How to perform individual operation on a Column of data from excel file?

I am intended to perform a separate operation (adding/subtracting some values) on Positive and Negative value from an Excel dataset using Matlab. To read the column I use the following code

num = xlsread('testdata.xls', 'B:B');

it reads fine and when I ask 'num' it shows the data. However, the data are having positive and negative values. I want to add some values to negative dataset and substruct from the positive dataset by keeping their position alike so that I can reconstruct a graph having exactly the same pattern. I have made two attempts:

1. Using logical indexing I can separate the positive and negative values and then perform the operation but can't combine them in the same order, something like following

idx = num<0;
neg = num(idx)+ 1.0e-08
pos = num(~idx)- 1.0e-08

2. If-else also not working at all:

if (num<0) 
num = num +1 
elseif(num>0) 
num = num +2
end

Any help, please?

Answer 1

[KSSV]

idx = num<0;
num(idx) = num(idx)+ 1.0e-08
num(~idx)= num(~idx)- 1.0e-08

Answer 2

[Walter Roberson]

idxneg = num<0;
num(idxneg) = num(idxneg)+ 1.0e-08;
idxpos = num>0;
num(idxpos) = num(idxpos)- 1.0e-08;

I did not use ~idxneg because ~idxneg would include num == 0 exactly, but exact zeros are not positive values.

There is a shortcut for this code:

num = num - sign(num) * 1.0e-08;

This even leaves 0 unchanged.

[Khondokar Fida Hasan]

This shortcut seems wonderful but in my case the operation (adding value) is different. Though in aforementioned case I use 1.0e-08 for every case, but I want to perform different adjustment for a different case.

[Walter Roberson]

offsets = [1.0e-08, 0, -8.7e-9];
num = num + offsets(2 + sign(num));

provided that the breakpoints are still negative, 0, positive