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Re: Difference between ; and , in for loop

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113957] Re: Difference between ; and , in for loop
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Fri, 19 Nov 2010 05:09:40 -0500 (EST)

On 11/18/10 at 7:03 AM, graser at gmail.com (graser) wrote:

>Would anyone explain me the difference between ; and , in
>conditional loop like for loop? For example,

>1) For[i = 1, i < 2, i++; Print[i]] Answer is 2

>But

>2)  For[i = 1, i < 2, i++, Print[i]] The Answer is 1

>Why does first loop get  2 instead of 1?

In Mathematica a semicolon is used to form a compound
expression. And Mathematica returns the result of the last part
of any compound expression encountered. So, when writing

For[i = 1, i < 2, i++; Print[i]]

the increment portion is the compound expression i++;Print[i]
and the body of the loop is Null since nothing is given. So, the
loop executes once then increments i to 2 and prints i.

Now when you write

For[i = 1, i < 2, i++, Print[i]]

the body of the loop is the Print[i] portion. So, i is set to 1.
The Print[i] prints a 1. Then I is incremented to 2 and the loop ends

In both cases the For loop works exactly the same. That is the
body is i is initialized to 1, the body is executed, the loop
variable is incremented then checked to see if the loop should end.



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