How to find the two nearest values related to a constant

Hello to all,

I've to find what are the two values and related indices respect to a constant.

I mean as example; I've a vector and a constant like follow:

a=[1250 2320 3520 4650 5550 6760];

b= 3700;

Therefore I would receive the values 3520 and 4650 and the equivalent indices 3 e 4.

Best regards and thanks in advance.

Answer 1

[Thomas]

a=[1250 2320 3520 4650 5550 6760];
b= 3700;

 % this will work even if 'a' is  in random order
  d=sort(abs(b-a));   
  lowest=find(abs(b-a)==d(1))
  sec_lowest=find(abs(b-a)==d(2))

[Juster]

Thank you Thomas for your troubleshooting.

It works fine because sorts the data also if there are negative numbers and as you sad also with random numbers.

Best regards.

[Trevor Wright]

This doesnt work if d(1) and d(2) are equal, meaning b sits exactly halfway between two values in a. An easy way to solve this is with an if statement.

d=sort(abs(b-a));

if (d(1) == d(2))
      vals = find(abs(b-a)==d(1))
      lowest = vals(1)
      second_lowest = vals(2)
else
      lowest=find(abs(b-a)==d(1))
      sec_lowest=find(abs(b-a)==d(2))
end

[Nimrod]

Tomas, instead of using find, you can use the second output of sort, which is the index for the sorting.

Answer 2

[Andrei Bobrov]

a=[1250 2320 3520 4650 5550 6760];
b= 3700;
[~,e] = histc(b,a)
out = [a(e+(0:1)) ; e+(0:1)]

Answer 3

[Image Analyst]

firstIndex = find(a<b, 1, 'last')
nextIndex = firstIndex + 1

[Philip Bates]

Only works if a is consistently increasing. If a= e.g. [0, -1, -2, -3] and b = -1.4 then firstIndex = find(a<b, 1, 'last') returns -2 and nextIndex returns -3 which is not correct. You can't simply check absoloute values in case the slope reverses at some point.

[Image Analyst]

Yes, you're right. A much better answer would have been:

a=[1250 2320 3520 4650 5550 6760]
b= 3700
differences = abs(a-b)
[minDiff, indexOfMinDiff] = min(differences);
closestValue = a(indexOfMinDiff);
message = sprintf('The value in "a" closest to "b" is %d', closestValue);
uiwait(helpdlg(message));