# Mathematica "Do" loop with "If" conditional

Tags:
1. May 20, 2016

### Monaliza Smile

Hi,

I have a function defined by:

F[x_,y_,z_]:=3 x+y-2Y[z]

where
Y[z_]:=3/z

I'd like to run Do loop to calculate the value of this function for specific regions of x,y,z.

I can use:

Do[F[x,y,z],{x,1,2,1},{y,-2,-1,1},{z,3,5,1}],

But I have two questions here:

First: How to save the output in .dat file ? I 'd like to have something like:

F $~~~~$ x $~~~~$ y $~~~~$ z

Second: How Do loop can contain a conditional If, to put constrain on Y[z] -> 0 <Y[z] < 1, before evaluating F.

Last edited: May 20, 2016
2. May 20, 2016

### Staff: Mentor

You will be better off to use Table here rather than Do.

Just put the If inside the definition of Y[z_]

3. May 20, 2016

### JorisL

With the export-to-file in mind I'd suggest creating the table as follows

Code ( (Unknown Language)):

data = Table[ If[ Y[z]>0 && Y[z]<1, {F[x,y,z], x, y, z}, 0 ], {x,1,2,1}, {y,-2,-1,1}, {z,3,5,1}] // DeleteCases[0];

Next you can use Export[ file, data, format].

Remark;
1. I'm not sure what would happen if you leave the else-case empty in the If[] statement. Perhaps it doesn't add an element or maybe a zero.
I'm lazy so I went for a way that certainly works.
2. For ranges like $\{x,1,2,1\}$ you don't have to add the step size if it is one.

Question;
Is the function $Y[z_]$ always the same? Then you can simply solve the constraint and avoid it.
Otherwise you could try to automate it by solving the inequalities and using the answer in the range.