# How can I extract specific rows based upon a logical expression?

0

If I have a matrix of observations, how can I create a new matrix that contains only the rows in which the   first element in the  row is greater than 0?

0
accepted

The GAUSS function selif will extract rows from a matrix that match a logical expression. For example:

```x = { 1 3,
-2 4,
2 1,
7 -9 };

mask = x[.,1] .> 0;
xout = selif(x, mask);```

After the code above the variables will have the following values:

```x =
1        3
-2        4
2        1
7       -9
1
0
1
1
xout =
1        3
2        1
7       -9```

Tags: asked October 8, 2012

0
accepted

The GAUSS function selif will extract rows from a matrix that match a logical expression. For example:

```x = { 1 3,
-2 4,
2 1,
7 -9 };

mask = x[.,1] .> 0;
xout = selif(x, mask);```

After the code above the variables will have the following values:

```x =
1        3
-2        4
2        1
7       -9
1
0
1
1
xout =
1        3
2        1
7       -9```

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