char confusion

This is a discussion on char confusion within the C Programming forums, part of the General Programming Boards category; I'm trying to find out the ranges of different data types through direct computation........this is what I did....... Code: #include ...

  1. #1
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    Post char confusion

    I'm trying to find out the ranges of different data types through direct computation........this is what I did.......

    Code:
    #include <stdio.h>
    main()
    {
    	char	ch;
    	signed short	sh;
    	signed int	in;
    	signed long	lo;
    	unsigned short	ush;
    	unsigned int	uin;
    	unsigned long	ulo;
    	ch = sh = in = lo = ush = uin = ulo = 0;
    
    	printf("Ranges through computation:\n\n");
    
    	while (ch < (ch+1))
    		ch++;
    	printf("The range of a char is from %d to %d\n", ch+1, ch);
    
    	while (sh < (sh+1))
    		sh++;
    	printf("The range of a short is from %d to %d\n", sh+1, sh);
    
    	while (in < (in+1))
    		in++;
    	printf("The range of an int is from %d to %d\n", in+1, in);
    
    	while (lo < (lo+1))
    		++lo;
    	printf("The range of a long is from %ld to %ld\n", lo+1, lo);
    
    	while (ush < (ush+1))
    		++ush;
    	printf("The range of an unsigned short is from %u to %u\n", ush+1, ush);
    
    	while (uin < (uin+1))
    		++uin;
    	printf("The range of an unsigned int is from %u to %u\n", uin+1, uin);
    
    	while (ulo < (ulo+1))
    		++ulo;
    	printf("The range of an unsigned long is from %lu to %lu\n", ulo+1, ulo);
    
    	return 0;
    }
    The ranges of all data types in the program print correctly, except for the char type (the corresponding code is red) in which the loop continues for ever ........why is it so?

    Thanks.

    PS: The program only works when the code in red is deleted
    Last edited by noodles; 06-21-2006 at 05:17 AM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    > while (ch < (ch+1))
    My guess is that the +1 silently promotes the whole thing to an integer expression, so on the supposedly last iteration, you end up with 255 < 256 rather than the modulo result of 255 < 0 (which is what you expected).
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    The Richness... Richie T's Avatar
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    The sign bit is causing trouble - for char, when ch is 127, ch++ is
    giving -128, so ch++ gives -127, -126.. all the way to +127 again,
    hence an infinite loop.

    Decimal -> Binary
    127 -> 0111 1111
    -128 -> 1000 0000
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  4. #4
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    Smile

    Thanks a lot everyone.........(especially Salem)

    The problem was that (ch+1) evaluated to 128 when ch equalled 127.

    Typecasting (ch+1) to char solved the problem:

    Code:
    	while (ch < (char)(ch+1))
    		ch++;
    	printf("The range of a char is from %d to %d\n", (char)(ch+1), ch);
    Last edited by noodles; 06-21-2006 at 07:01 AM.

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    You can use CHAR_MAX and look-alikes in <limits.h> instead of computing the numbers directly.
    dwk

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  6. #6
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    You do realize that it would take you in excess of tens of years to finish the program

    A better way is to take advantage of the fact that, due to the way modern computers handle numbers, the limits are usually one less than a power of two.
    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;}

  7. #7
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    Thanks for information, jafet!!

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