Getting first three digits from int/float

Question

I'd like to get the first three digits from large floats or integers and on some insert a decimal. For example:

KB
----------
32589 >> 325
43266 >> 432

MB
----------
1234599 >> 1.23
3422847 >> 3.42

For the particular number, I will have the "KB" and "MB" strings. This will let me know if the decimal is required, as in "MB" examples. I looked at NSNumberFormatter but wasn't sure what on there would help. Any suggestions?

Answer 1

You can do this by converting the number into a text representation using itoa or sprintf or implementing your own version of this conversion technique. Once this is done, you can pull the first three characters from the text buffer to use.

Answer 2

Probably the easiest way to deal with this is to convert it to a string and use one of the cocoadev regex libraries to do the formatting

Answer 3

Wait, are you trying to say 32589 bytes is 325 KB?

Wouldn't it make sense to divide the number by 1000 repeatedly until the result is less than 1000, then format with whatever printf equivalent you have?

Answer 4

Here is some quick & dirty code to pull off the top three digits of the number (in NSString format):

long someNumber = 1234599;
NSString * allDigits = [NSString stringWithFormat:@"%l", someNumber];
NSString * topDigits = [allDigits substringToIndex:3];

NSLog(@"%@", topDigits); // will output 123

Answer 5

Actually, decided to chime in with my own answer. This is my routine for generating strings for outputting human sizes from byte counts:

#include <math.h>    // for isgreater()
static NSString * MemorySizeString( mach_vm_size_t size )
{
    enum
    {
        kSizeIsBytes        = 0,
        kSizeIsKilobytes,
        kSizeIsMegabytes,
        kSizeIsGigabytes,
        kSizeIsTerabytes,
        kSizeIsPetabytes,
        kSizeIsExabytes
    };

    int sizeType = kSizeIsBytes;
    double dSize = (double) size;

    while ( isgreater(dSize, 1024.0) )
    {
        dSize = dSize / 1024.0;
        sizeType++;
    }

    NSMutableString * str = [[NSMutableString alloc] initWithFormat: (sizeType == kSizeIsBytes ? @"%.00f" : @"%.02f"), dSize];
    switch ( sizeType )
    {
        default:
        case kSizeIsBytes:
            [str appendString: @" bytes"];
            break;

        case kSizeIsKilobytes:
            [str appendString: @"KB"];
            break;

        case kSizeIsMegabytes:
            [str appendString: @"MB"];
            break;

        case kSizeIsGigabytes:
            [str appendString: @"GB"];
            break;

        case kSizeIsTerabytes:
            [str appendString: @"TB"];
            break;

        case kSizeIsPetabytes:
            [str appendString: @"PB"];
            break;

        case kSizeIsExabytes:
            [str appendString: @"EB"];
            break;
    }

    NSString * result = [str copy];
    [str release];

    return ( [result autorelease] );
}

It works out the correct size by seeing what order of binary magnitude it is, using 1024 as the base (1024 bytes = 1KB, 1024KB = 1MB, etc.). While doing this, it shrinks the input value (using floating-point arithmetic) such that once it's below 1024, it has both a human-readable value and a magnitude specifier. It then generates a string containing the formatted value (no decimal places for bytes, 2 decimal places for any larger magnitude), and inspects the magnitude constant to determine what suffix to attach.