Thread: Converting a String of Characters to a Negative Integer

I have a function called strToInt that accepts a string of characters and converts it to an integer:

insert

Code:

int strToInt (const char str[])
{
	int i, intValue, result = 0;
	
	for (i = 0; str[i] >= '0' && str[i] <= '9'; ++i)
	{
		intValue = str[i] - '0';
		result = result * 10 + intValue;
	}
	return result;
}

I am trying to add to this function so that it detects a '-' as the first character in the string and converts the string to a negative number instead. How would I implement that? Thanks for any help!

Use a flag; if the first character is a '-', then set the flag; if the flag is set at the end, multiply by -1.

OK I added this code but it still doesn't seem to be working correctly:

insert

Code:

int strToInt (const char str[])
{
	int i, intValue, result = 0;
	int negative;
	
	for (i = 0; str[i] >= '0' && str[i] <= '9'; ++i)
	{
		if (str[0] == '-')
		{
			negative = 1;
		}
		else
		{
			negative = 0;
		}
		
		intValue = str[i] - '0';
		result = result * 10 + intValue;
		
		if (negative == 1)
		{
			result = result*(-1);
		}
		else
		{
			result = result;
		}
	}
	return result;
}

Notice that if your leading character is '-', your for loop never happens. Do the check outside the for-loop; and in that case make sure your loop starts at 1 instead of 0.

OK so I made these modifications:

Code:

int strToInt (const char str[])
{
	int i, intValue, result, posResult = 0;
	int negative;
	
	if (str[0] == '-')
	{
		for (i = 1; str[i] >= '0' && str[i] <= '9'; ++i)
		{	
			intValue = str[i] - '0';
			posResult = posResult * 10 + intValue;
			result = posResult * (-1);
		}
	}
	else
	{
		for (i = 0; str[i] >= '0' && str[i] <= '9'; ++i)
		{	
			intValue = str[i] - '0';
			result = result * 10 + intValue;
		}
	}
	return result;
}

However, now the only conversion that works correctly is the one for negative numbers. Each positive number that is returned has a 36 in front of it.

And you initialize result where?

I use the line

Code:

printf("%i\n",strToInt(var1));

where var1 is the name of the string that I am converting. The string contains the characters 5 and 8, but it is printing out 3658.

I set result to 0 at the beginning of the function.

Originally Posted by FJRRulz

I set result to 0 at the beginning of the function.

If you did, I wouldn't have asked the question.

Wow, quite the stupid mistake. Thank you good sir, it works.