Objective-C: Convert Hex Strings to Integers to Compare Which is Greater

Question

My goal is to compare two hex strings and determine which number is higher. I assume I need to convert those hex strings to integers to be able to compare them mathematically, but the conversion to unsigned isn't working. Here's what I've tried:

NSString *firstHex = @"35c1f029684fe";
NSString *secondHex = @"35c1f029684ff";

unsigned first = 0;
unsigned second = 0;

NSScanner *firstScanner = [NSScanner scannerWithString:firstHex];
NSScanner *secondScanner = [NSScanner scannerWithString:secondHex];

[firstScanner scanHexInt:&first];
[secondScanner scanHexInt:&second];

NSLog(@"First: %d",first);
NSLog(@"Second: %d",second);

But the log output gives me:

First: -1
Second: -1

I can't figure out what I'm doing wrong. Am I using NSScanner correctly here? Thanks in advance.

That's a seven byte integer. Are you sure that there isn't overflow? What happes if you try it with half length strings (3 byte ints)? — Linuxios, Jan 16 '14 at 00:28

score 6 · Accepted Answer · edited Jan 16 '14 at 01:05

6

Your hex numbers are 13 digits long - 52 binary bits. This is longer than 32 bits, use long long variables and scanHexLongLong: instead.

edited Jan 16 '14 at 01:05

Andrew Madsen

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answered Jan 16 '14 at 00:31

CRD

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score 0 · Answer 2 · answered Jan 16 '14 at 05:25

For the sake of completeness, here's the working code using the advice from the above answer:

NSString *firstHex = @"35c1f029684fe";
NSString *secondHex = @"35c1f029684ff";

unsigned long long first = 0;
unsigned long long second = 0;

NSScanner *firstScanner = [NSScanner scannerWithString:firstHex];
NSScanner *secondScanner = [NSScanner scannerWithString:secondHex];

[firstScanner scanHexLongLong:&first];
[secondScanner scanHexLongLong:&second];

NSLog(@"First: %llu",first);
NSLog(@"Second: %llu",second);

if(first > second){
  NSLog(@"First is greater");
}else{
  NSLog(@"Second is greater");
}

score 0 · Answer 3 · answered Jan 16 '14 at 23:45

0

it must be faster to just find out which one is larger as a string:

the longer string is bigger (ignoring leading 0's)
if they are the same then you can convert each char and compare, Repeat for each char...

answered Jan 16 '14 at 23:45

Grady Player

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Objective-C: Convert Hex Strings to Integers to Compare Which is Greater

3 Answers3