Look-up tables: finding index of a value below a given number

[nigelmercier]

Hi guys, I'm a C newbie (a couple of months ago I was a virgin!) working with PIC microcontrollers. I need to do a look-up table to change one value to another, probably best to give an example in pseudocode:

code const integer reading[5] ={12, 96, 147, 245, 747}; // sorted numerically
code const integer result[5] ={5, 10, 15, 20, 25}; // sorted numerically
get look-up value for reading[] (for example 120)
find last value in reading[] below this (for example 96)
return equivalent value from result[] (for example 10)

The last part I can deal with, and may not even be required (as you can see, the values in result[] are at intervals of 5, at least for the moment).

The tables will in reality have up to 100 entries. My plan was to iterate through reading[] and find when the number is bigger, then step back one. Is this OK, or is there a better way? Should I consider hash tables?

[newb16]

If values in reading[] are sorted, use binary search.

[nigelmercier]

Binary search, you mean keep splitting the array? Can you point me to some code to do this?

[puneetsardana88]

         int binarySearch( int [] reading , int target )
         {
                int found = 0 ;
            int start=0 , mid =-1 , end = 100 ; 
                while( !found &&  end >= start )
                  {
                    mid = (start+end) / 2 ;
                    if(reading[mid] == target)
                        found = 1 ;
                    else if (reading[mid] > target) 
                         end = mid - 1 ;
                    else
                        start = mid + 1 ;
                 }
                        return mid - 1;
 }
 

The index you are looking is mid-1.

[nigelmercier]

Many thanks, I'll give it a go.

What a helpful forum this is.