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question about Block
*To*: mathgroup at smc.vnet.net
*Subject*: [mg69661] question about Block
*From*: dimmechan at yahoo.com
*Date*: Wed, 20 Sep 2006 02:44:07 -0400 (EDT)
Hello to all.
Block as it is well known to the members of this forum does have some
nice applications.
For example
$Version
"5.2 for Microsoft Windows (June 20, 2005)"
Block[{Message}, Integrate[1/x, {x, -1, 2}]]
Infinity
Block[{Message}, NIntegrate[Sin[x]/x, {x, 0, Infinity}]]
2.6457805920701
(execute the commands without Block to see the messages)
Let me now define a list of two functions.
lst = {1/x, 1/x^2};
Then
NIntegrate[lst, {x, 1, 2}]
\!\(NIntegrate::"inum" : "Integrand \!\({1\/x,
1\/x\^2}\) is not numerical at \!\({x}\) = {1.5`}."\)
NIntegrate[lst, {x, 1, 2}]
because NIntegrate has the attribute HoldAll
Attributes[NIntegrate]
{HoldAll, Protected}
Of course
NIntegrate[Evaluate[lst], {x, 1, 2}]
{0.693147180559947, 0.5000000000000211}
Now to my surprise I discovered that the following commands fail to
prevent the appearance of NIntegrate::inum message.
Block[{Message}, NIntegrate[lst, {x, 1, 2}]]
\!\(NIntegrate::"inum" : "Integrand \!\({1\/x,
1\/x\^2}\) is not numerical at \!\({x}\) = {1.5`}."\)
NIntegrate[lst, {x, 1, 2}]
Block[{$Messages = {}}, NIntegrate[lst, {x, 1, 2}]]
\!\(NIntegrate::"inum" : "Integrand \!\({1\/x,
1\/x\^2}\) is not numerical at \!\({x}\) = {1.5`}."\)
NIntegrate[lst, {x, 1, 2}]
What is more strange is that the relevant Plot::plnr message can be
prevented.
Plot[lst, {x, 1, 2}, DisplayFunction -> Identity];
Plot::plnr : lst is not a machine-size real number at x =
1.0000000416666666`. (...3 times...)
General::stop : Further output of Plot::plnr will be suppressed during
this \
calculation.
Attributes[Plot]
{HoldAll, Protected}
Block[{Message}, Plot[lst, {x, 1, 2}, DisplayFunction -> Identity]];
Block[{$Messages = {}}, Plot[lst, {x, 1, 2}, DisplayFunction ->
Identity]];
Of course someone can simply switch off the message.
Off[NIntegrate::inum]
NIntegrate[lst, {x, 1, 2}]
NIntegrate[lst, {x, 1, 2}]
On[NIntegrate::inum]
but the question remains why exist this strange (at least for me)
situation.
Any comments/suggestions?
Dimitris
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