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;
}

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.

what exactly does the -'0' do?

The numbers in ASCII are contiguous so by subtracting the value of '0' you turn a char into its integer representation.

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

> for (;y < 20; y++)
So explain exactly why you didn't do
for (y = 0 ;y < 20; y++)

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.

Odd... I thought I explained this in my reply to your earlier post.

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 ++)

Both are poor use of for loops.

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.

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.

Yes, it's just a choice of what is more readable and clean

Code:

for (; expr1; expr2)
{
    statements
}

or

Code:

while (expr1)
{
    statements
    expr2
}

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.

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."