LinearAlgebra`BLAS`

LinearAlgebra`BLAS`

IAMAX

IAMAX[x]

gives the position of the element with the maximum absolute value in a vector x.

Details and Options

To use IAMAX, you first need to load the BLAS Package using Needs["LinearAlgebra`BLAS`"].
The following argument must be given:
x input expression vector
For complex vectors x, IAMAX locates the element with the greatest sum of absolute values of the real and imaginary parts.
If x contains non-numeric elements, then IAMAX[x] always returns $Failed.
For duplicate entries the first position found is returned.

Examples

open allclose all

Basic Examples (1)Summary of the most common use cases

Load the BLAS package:

In[1]:=1

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-nluht4

Find the position of the element with the largest absolute value in a vector:

In[2]:=2

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-4piglx

Out[2]=2

Scope (4)Survey of the scope of standard use cases

A real vector:

In[1]:=1

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-0i0euc

Out[1]=1

A complex vector:

In[1]:=1

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-69hbpj

Out[1]=1

An arbitrary-precision vector:

In[1]:=1

✖

https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-mj87da

Out[1]=1

Vectors with symbolic entries will return $Failed:

In[1]:=1

✖

https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-0z7tup

Out[1]=1

Properties & Relations (1)Properties of the function, and connections to other functions

IAMAX[x] is equivalent to Part[Position[Abs[x],Max[Abs[x]]],1,1] for real-valued vectors:

In[1]:=1

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-29ji2k

Out[1]=1

A similar relation is true for complex-valued vectors :

In[2]:=2

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https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-f694zr

Out[2]=2

Possible Issues (2)Common pitfalls and unexpected behavior

If there are multiple elements with the same maximum absolute value, the position of the first one is returned:

In[1]:=1

✖

https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-da5kto

Out[1]=1

If a vector contains a symbol, IAMAX returns $Failed:

In[1]:=1

✖

https://wolfram.com/xid/0gcd7rjqvunx9h3oe7x1p8-egjmlj

Out[1]=1

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