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if statements causing program to crash

This is a discussion on if statements causing program to crash within the C++ Programming forums, part of the General Programming Boards category; My program crashes on runtime. It can compile, but crashes at .exe. But everything works, right up to until I ...

  1. #1
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    if statements causing program to crash

    My program crashes on runtime. It can compile, but crashes at .exe. But everything works, right up to until I run the following code:
    Code:
    cout<< "centroid y: "<<trialone.centroidy()<<endl;
    the method centroidy is defined as follows:
    Code:
    double EarthCalc::centroidy()
    {
        double firstmoment = 0.0;
        double mass1 = 0.0;
        double mass2 = 0.0;
        double count = 2159;
    
    
        for (int i = 0; i<colNum; i++ )
        {
            count--;
    
    
            for ( int j = 0; j<rowNum; j++ )
            {
                
                if ( array[i][j] > 0 )
                {
                    mass1 = mass1 + (LONDISTcalc(count/12.0 - 90)) *(LATDISTcalc(count/12.0 - 90))*(array[i][j])*2.5;
                }
                if ( array[i][j] < 0 )
                {
                    mass2 = mass2 + (LONDISTcalc(count/12.0 - 90) * LATDISTcalc(count/12.0 - 90) *array[i][j]);
                }
    
    
                firstmoment = firstmoment + (j/12.0 - 90)*LONDISTcalc(count/12.0 - 90)*LATDISTcalc(count/12.0 - 90);
            }
        }
        return (firstmoment/(mass1 + mass2));
    }
    Ive narrowed the problem down to the 2 if statements. For some reason, the if statements crash the program, even though I checked the code like 50 times and saw nothing wrong. I am accessing a huge array of like 18 MB, but still, I accessed that array before in other methods of the class as well. But it didnt crash. WTF?

    Anyways, I've also attached the 3 files with all the code as well. If anyone can help me, thanks.

    UPDATE: I fixed the code by deleting the if statements entirely. I used some of the other methods in the class to work around using ifs and only using the fors. But it would be still nice if someone can tell me why it crashed me
    Attached Files Attached Files
    Last edited by mantracker; 11-10-2012 at 07:23 PM.

  2. #2
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    if statements don't cause a program to crash. Mistakes by programmers do. Always.

    I haven't even bothered to look at the code. The fact you are "accessing a huge array" gives an indication of where your problem is.


    Look at the code that creates (or allocates) the arrays, and sets the values of rowNum and colNum. For an array access to work, the array must be successfully created with the right size (or sizes, since you are using two-dimensional indices) and the recorded dimensions need to be correct.

    If the array was created on the stack, it is quite possibly larger than the stack. That might be one cause. Similarly, if the array is created globally - it may be too large. Reduce the array dimensions (say to a 3x3 array) and see if the code runs. If so, you are exhausting capabilities of your host system. The only solution is to make the arrays smaller, and use some smarter (less lazy) technique for processing your data that does not require such large arrays.

    If you are using dynamic memory allocation (operator new in C++ or malloc() if you are using C techniques) to create the array, then you need to check that the allocation succeeded. If the allocation failed, then then you should not be accessing array elements.
    Right 98% of the time, and don't care about the other 3%.

  3. #3
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    Quote Originally Posted by grumpy View Post
    if statements don't cause a program to crash. Mistakes by programmers do. Always.

    I haven't even bothered to look at the code. The fact you are "accessing a huge array" gives an indication of where your problem is.


    Look at the code that creates (or allocates) the arrays, and sets the values of rowNum and colNum. For an array access to work, the array must be successfully created with the right size (or sizes, since you are using two-dimensional indices) and the recorded dimensions need to be correct.

    If the array was created on the stack, it is quite possibly larger than the stack. That might be one cause. Similarly, if the array is created globally - it may be too large. Reduce the array dimensions (say to a 3x3 array) and see if the code runs. If so, you are exhausting capabilities of your host system. The only solution is to make the arrays smaller, and use some smarter (less lazy) technique for processing your data that does not require such large arrays.

