Converting Strings of Numbers to Integers

**Adrian** · 11-21-2007

How would I go about changing a string of numbers like "2546" and return the integer value 2546.

This is what I have so far (this is just my function to convert the string to an integer)

The 48 is in there because the value of 0 is 48, 1 is 49, etc

I realize my "power" will depend on the length of the string of numbers but for now I have it set to 1 so it should work with any 2-digit integers.

Code:

int stringToInt ( char s[] )
{
	int i = 0;

	int place;
	int value = 0;
	int power = 1;

	for (i=0; s[i]!='\0' ; i++ )
	{
		place = s[i] - 48;
		value = (value + (place*(10^power)));

		power = power-1;

	}

	return place;
}

For some reason I'm only getting the last number as an integer from every string for instance, if the string is 48, I will get 8 returned (again, I'm just testing on 2-digit numbers right now)

I will need this to work with any size string of numbers including negative numbers too.

Any help would be great.

Thanks

Edit: I know I probably should have returned value at the end, but that's still wrong, it gives me a result of 61 when the string is 15, and 52 when the string is 23.

**cpjust** · 11-21-2007

Try atoi() or sscanf()

**Adrian** · 11-21-2007

Originally Posted by cpjust

Try atoi() or sscanf()

Sorry, but I don't know what either of those would do or where to implement them.

**whiteflags** · 11-21-2007

Uh huh, well I think the problem is actually your math. C doesn't have an exponent operator. operator ^ is bitwise exclusive OR, not the pow() function. I think you can do value *= 10 before you add the next place value to move along the conversion anyway so it shouldn't matter.

consider something like this too.

Code:

int sign = 0;
if ( i ==0 && s[i] == '-' ) {
   sign = 1;
   continue;
}
...

if( sign ) 
   value = -value;

You can always just use the other functions that convert strings to numbers, like strtol(). it's useful to learn them.

**Adrian** · 11-21-2007

Originally Posted by citizen

Uh huh, well I think the problem is actually your math. C doesn't have an exponent operator. operator ^ is bitwise exclusive OR, not the pow() function. I think you can do value *= 10 before you add the next place value to move along the conversion anyway so it shouldn't matter.

consider something like this too.

Code:

int sign = 0;
if ( i ==0 && s[i] == '-' ) {
   sign = 1;
   continue;
}
...

if( sign ) 
   value = -value;

You can always just use the other functions that convert strings to numbers, like strtol(). it's useful to learn them.

Brilliant, thanks a lot, this works very well.

**Adrian** · 11-21-2007

Actually, it gives me 0 for negative numbers because I'm returning 0 (since value = 0 before the for loop, which won't run if they have a '-' at the beginning of the string). How would I get the integer value for a negative number? I'm working on it now, but if you could help that would be awesome.

**Adrian** · 11-21-2007

Somehow I have to ignore the '-' and do the same process but multiply it by -1 at the end. I think I can do this.

Edit: I got it, thanks again for your help.

**whiteflags** · 11-21-2007

Nah. The point was to check the first character, and then wait until the end to negate the whole return value.

1. Check for negative and set a flag
2. Do the rest of the conversion work.
3. If the flag is set, negate the value.
4. return the value.

**Salem** · 11-22-2007

> value = (value + (place*(10^power)));
Erm, ^ isn't "raise to power", it's exclusive-or.

**Elysia** · 11-22-2007

Originally Posted by cpjust

Try atoi() or sscanf()

IIRC, atoi is not part of the standard, is it? It's better to use sprintf, or some function in that family.

**laserlight** · 11-22-2007

atoi is not part of the standard, is it?

It is. itoa is the one that is not standard.

**Elysia** · 11-22-2007

Right, right, I confused those two.

Thread: Converting Strings of Numbers to Integers

Thread Tools

Search Thread

Display

Converting Strings of Numbers to Integers

Similar Threads

Converting strings to chars

converting a vector of strings into an array.

converting c style strings to c++ strings

convert numbers to strings