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How to read in a single digit integer from file

Posted on 2003-03-15
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Last Modified: 2012-05-05
I have a file of a very long integer like : 43242348913123141341321487398432
How do I using ifstream to read in only 1 digit at a time?
I tried
inFile >> number; but i think it reads in the whole line and does not give me a single digit number.
I have tried using getline first to store it in array, but I cannot get the correct integer value from the array because they'd be stored in char.

Is there a simple way to get just 1 digit at a time, without having to store it in char array then somehow convert it back to int again?

thank you very much
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Question by:lordiano
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7 Comments
 
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Accepted Solution

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limestar earned 105 total points
ID: 8142833
What you want is actually to read in one character at a time. When you have a character, say '5', it can easily be converted to an integer with the value 5 by doing something like this:

char digitChar = '5';
int digit = digitChar - '0';

Things will go wrong if you read an 'A' or something, but if you know your input is '0'-'9', then the above method works fine.

Now that you have a method for converting a character to a digit it's up to you if you want to read one character at a time or read them all into an array first.
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Author Comment

by:lordiano
ID: 8142841
How do I read 1 character at a time?
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Author Comment

by:lordiano
ID: 8142853
How do I read 1 character at a time?
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Expert Comment

by:fivesigmaevent
ID: 8142863
lordiano,

That number in your file is much too large to store in an int. A signed 32 bit integer can only hold a number as large as 2147483648 and 32 bit unsigned int is double that.

For extremely large numbers that require calculations, you can just retrieve the number as a string of characters and write a class with member functions that performs math on that long string of digit chars.

I know you wanted something simpler, but I don't think it's possible any other way with such large numbers.
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LVL 4

Expert Comment

by:n_fortynine
ID: 8142899
fivesigmaevent's is a good suggestion. If you really just want to get one at a time, use cin.get(ch) with ch being a character, then convert that character to an int the way limestar said.
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Expert Comment

by:fivesigmaevent
ID: 8142938
I guess I misunderstood what you are trying to accomplish exactly. From your first sentence

"I have a file of a very long integer like : 43242348913123141341321487398432"

It sounded like you wanted to store the entire value as an int. It is impossible to store this in a primitive int variable.
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Author Comment

by:lordiano
ID: 8142956
Thanks fivesigmaevent and n_fortynine for your input.
Appreciate the help, thanks again
And Limestar, thanks for your comment it works flawlessly.
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