Determine if an element in array of doubles start with patten

Hey,

I need to find the index of the first element that starts with a specific number. So in this picture for example, if i give 8 as an input, i want the code to give me back the index of the first 8. that it meets which would be 163 in this case.

Answer 1

[Voss]

idx = find(floor(data)==val,1)

where data is your array and val is what you're looking for, e.g., 8.

[Adam Danz]

Neither of our current answers work with negative values although that may not be a problem in the OP's use case.

The benefit of this answer over mine is that val can be any number of digits. You could search for "82" in an array of values. It would match the third element of [8.2 820 82 8200].

The benefit of my answer over this one is that only the first digit is considered in the search but is limited to single-digit target values (which could easily by adapted). You could search for the number 8 in this vector and it would match the 3rd element. [0.8, 18, 8203, 8, 80].

Answer 2

[Adam Danz]

Here's a demo that finds the first value in a vector that has a first digit equal to 4. The values can have any number of digits to the left of the decimal although this doesn't work with negative values.

rng default
val = sort(rand(1,10).*randi(500,1,10))'

val = 10×1

6.9254 58.7631 60.8267 64.3632 221.9503 250.4717 253.5761 380.1302 440.2149 463.1465

target = 4; 
firstDigit = cellfun(@(v)v(1),""+val)-'0'

firstDigit = 10×1

6 5 6 6 2 2 2 3 4 4

firstDigitIdx = find(firstDigit == target,1,'first')

firstDigitIdx = 9