You're better off just converting it to a string and reversing it.
NSString *inputString = [@(input) stringValue]; // Eg 1230
NSMutableString *reversedString = [NSMutableString string];
NSInteger charIndex = [inputString length];
while (charIndex > 0) {
charIndex--;
NSRange subStrRange = NSMakeRange(charIndex, 1);
[reversedString appendString:[inputString substringWithRange:subStrRange]];
}
double result = [reversedString doubleValue]; // Eg 0321 -> 321
// Go back to NSString to get the missing length
NSString *resultString = [@(result) stringValue]; // Eg. 321
// Multiple by factors of 10 to add zeros
result *= exp(10, [inputString length] - [resultString length]); // 3210
NSLog(@"reversed value %f", result);
Reverse string method from this answer.