# Help with "Which[] in a For loop using Mathematica

• Mathematica
Hello,
I am a novice in Mathematica, but insisting to learn it. I would appreciate help in solving the following problem:

(*a=1;Which[a==1,x,a==2,b]*)

list := List[1611, 1612, 1613, 1614, 1615, 1616, 1617, 1618, 32, 10]

For[i = 1, i < 4, i++,

a = list[];

Print[a];

Print[list[]];

Which[a == 1611, x, a == 1612, y];

Print[%]]

When I run the code, I get the following result:

1611

1611

Null

1612

1612

Null

1613

1613

Null

While I think I am faithfully mimicking the Mathematica "Which statement" quoted above, it seems including the "Which" statement in the "For" loop does not make work as intended. Is this right? If not, please point out my mistake.

Best Regards

## Answers and Replies

Compare these two

Code:
In:= q = 1611;
Which[q == 1611, x,
q == 1612, y]

Out= x

In:= q = 1611;
Which[q == 1611, x,
q == 1612, y];

In the first the value of the Which is x. In the second the semicolon, after after that Which, discards the output and the result is simply nothing (or Null). So in your example you are Printing the result returned from the previous expresssion, the Which, and that is nothing or Null. Thus Null is Printed.

If you changed your code to be

Code:
w = Which[a == 1611, x,
a == 1612, y];
Print[w]

then you might get what you are hoping for.

Thank you so much. It works for me. :)