Is there an explanation for the following pascal code? - pascal

Question

I wrote the following pascal code. Its main purpose is to convert a number given by letters to an integer given by digits using a function that does the opposite (convert a number given by digits to letters) :
Program number_to_letters;
Uses crt;
// beginning of the function convert
// the following function converts a number given by the user to letters.
// For 12 ==> 'Twelve'
Function convert(user_input:Integer) : String;
Var
units,tens,thousands,hundreds: Integer;
s: String;
Begin
units := user_input Mod 10;
tens := ( user_input Div 10) Mod 10;
hundreds := ( user_input Div 100 ) Mod 10;
thousands := user_input Div 1000;
//*** handling units first ***
case ( units ) of
1 : s := 'One';
2 : s := 'Two';
3 : s := 'Three';
4 : s := 'Four';
5 : s := 'Five';
6 : s := 'Six';
7 : s := 'Seven';
8 : s := 'Eight';
9 : s := 'Nine';
end;
//*** handling tens
Case ( tens ) Of
2 : s := 'Twenty' + s;
3 : s := 'Thirty' + s;
4 : s := 'Forty' + s;
5 : s := 'Fifty' + s;
6 : s := 'Sixty' + s;
8 : s := 'Eighty' + s;
end;
convert := s;
end;
//* end of the function **
// begin of the program
//** the program reveive a number from the user written with letters and converts it to a number using the Function above
var
user_input : string;
i:integer ;
begin
write('Enter your number : ');
readln(user_input);
for i:=1 to 80 do
begin
if (convert(i) = user_input ) then write(i);
end;
end.
I get the result I want for numbers between 20-80 except when I enter 20,30,40,50,60,80.
For example, when I go from 29 to 41 and I enter Thirty I get as result "ThirtyTwentyNine" instead of "Thirty" knowing that if I enter another number for example "TwentyNine" I get 29.
And if do writeln(convert(30)) I get "Thirty". So why it doesn't work inside the for loop?

Answer 1

It seems that Pascal clears the region of function variables before the first run, not before each run. Generally, Pascal is used for good programming, and it is not a good idea to make any operation on a variable of no assigned value (the situation when the last digit is equal to 0). Because "s" is used in the case of tens, it should have a value, i.e. s:='' should be written somewhere before, e.g. before the case of units.
By the way, it is an interesting side effect - declared but not assigned variable keeps a value from the previous run of the function.

Answer 2

In the case (units) of in the function, you can add 0 as a case label with an empty string.
So you can get the result when you enter 20,30,40,50,60,80, because these numbers will produce 0 when you mod by 10 on units variable. (units := user_input Mod 10;)
0 : s := '';
Like this
case ( units ) of
0 : s := '';
1 : s := 'One';
2 : s := 'Two';
3 : s := 'Three';
4 : s := 'Four';
5 : s := 'Five';
6 : s := 'Six';
7 : s := 'Seven';
8 : s := 'Eight';
9 : s := 'Nine';
end;
Then in the Case ( tens ) Of, you can add 1 as a case label with s+'teen' to handle 14-19 (except 15). And you can also add 7 as a case label, to avoid being printed twice when user_input = 1-9 and 70-79.
1 : s := s+'teen';
7 : s := 'Seventy' + s;
Like this
Case ( tens ) Of
1 : s := s+'teen';
2 : s := 'Twenty' + s;
3 : s := 'Thirty' + s;
4 : s := 'Forty' + s;
5 : s := 'Fifty' + s;
6 : s := 'Sixty' + s;
7 : s := 'Seventy' + s;
8 : s := 'Eighty' + s;
end;
Then for the number 10, 11, 12, 13, 15, you can use a separate case..of statement.
Like this
case (user_input) of
10 : s:= 'Ten';
11 : s:= 'Eleven';
12 : s:= 'Twelve';
13 : s:= 'Thirteen';
15 : s:= 'Fifteen';
end;
This is the full code
Program number_to_letters;
Uses crt;
Function convert(user_input:Integer) : String;
Var
units,tens,thousands,hundreds: Integer;
s: String;
Begin
units := user_input Mod 10;
tens := ( user_input Div 10) Mod 10;
hundreds := ( user_input Div 100 ) Mod 10;
thousands := user_input Div 1000;
//*** handling units first ***
case ( units ) of
0 : s := '';
1 : s := 'One';
2 : s := 'Two';
3 : s := 'Three';
4 : s := 'Four';
5 : s := 'Five';
6 : s := 'Six';
7 : s := 'Seven';
8 : s := 'Eight';
9 : s := 'Nine';
end;
//*** handling tens
Case ( tens ) Of
1 : s := s+'teen';
2 : s := 'Twenty' + s;
3 : s := 'Thirty' + s;
4 : s := 'Forty' + s;
5 : s := 'Fifty' + s;
6 : s := 'Sixty' + s;
7 : s := 'Seventy' + s;
8 : s := 'Eighty' + s;
end;
case (user_input) of
10 : s:= 'Ten';
11 : s:= 'Eleven';
12 : s:= 'Twelve';
13 : s:= 'Thirteen';
15 : s:= 'Fifteen';
end;
convert := s;
end;
var
user_input : string;
i:integer ;
begin
write('Enter your number : ');
readln(user_input);
for i:=1 to 80 do
begin
if (convert(i) = user_input ) then write(i);
end;
readln;
end.
You can convert 1 - 80