedures
STR
VAL
STR(123, strg);
stores '123' to strg
VAL('123', num, errorpos);
stores 123 to num, errorpos = 0 (because no error occurs)
VAL('123abc', num, errorpos);
stores 0 to num, errorpos = 4 (because error occurs at 4th char)
STR(3.1416 :4:2, strg);
stores 3.14 to strg
String as Array of characters
if S = 'TRUE LIGHT', then
S[1] = 'T'
S[4] = 'E'
S[5] = ' '
for count := 1 to length(S) do
write( S[count] );
output: TRUE LIGHT
Arrays
Declaration
mark : array[1..10] of integer;
grade : array[1..20] of char;
gradecount : array['A' ..'E'] of integer;
var score : array[1..40] of integer;
max := score[1];
min := score[1];
for count := 2 to 40 do
if score[count] > max then max := score[count];
if score[count] < min then min := score[count];
click to edit
Adding up values and finding average
var score : array[1..40] of integer;
sum := 0;
for count := 1 to 40 do
sum := sum + score[count];
average := sum/40;
click to edit
Searching an array
var name : array[1..40] of string;
found := false;
target := 'CHAN SIU LING';
for count := 1 to 40 do
if target = name[count] then found := true;
if found then writeln('name found!');
click to edit
2D array
Declaration
var X : array[1..2, 1..3] of char;
X[1, 2] \( \neq \) X[2, 1]
Tabular output
for row := 1 to 2 do
begin
for col := 1 to 3 do
write(X[row, col])
writeln;
end;
Tabular input
for row := 1 to 2 do
begin
for col := 1 to 3 do
read(X[row, col])
readln;
end;
Procedures
Declaration
Must be declared before main program
procedure <title>;
var <variables>;
begin
<statement>
end;
Variables
Global variable
Local variable
program test2;
var X : integer; { global var }
procedure addone;
begin
X := X + 1;
end;
begin { main program }
X := 10;
addone;
writeln(X)
end.
output: 11
program test1;
var X, Y : integer; { global var }
procedure swap;
var temp : integer; { local var }
begin
temp := X;
X := Y;
Y := temp
end;
begin { main program }
X := 1; Y := 2;
swap;
writeln(X, Y)
end.
output: 21
Parameters
Using parameters make procedures more standalone, more modular
program test3;
var X : integer;
procedure addone(Y : integer);
begin
Y := Y + 1;
end;
begin { main program }
X := 10;
addone(X);
writeln(X)
end.
output: 10
program test4;
var X : integer;
procedure addone(var Y : integer);
begin
Y := Y + 1;
end;
begin { main program }
X := 10;
addone(X);
writeln(X)
end.
output: 11
Advantages
- Breaking long programs into smaller manageable parts called modules, easier to write, test, debug, maintain
- More efficient program development
- Procedures/sub programs can be re-used
- Facilitate division of labour in a coding team, large projects can be developed in parallel
User-defined function
function cube(N : integer) : real;
begin
cube := N＊N＊N
end;
begin { main program }
writeln( cube(2) )
end.
output: 8.0