Palindromic numbers problem

This is a discussion on Palindromic numbers problem within the C Programming forums, part of the General Programming Boards category; I wrote a program to find the largest palindromic number one could produce as the product of two three digit ...

  1. #1
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    Palindromic numbers problem

    I wrote a program to find the largest palindromic number one could produce as the product of two three digit numbers.

    Code:
    int main (void)
    {
    	int i, j, z;
    	int num;
    	char buf[6];
    	char test[6];
    
    	for(i=100; i<=999; i++)
    	{
    		for(j=100; j<=999; j++)
    		{
    			num = i * j;
    
    			sprintf(buf, "%d", num);
    
    			for(z=0; z<=6; z++)
    				test[z]=buf[z];
    			
    			int len=strlen(buf);
    
    			for(z=0; z<len/2;z++)
    			{
    				test[z]^=test[len-z-1];
    				test[len-z-1]^=test[z];
    				test[z]^=test[len-z-1];
    			}
    
    			/*puts(buf);*/
    			/*puts(test);*/
    			/*printf("\n");*/
    			if(test == buf)
    				puts(buf);
    
    		}
    	}
    
    	return 0;
    }
    I'm having problems withe the
    Code:
    if(test == buf)
    part. I know the program produces the correct answer (906609), but when I run it, nothing ever ends up being printed. Should I be comparing the two arrays differently?

  2. #2
    C++ Witch laserlight's Avatar
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    This accesses the arrays out of bounds because of the use of <= instead of < :
    Code:
    for(z=0; z<=6; z++)
        test[z]=buf[z];
    Actually, you have another problem. 999 * 999 = 998001, which when converted to a numeric string would require 6+1=7 characters, so your arrays are not big enough.

    Also:
    Code:
    test[z]^=test[len-z-1];
    test[len-z-1]^=test[z];
    test[z]^=test[len-z-1];
    Use a conventional swap involving a "temporary" variable instead.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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