How to give a variable output name in a shell script inside a for loop

Hi all

I run my program prog.c in the following way :

$ ./prog 1 > output.txt

where 1 is a user defined initial value used by the program.

But now I want to run it for many a thousand initial values, 1-1000, and store all the outputs in different files.
Like

$ ./prog 1 > output1.txt 
$ ./prog 2 > output2.txt 
$ ./prog 3 > output3.txt 
.
 
.
$ ./prog 1000 > output1000.txt  

I know how to use the for loop in a shell script but dont know how to give a variable output name.

Please give me a small example
PLEASE HELP

Regards
Alice

Using man bash ("bash") shell

 for i in {1..1000};do ./prog $i > output$i.txt;done