How to convert a string into double and vice versa?

To really convert from a string to a number properly, you need to use an instance of NSNumberFormatter configured for the locale from which you're reading the string.

Different locales will format numbers differently. For example, in some parts of the world, COMMA is used as a decimal separator while in others it is PERIOD — and the thousands separator (when used) is reversed. Except when it's a space. Or not present at all.

It really depends on the provenance of the input. The safest thing to do is configure an NSNumberFormatter for the way your input is formatted and use -[NSFormatter numberFromString:] to get an NSNumber from it. If you want to handle conversion errors, you can use -[NSFormatter getObjectValue:forString:range:error:] instead.

You can convert an NSString into a double with

double myDouble = [myString doubleValue];

Rounding to the nearest int can then be done as

int myInt = (int)(myDouble + (myDouble>0 ? 0.5 : -0.5))

I'm honestly not sure if there's a more streamlined way to convert back into a string than

NSString* myNewString = [NSString stringWithFormat:@"%d", myInt];