string misbehaving for ifstream

This is a discussion on string misbehaving for ifstream within the C++ Programming forums, part of the General Programming Boards category; Code: int MP4010::get_rows( string& file_name ) { int count; int val; ifstream inFile(file_name); if(!inFile) { cerr<<"ERROR: The input file can ...

  1. #1
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    string misbehaving for ifstream

    Code:
    int MP4010::get_rows( string& file_name )
    {
    	int count;
    	int val;
    	ifstream inFile(file_name);
    	if(!inFile)	{
    		cerr<<"ERROR: The input file can not be opened."<<endl;
    		exit(1);
    	}
    	while (inFile) { 
    	    inFile.get(val);
    		count += val == '\n';
    	}
    
    	inFile.close();
    	return count;
    }
    Each time I try to compile I get the following 2 errors (lines 88 and 94 are highlighted in the code above):
    c:\...\mp_4010.cpp(88): error C2664: 'std::basic_ifstream<_Elem,_Traits>::basic_ifstrea m(const char *,std::_Iosb<_Dummy>:penmode)' : cannot convert parameter 1 from 'std::string' to 'const char *'
    with
    [
    _Elem=char,
    _Traits=std::char_traits<char>,
    _Dummy=int
    ]
    c:\...\mp_4010.cpp(94): error C2664: 'std::basic_istream<_Elem,_Traits>::_Myt &std::basic_istream<_Elem,_Traits>::get(_Elem &)' : cannot convert parameter 1 from 'int' to 'char &'
    with
    [
    _Elem=char,
    _Traits=std::char_traits<char>
    ]
    A reference that is not to 'const' cannot be bound to a non-lvalue
    I have no idea why this is happening. I am not converting between an int and a char, etc. Any ideas?

  2. #2
    The superhaterodyne twomers's Avatar
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    ifstream inFile(file_name.c_str());

    The function takes a char*, which <string>'s .c_str() member function returns.

  3. #3
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    cheers for the lightning response! that solved the first problem (line 88 fixed) but the other error still appears.

  4. #4
    (?<!re)tired Mario F.'s Avatar
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    > cannot convert parameter 1 from 'std::string' to 'const char *'

    It means you are feeding it an std::string and it can't just convert that to a const char*, which is what it expects.

    The error happens here:
    ifstream inFile(file_name);

    And you only need to feed ifstream constructor with a char*. That can be easily achieved because strings have a member function called c_str() that returns a char* representation of the string.

    ifstream inFile(file_name.c_str());

    The second error I will leave to you to figure out. I think you can now interpret it. Give it a shot. And see what you can do to fix it.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  5. #5
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    well i think if i replace int value; with char the 2nd error will be fixed. but i'm just wondering why i can't give it an int.

  6. #6
    MFC killed my cat! manutd's Avatar
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    Because the function doesn't take an int.
    Silence is better than unmeaning words.
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  7. #7
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    right right, thanks! but the function gives strange results. when i input the following txt file:
    1 2 3 4 5 6 5 4 3 2 1
    6 5 4 3 2 1
    11 2 3 55 3 22 3 4
    the function should return 3, but it returns 2. now when i add a new line at the end (an empty 4th line basically) it returns 4. i don't understand why it's doing so.

  8. #8
    (?<!re)tired Mario F.'s Avatar
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    Because it expects an int

    But also because C++ will not automagically convert an int to char. You have to tell it to do it. C++ wouldn't know what to do, the poor fella. What would you want? The screen repesentation of the int? Or use it as an ascii value of the character to display?

    http://www.cppreference.com/cppio/get.html

    If then you need to use that char as an int (do ome calculations or something) you can convert it to int with a function like atoi() or using stringstreams.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  9. #9
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    Is there a reason you don't use inFile >> val to read in the integers?

  10. #10
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    the function is supposed to return the # of lines within the txt file. I have tried using that but it didn't work (always returned 0), so I switched and now it's barely working (like I said, returns incorrect values).

  11. #11
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    In that case see my reply in the other thread for what I think is the best solution.

    A more low-level (beginner) solution is to get one character at a time and process it that way. When you read in a character that is a digit, then you can convert it to a number by subtracting the character '0' from it. So if you have a character ch and isdigit(ch) is true, you can convert it to an int with ch - '0'. Since your numbers can be multiple digits, you then have to combine them until you read in a non-digit. Combine them by multiplying the previous value by 10 and then adding the current digit.

    Using getline and stringstreams are easier and less error prone, but if you aren't allowed to use them you can try that method.

  12. #12
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    A more low-level (beginner) solution
    I don't seem to recall that "low-level" == "beginner"
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  13. #13
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    I got a php error when posting on this thread which caused duplicate entries, administrator.php for got the line number something about ';', i'd go look again but i might cause 10 more replies to show.

  14. #14
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    I don't seem to recall that "low-level" == "beginner"
    only 'n00bs' do ASM.

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