converting char to int

This is a discussion on converting char to int within the C++ Programming forums, part of the General Programming Boards category; hello I want to convert a char array containing only numbers like char aa[50]="123456789" into a int with the value ...

  1. #1
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    converting char to int

    hello

    I want to convert a char array containing only numbers like char aa[50]="123456789" into a int with the value 123456789.

    i thought that i could to it by simply going: number in place* 10^place, and reapeating that for every place: like so 9+8*10^2=89 , 89+7*10^2=789, 789+6*10^3=6789 etc

    i coded the following to do this:

    Code:
    int char_to_int ( char IT[100] )
    {
    	cout<<"----debug int char_to_int ( char IT[100] )----\n";
    	cout<<"IT = "<<IT<<endl;
    	int result=0;
    	int bb = 0;
    	int aa = char_length ( IT );
    	cout<<"char_length ( IT ) = "<< char_length ( IT ) <<endl;
    	while ( aa > 0 )
    	{
    		if ( bb == 0 )
    		{
    			cout<<"IT[aa] = "<<IT[aa]<<endl;
    			cout<<"IT[aa]-48 = "<<IT[aa] - 48<<endl;
    			result = result + ( IT[aa] - 48 );
    			cout<<"result = "<<result<<endl;
    			aa--;
    			bb++;
    		}
    		else
    		{
    			cout<<"IT[aa] = "<<IT[aa]<<endl;
    			cout<<"IT[aa]-48 = "<<IT[aa] - 48<<endl;
    			result = result + ( IT[aa] - 48 ) * 10^bb;
    			cout<<"result = "<<result<<endl;
    			aa--;
    			bb++;
    		}
    	}
    	cout<<"----debug int char_to_int ( char IT[100] ) END----\n";
    	return result;
    }
    but it dosent work (surprised)

    this is the output:

    Code:
    ----debug int char_to_int ( char IT[100] )----
    IT = 112233
    char_length ( IT ) = 6
    IT[aa] =
    IT[aa]-48 = -48
    result = -48
    IT[aa] = 3
    IT[aa]-48 = 3
    result = -17
    IT[aa] = 3
    IT[aa]-48 = 3
    result = 15
    IT[aa] = 2
    IT[aa]-48 = 2
    result = 32
    IT[aa] = 2
    IT[aa]-48 = 2
    result = 48
    IT[aa] = 1
    IT[aa]-48 = 1
    result = 63
    ----debug int char_to_int ( char IT[100] ) END----
    look at the out put see this line IT[aa] = why isent there anything printed there?

    thanks for your help.

    EDIT: i know this method wont work if the input includes a 0
    Last edited by IM back!; 11-27-2008 at 02:59 PM.

  2. #2
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    In C, x ^ y doesn't do x to the power of y. You can do that with the cmath function pow(), but for converting digits, it's actually easier to just start from the left, grab one digit, multiply the result so far by 10, and add the current digit. Keep going until there is no more digits.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    C++ Witch laserlight's Avatar
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    It would be easier to use a stringstream, e.g.,
    Code:
    char aa[50] = "123456789";
    std::stringstream ss(aa);
    int num;
    ss >> num;
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  4. #4
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    Quote Originally Posted by laserlight View Post
    It would be easier to use a stringstream, e.g.,
    Code:
    char aa[50] = "123456789";
    std::stringstream ss(aa);
    int num;
    ss >> num;
    Of course! However, I got the impression that the original post was some sort of asignment or such, and that such a solution is "cheating". I could be wrong.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  5. #5
    C++まいる!Cをこわせ!
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    Or my favorite, boost::lexical_cast:
    Code:
    int x = boost::lexical_cast<int>("12345678");
    That is, if you are allowed to use these shortcuts, and it is not an assignment.
    It's part of the Boost library (boost.org).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    Cat without Hat CornedBee's Avatar
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    atoi is about four lines of code. matsp described the algorithm.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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