I have this simple test code:
n1 = 0;
n2 = 0;
m = 0;
If[n1 == 0, n2 == 1]
which evaluates as False, as it should. However, if I do:
If[n1 == 0, n2 == 1; m = 1];
m
I get 1 instead of 0 (m value shouldn't change, if the expression evaluates to False). What am I doing wrong?
Translation to make it easier to see. When you write
If[n1 == 0, n2 == 1]
This translates to
IF n1==0 THEN
n2==1 (*this gives False*)
END IF
Since last expression of IF is its value, then the above returns False which is what you got.
Now when you write
If[n1 == 0, n2 == 1; m = 1 ];
This translates to
IF n1 == 0 THEN
n2==1; (*this does nothing, it just gives False, but not used*)
m=1 (*this assigned m =1, over writing its old value of zero*)
END IF
The last expression gives 1 now, which is value of m after it was assigned.
Now when you check you find m is 1 as expected.
m value shouldn't change
Why? You assigned 1 to m, so it should change? Mathematica is correct.
what I want to do is, if n1=0 and n2=1 them m=1. I tried using && and "and" instead of the come between n1=0 , n2=1, but they didn't work either
It seems you thought that n1==0 , n2==1 means n1==0 and n2==1 but that is not the case. It translates to n1==0 THEN n2==1. The first comma inside the If means THEN and not logical and and the second comma means ELSE.
So to do what you want use
If[n1 == 0 && n2 == 1, m = 1]
Now m stays zero
The above can also be written as
If[And[n1==0,n2==1],m=1]
Which translates to
IF n1==0 AND n2==1 THEN
m=1
END IF
The structure of If is this
If[ booleanValuedExpression , truePart, falsePart ]
Which translates to
IF booleanValuedExpression THEN
truePart expressions separated by `;'
ELSE
falsePart expressions separated by `;' but do not end last one by `;`
END IF
So you see that the first comma inside If translates to THEN and the second comma translates to ELSE and the closing ] translates to END IF.
The above is for the two argument If. Mathematica also also has the three arguments If.
When writing my complicated if then else in Mathematica, I like to do alignment as follows to make it clear to me
If[ booleanValued ,
stuff;
stuff;
more stuff
,
stuff;
stuff;
more stuff
]
I use code cell and not input cell when writing in notebooks (for .m files (i.e package files), I use notepad++ so I am free to do any alignment I want).
Using code cell in notebook makes it easier to do the above.
I always put the ELSE (which is the second comma in Mathematica) shifted left to align under the If so it is more clear to me reading the code.
I must admit I old fashioned and prefer explicit THEN, ELSE, END IF in code instead of commas and braces or spaces (as in Python). These make the logic more clear to me.
