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File window character eating
Posted on 1999-01-18
I'm trying to get into Pascal and I'm having some trouble understanding the file window.
1/ I want to skip leading non-digits from an input stream and I have the following code
read (c);
while not c in ['0'..'9'] do read (c);
This isn't working, properly, I'm not processing all the characters it seems. I'm not sure how to rewrite that loop using the input window. I think I must need it because I think my read statement is going too far.
2/ I'm trying to use the file window to slip blanks, like so
while input^ <> ' ' do read (c);
This sometimes crashes. What am I doing wrong?
1/ I want to skip leading non-digits from an input stream and I have the following code
read (c);
while not c in ['0'..'9'] do read (c);
This isn't working, properly, I'm not processing all the characters it seems. I'm not sure how to rewrite that loop using the input window. I think I must need it because I think my read statement is going too far.
2/ I'm trying to use the file window to slip blanks, like so
while input^ <> ' ' do read (c);
This sometimes crashes. What am I doing wrong?
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2 Comments
1/ You're right, your input goes too far because when you stop reading, you've already given c the first digit of the number. You do need to use the file window as you suspected, like this -
while not (input^ in ['0' .. '9'] do
read (c);
2/ I expect your code crashes on empty files, or if the file is padded with blanks - because you will try to read past the end of the file.
Try this routine -
Finished := False;
repeat
if eof then
Finished := True
else if input^ = ' ' then
read (c)
else
Finished := True
until Finished;
This is a bit more complicated, but more robust !
Were these perhaps homework questions? They actually are examples from a Pascal book called 'Oh, Pascal' ! That takes me back to COMP101 ....
while not (input^ in ['0' .. '9'] do
read (c);
2/ I expect your code crashes on empty files, or if the file is padded with blanks - because you will try to read past the end of the file.
Try this routine -
Finished := False;
repeat
if eof then
Finished := True
else if input^ = ' ' then
read (c)
else
Finished := True
until Finished;
This is a bit more complicated, but more robust !
Were these perhaps homework questions? They actually are examples from a Pascal book called 'Oh, Pascal' ! That takes me back to COMP101 ....
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