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MathGroup Archive 2006

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Re: decrementing problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72306] Re: decrementing problem
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Tue, 26 Dec 2006 04:42:06 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <em60nv$3ar$1@smc.vnet.net>

jltrahan at mail.usf.edu wrote:
> Hi,
> I am having a really difficult time with decrementing values in my For
> loop. I have tried i--, --i, i-=1, i=i-1, and nothing is working. The loop
> is as follows:
> For[i=n-1, i<=1, i--, summ=0;

As written, the outer loop is flawed since either i is greater than 1 
and the body of the loop is never executed, or i is less than 1 and the 
body will be executed forever because it decremented.

For instance, say n = 3, then i = 2, i is not less or equal than 1 
therefore the loop ends here.

Now, let n = 0, then i = -1, -1 is less than 1, therefore the body of 
the loop is executed, then i is decremented and i = -2. Again i is less 
than 1, the body is executed, etc.

>    For[j=i+1, j<=n, j++, summ=summ + U[[i,j]]*X[[j]]]
> ]
> When I try to print i or j, nothing shows up.
> Can anyone please help?
> 
> Thank you in advance,
> Jamie
> 

Compare with

In[4]:=
With[{n = 3},
	For[i = n - 1, i >= 1, i--,
		summ = 0;
		Print["i: ", i];
		For[j = i + 1, j <= n, j++,
			Print["j: ", j];
			summ = summ + U[[i,j]]*X[[j]]
		]
	]
]

 From In[4]:=
i: \[InvisibleSpace]2

 From In[4]:=
j: \[InvisibleSpace]3

 From In[4]:=
i: \[InvisibleSpace]1

 From In[4]:=
j: \[InvisibleSpace]2

 From In[4]:=
j: \[InvisibleSpace]3

Regards,
Jean-Marc


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