Calculate log return between the first and last observation in a day ?

I have 1 min prices for each day begining 09:30 to 4:00pm and i need to calculate daily log return i.e. log(close price on the day / opening price on the day)?

Since its possible that some day may not have 09:30 or 4:00 , then i want it to take next time 9:31 if 9:30 is not there and 3:59 if 4:00 pm time is not there . How to do it ? If i split my data into days and wish to calculate log return i.e log(last observation in that day/first observation in that day). How to code this in matlab R2016a?

my data looks like

2014-02-03 09:30:00    10.450000
2014-02-03 09:31:00    10.450000
2014-02-03 09:32:00    10.326600
2014-02-03 09:33:00    10.290000
.
.
2014-02-03 04:00:00    10.326500
...
2014-07-31 09:30:00    15.8500
2014-07-31 09:31:00    15.8600
2014-07-31 09:32:00    15.8600
.
2014-07-31 03:50:00    15.9101
2014-07-31 04:00:00    15.9300

[NS]

Any help on this ?

[NS]

any help on this ??

Answer 1

[KSSV]

fid = fopen('data.txt','rt') ;
S = textscan(fid,'%s','delimiter','\n') ;
fclose(fid) ; 
S = S{1} ;

firstline = S{1}
lastline = S{end}

[NS]

Thanks Sir