Question

I am trying to create an array of values. These values should be “2.4,1.6,.8,0”. I am subtracting .8 at every step.

This is how I am doing it (code snippet):

float mean = [[_scalesDictionary objectForKey:@"M1"] floatValue];  //3.2f
float sD = [[_scalesDictionary objectForKey:@"SD1"] floatValue];   //0.8f

nextRegion = mean;
hitWall = NO;
NSMutableArray *minusRegion = [NSMutableArray array];


while (!hitWall) {

    nextRegion -= sD;

if(nextRegion<0.0f){
    nextRegion = 0.0f;
    hitWall = YES;
}

[minusRegion addObject:[NSNumber numberWithFloat:nextRegion]];

}

I am getting this output:

minusRegion = (
    "2.4",
    "1.6",
    "0.8000001",
    "1.192093e-07",
    0
)

I do not want the incredibly small number between .8 and 0. Is there a standard way to truncate these values?

Answer 1

You’re looking at good old floating-point rounding error. Fortunately, in your case it should be simple to deal with. Just clamp:

if( val < increment ){
    val = 0.0;
}

Although, as Eric Postpischil explained below:

Clamping in this way is a bad idea, because sometimes rounding will cause the iteration variable to be slightly less than the increment instead of slightly more, and this clamping will effectively skip an iteration. For example, if the initial value were 3.6f (instead of 3.2f), and the step were .9f (instead of .8f), then the values in each iteration would be slightly below 3.6, 2.7, 1.8, and .9. At that point, clamping converts the value slightly below .9 to zero, and an iteration is skipped.

Therefore it might be necessary to subtract a small amount when doing the comparison.

A better option which you should consider is doing your calculations with integers rather than floats, then converting later.

int increment = 8;
int val = 32;

while( val > 0 ){
    val -= increment;

    float new_float_val = val / 10.0;
};