Programmer Guide/Command Reference/EVAL/vsubn: Difference between revisions
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{{DISPLAYTITLE:{{SUBPAGENAME}}}} | {{DISPLAYTITLE:{{SUBPAGENAME}}}} | ||
Extract rows from a vector or matrix. | Extract rows from a vector or matrix. | ||
;Usage: | ;Usage: <code>vsubn(<var>x</var> {, <var>o</var> {, <var>n</var>}})</code> | ||
:;<var>x</var>: source vector or matrix | :;<var>x</var>: source vector or matrix | ||
:;<var>o</var>: offset, 0 | :;<var>o</var>: offset, 0 ≤ ''o'' < <code>nrow(''x'')</code> (default=0) | ||
:;<var>n</var>: length, ''n'' > 0 and ''o''+''n'' | :;<var>n</var>: length, ''n'' > 0 and ''o''+''n'' ≤ <code>nrow(''x'')</code> (default=<code>nrow(''x'')-''o''</code>) | ||
;Result: A numerical object with <code>ncol(''x'')</code> columns and ''n'' rows, consisting of the rows ''o'' to ''o''+''n''-1 of the argument ''x''. | ;Result: A numerical object with <code>ncol(''x'')</code> columns and ''n'' rows, consisting of the rows ''o'' to ''o''+''n''-1 of the argument ''x''. | ||
;See also: [[ | ;See also: [[../vsubc|vsubc]], [[../select|select]], [[../vvset|vv]] | ||
[[../#Functions|<function list>]] | |||
Example: | Example: | ||
| Line 22: | Line 25: | ||
</pre> | </pre> | ||
[[ | Use the [http://en.wikipedia.org/wiki/Trapezoidal_rule trapezoidal rule] to approximate technique definite integral of a function. | ||
<pre> | |||
// The function y=f(x) is given by the points y[i]=f(x[i]). | |||
// Approximate the definite integral of the function: | |||
#n := int $#y[!nrow]-1 | |||
#a = eval (vsubn($#x,1,$#n)-vsubn($#x,0,$#n)) * (vsubn($#y,1,$#n)+vsubn($#y,0,$#n)) / 2 | |||
</pre> | |||
Latest revision as of 12:12, 21 April 2011
Extract rows from a vector or matrix.
- Usage
vsubn(x {, o {, n}})- x
- source vector or matrix
- o
- offset, 0 ≤ o <
nrow(x)(default=0) - n
- length, n > 0 and o+n ≤
nrow(x)(default=nrow(x)-o)
- Result
- A numerical object with
ncol(x)columns and n rows, consisting of the rows o to o+n-1 of the argument x. - See also
- vsubc, select, vv
Example:
#a := eval vv(1,2,3,4,5)
#b := eval vmcol($#a, vv(5,4,3,2,1))
#c := vsubn($#a,3)
// -> vector: $#c = { 4 , 5 }
#d := eval vsubn($#b, 1 , 3)
// -> matrix: $#d[*,0] = { 2 , 3 , 4 }, $#d[*,1] = { 4 , 3 , 2 }
#e := eval vsubn($#a, 2, 1)
// -> scalar: $#e = 2, this is equivalent to $#a[2]
#f := eval vsubn($#b, 2, 1)
// -> vector (!!): $#f = { 3 , 3 }, this is equivalent to $#b[2,*]
Use the trapezoidal rule to approximate technique definite integral of a function.
// The function y=f(x) is given by the points y[i]=f(x[i]). // Approximate the definite integral of the function: #n := int $#y[!nrow]-1 #a = eval (vsubn($#x,1,$#n)-vsubn($#x,0,$#n)) * (vsubn($#y,1,$#n)+vsubn($#y,0,$#n)) / 2
