# Assign a variable to a condition only if the condition is met using Which

If I have a conditional of the form:

Which[Round[b[i], 0.1] == 74,1,40 < b[i] < x,2] where b[i] represents a number that gets updated in a loop and x is the variable I am asking for below.

How can I save the value of Round[b[i], 0.1] == 74 as a variable (let's say x as in the code) only for when the condition Round[b[i], 0.1] == 74 is met. So, to be more specific if b[i]=73.99, for example, then the code would translate to Which[Round[73.99, 0.1] (*which is equal to 74*) == 74,1,40 < b[i] < 73.99,2] but if b[i]=75, for example, then it will keep being Which[Round[75, 0.1] == 74,1,40 < b[i] < x,2] as Round[b[i]] is still not 74 and hence x is still a variable with nothing assigned to. Is this possible?

Edit: I will also appreciate comments on getting Round[b[i], 0.1] == 74 equal to a variable even when the condition is not met, which should be simpler to do. For this case I tried naively Which[x==Round[b[i], 0.1] == 74,1,40 < b[i] < x,2] but here x will be "True" or "False" but not the number.

I hope this makes sense and I appreciate in advanced your help

Thank you!

• why can't you just do x = Round[73.99, 0.1]; If[x == 74, .etc....] ? Commented May 7, 2020 at 23:31
• What are you expecting the Which to evaluate to when the x variable is unassigned? This feels like an XY problem.. Commented May 7, 2020 at 23:46

If you are already using a Which statement to control the flow, you can do more than one thing at a time by using ;. While it might seem like the purpose of ; is just to suppress output, it's actually the infix form of CompoundExpression which lets you join many expressions together where only one overall expression is allowed.

Which[
Round[b[i], 0.1] == 74,
x = Round[b[i], 0.1]; 1, (* you could also just put x = 74 *)
40 < b[i] < x,
2
]


You could also use brackets to set the assignment apart from the comparison. I think this would allow you to complete both tasks that you wanted:

Which[
(x = Round[b[i], 0.1]) == 74,
1,
40 < b[i] < x,
2
]


This is pretty close to what you had in your edit, I think you just needed the brackets.

• Thank you @MassDefect your first code using ; (which I did not know was possible) is exactly what I wanted!
– John
Commented May 8, 2020 at 0:42
• @John No problem! Note that in the first code, x only gets an assignment if the first condition is met, while in the second code x always gets assigned regardless of which condition is met. Commented May 8, 2020 at 0:52

Do you mean:

Which[
If[Round[b[i], 0.1] == 74, x = Round[b[i], 0.1]]; Round[b[i], 0.1] == 74, 1,
40 < bi < x, 2
]


You can remove the value of x by using x=..