# How to apply a equation to specific rows in a matrix

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Philippe Corner on 28 Dec 2018
Commented: Stephen23 on 28 Dec 2018
If I have a matrix m like:
m=[0 340.216815000000
1.56250000000000 570.718050000000
4.68750000000000 769.256473000000
7.81250000000000 1176.42951000000
10.9375000000000 1632.88855600000
18.4375000000000 2099.99990500000
25.9375000000000 2099.99990500000
33.4375000000000 2099.99990500000
40.9375000000000 2099.99990500000
48.4375000000000 2099.99990500000
0 331.115127000000
1.53698699999995 510.419428000000
4.61084000000005 719.346166000000
7.68469200000004 1145.58672900000
10.7585449999999 1583.81915100000
18.1358640000001 2099.99990500000
25.5130620000000 2099.99990500000
32.8903809999999 2099.99990500000
40.2675780000000 2099.99990500000
47.6448969999999 2099.99990500000];
and a matrix n like:
n=[0 159.846351000000
1.52099599999997 343.757153000000
4.56286599999999 412.372440000000
7.60473600000000 926.872313000000
10.6466060000000 1063.47274800000
17.9471440000000 1079.82945400000
25.2475589999999 1200.00004800000
32.5480960000000 1200.00004800000
39.8486330000001 1200.00004800000
47.1491699999999 1200.00004800000
0 186.247796000000
1.49536100000000 386.090606000000
4.48608399999989 477.258742000000
7.47680700000001 775.100648000000
10.4676509999999 1004.91941000000
17.6453859999999 1075.44088400000
24.8232419999999 1200.00004800000
32.0009769999999 1200.00004800000
39.1787110000000 1200.00004800000
46.3565670000000 1200.00004800000];
how to build a vector E which matches that when m(:,1) is lower than 10, then use this formula:
1. (m(:,2).^2).*(3*(n(:,2).^2));
and when 10 < m(:,1) <= 25, use:
2. (m(:,2).*2)
the results would be this:
E = [8872341649.18581
115469679048.928
301885906398.286
3566916094097.57
3265.77711200000
4199.99981000000
4199.99981000000
4199.99981000000
4199.99981000000
4199.99981000000
662.230254000000
1020.83885600000
1438.69233200000
2291.17345800000
3167.63830200000
4199.99981000000
4199.99981000000
4199.99981000000
4199.99981000000
4199.99981000000];
Stephen23 on 28 Dec 2018
Edited: Stephen23 on 28 Dec 2018
Your example output does not match your description: rows 7, 8, 9, 10, 17, 18, 19, and 20 of m do not match either of the conditions that you describe, and yet you show data in those rows of the output matrix. Please explain how that data should be generated.

Stephen23 on 28 Dec 2018
Edited: Stephen23 on 28 Dec 2018
X = m(:,1)<=10;
Y = 10<m(:,1) & m(:,1)<=25;
Z = nan(size(m,1),1);
Z(X) = m(X,2).^2 .* 3.*n(X,2).^2;
Z(Y) = m(Y,2).*2;
Giving:
>> Z
Z =
8872341649.185812
115469679048.9282
301885906398.2859
3566916094097.571
3265.777112
4199.99981
NaN
NaN
NaN
NaN
11409367861.74257
116507562823.7022
353594015029.8446
2365339054254.324
3167.638302
4199.99981
NaN
NaN
NaN
NaN
Philippe Corner on 28 Dec 2018
Sorry you are right, they should be filled with (m(:,2).*2)
Stephen23 on 28 Dec 2018
X = m(:,1)<=10;
Z = m(:,2).*2;
Z(X) = m(X,2).^2 .* 3.*n(X,2).^2;
Giving:
>> Z
Z =
8872341649.185812
115469679048.9282
301885906398.2859
3566916094097.571
3265.777112
4199.99981
4199.99981
4199.99981
4199.99981
4199.99981
11409367861.74257
116507562823.7022
353594015029.8446
2365339054254.324
3167.638302
4199.99981
4199.99981
4199.99981
4199.99981
4199.99981