    If you are using dynamic memory allocation (operator new in C++ or malloc() if you are using C techniques) to create the array, then you need to check that the allocation succeeded. If the allocation failed, then then you should not be accessing array elements.
    If you look at some of my other methods in the class, you would notice that the array was accessed in the exactly same way: "If ( array[i][j] > 0 )", none of those caused me to crash, so that's what confused me. It is an array of size 2196x4000 something, but still you should be able to check it w/o pointers right?

  4. #4
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    You have your heart set on the belief that the problem is with the particular "if" statement, and don't want to accept advice that the problem occurs before that, eh?

    The fact that a statement ("if") or expression (array[i][j] > 0) causes a crash in one place, but (under similar conditions, such as the same values of i and j) doesn't cause a crash, is a sign that you're looking in the wrong spot.

    A common mistake for beginners, which you are making, is to believe that the code where a crash occurs is also the cause of the crash. In reality, any and call code code, and code interactions, executed before the crash occurs are potential contributors to it.

    But, okay, you can insist that the problem is with the if statement, and persist in looking there. It's your time to waste.

    When problems occur with arrays, the root cause is most often with dimensions (eg trying to access 10th element of a 3 element array) or with creation of the array (eg the system can't allocate all memory the array needs, so code which accesses array elements eventually - not immediately - fails).
    rags_to_riches likes this.
    Right 98% of the time, and don't care about the other 3%.

  5. #5
    and the hat of wrongness Salem's Avatar
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    Use a debugger!
    Code:
    $ g++ -g DTM.cpp DTM-driver.cpp
    $ gdb ./a.out 
    GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
    Copyright (C) 2011 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
    and "show warranty" for details.
    This GDB was configured as "x86_64-linux-gnu".
    For bug reporting instructions, please see:
    <http://bugs.launchpad.net/gdb-linaro/>...
    Reading symbols from /tmp/a.out...done.
    (gdb) run
    Starting program: /tmp/a.out 
    Water percentage :-nan
    Water volume:0
    Highest elevation:0
    Lattitude highest: 2.96439e-323
    Longitutde highest: 3.11455e-317
    Deepest elevation: 0
    Lattitude lowest: 2.07378e-317
    Longitutde lowest: 0
    Average elev: 0
    Average above sea elev: -nan
    
    Program received signal SIGSEGV, Segmentation fault.
    0x000000000040195a in EarthCalc::centroidy (this=0x7fffffffe100) at DTM.cpp:188
    188				if ( array[i][j] > 0 )
    (gdb) print i
    $1 = 2176
    (gdb) print j
    $2 = 0
    Woah - 2176!?
    But you have
    static const int rowNum = 2160;

    A large number of your for loops are whacked.
    Code:
    $ grep 'for' DTM.cpp
      for( int i=0; i < columns; i++)
    	  for( int j=0; j < rows; j++ )
    	for (int i = 0; i<rowNum; i++ )
    		for(int j = 0; j<colNum; j++ )
    	for(int i = 0; i<rowNum; i++ )
    		for(int j = 0; j<colNum; j++ )
    	for(int i = 0; i<rowNum; i++ )
    		for(int j = 0; j<colNum; j++ )
    	for(int i = 0; i<rowNum; i++ )
    		for(int j = 0; j<colNum; j++ )
    	for( int i = 0; i<rowNum; i++ )
    		for( int j = 0; j<rowNum; j++ )
    	for ( int i = 0; i<rowNum; i++ )
    		for( int j = 0; j<rowNum; j++ )
    	for (int i = 0; i<colNum; i++ )
    		for ( int j = 0; j<rowNum; j++ )
    	for (int i = 0; i<colNum; i++ )
    		for ( int j = 0; j<rowNum; j++ )
    It's supposed to be row by col, not
    row by row
    or
    col by row
    stahta01 likes this.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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