Unwelcome return value.

This is a discussion on Unwelcome return value. within the C Programming forums, part of the General Programming Boards category; My question is why atoi returns a value of -954337623 or something like that in the following function. The fgets ...

  1. #1
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    Unwelcome return value.

    My question is why atoi returns a value of -954337623 or something like that in the following function.

    The fgets function reads from a textfile which holds 20 rows of 1s, 0s and newline chars.

    Code:
    int getmap(int map[20][50])
    {
    	int x = 0, y = 0;
    	char rdBuffer[50], c;
    	FILE* mapfile = fopen(MAP_NAME, "r");
    	if (mapfile == NULL)
    	{
    		printf("Could not open file properly");
    		return MAPERROR;
    	}
    	for (;y < 20; y++)
    	{
    		fgets(rdBuffer, 50, mapfile);
    		for (; x < 50; x++)
    		{
    			c = rdBuffer[x];
    			map[y][x] = atoi(&c); // Need help here, please!
    			if (map[y][x] != 1 && map[y][x] != 0)
    			{
    				return MAPERROR;
    			}
    			else
    			{
    				switch(map[y][x])
    				{
    					case 1: printf("#"); break;
    					case 0: printf(" "); break;
    					default: ;
    				}
    			}
    		}
    		printf("\n");
    		x = 0;
    	}
    	fclose(mapfile);
    	return 0;
    }

  2. #2
    cwr
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    Because you're very confused.

    You're declaring c as a char, and passing the address of it to atoi. But atoi is looking for a char array that is terminated with a null character. If it's just one character that you want to convert to a number, consider using rdBuffer[x]-'0'. atoi is suitable for strings like "514", not a single character.

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    what exactly does the -'0' do?

  4. #4
    Skunkmeister Stoned_Coder's Avatar
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    The numbers in ASCII are contiguous so by subtracting the value of '0' you turn a char into its integer representation.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  5. #5
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    Thank you both for replying, I'll look into this now and hopefully I will learn something...

  6. #6
    and the hat of wrongness Salem's Avatar
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    > for (;y < 20; y++)
    So explain exactly why you didn't do
    for (y = 0 ;y < 20; y++)
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  7. #7
    Supermassive black hole cboard_member's Avatar
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    Quote Originally Posted by Salem
    > for (;y < 20; y++)
    So explain exactly why you didn't do
    for (y = 0 ;y < 20; y++)
    He initialised y to zero on declaration.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  8. #8
    Sr. Software Engineer filker0's Avatar
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    Odd... I thought I explained this in my reply to your earlier post.

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    Code:
    for (;y < 20; y++)
    This one is okay. But
    Code:
    for (; x < 50; x++)
    this one isn't, since it is itself in a loop.

    Change it to
    Code:
    for(x = 0; x < 50; x ++)
    dwk

    Seek and ye shall find. quaere et invenies.

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  10. #10
    and the hat of wrongness Salem's Avatar
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    Both are poor use of for loops.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  11. #11
    Frequently Quite Prolix dwks's Avatar
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    Both are poor use of for loops.
    The ones that look like this
    Code:
    for (; x < 50; x++)
    are. But these
    Code:
    for(x = 0; x < 50; x ++)
    are okay.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


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    Sometimes it is useful to have a for loop without an initialising statement. For instance, if you want to start doing something at the point where you left off something else previously, or if there is nothing you actually want to initialise there. In this case, it's probably better for readability to initialise y in the for loop rather than when it's declared, but that doesn't mean that leaving out the first statement is in general a poor use of for loops.

  13. #13
    cwr
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    Yes, it's just a choice of what is more readable and clean
    Code:
    for (; expr1; expr2)
    {
        statements
    }
    or
    Code:
    while (expr1)
    {
        statements
        expr2
    }

  14. #14
    & the hat of GPL slaying Thantos's Avatar
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    Quote Originally Posted by Stoned_Coder
    The numbers in ASCII are contiguous so by subtracting the value of '0' you turn a char into its integer representation.
    IIRC Prelude or Quzah said that the standard requires that the numbers be contiguous in a representation.

  15. #15
    cwr
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    Quote Originally Posted by Thantos
    IIRC Prelude or Quzah said that the standard requires that the numbers be contiguous in a representation.
    Indeed, from ISO 9899:1999 5.2.1:

    "In both the source and execution basic character sets, the
    value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous."

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