列表理解Python的多个条件语句

Question

I have a list with elements that I want to do operations on with 2 conditions .

One condition is to multiply a norm function by -1 if the element is negative or leaving the norm function positive if the element is positive.

The list of values looks like this:

print(values)

-97476.70633454417
-93170.30642401175
-89901.82679086612
-87187.62533194348
-87269.09594982903
-85513.31676486236
-83545.26529198853
-82411.91255123452
-81620.01849452594

As you can see they are all negative (in this experiment).

The code looks like this:

norm_BIC = [(-1.0) * float(i)/max(values) for i in values if i < 0 or float(i)/max(values) for i in values if i > 0]

If I run the code before the or statement it works:

norm_BIC = [(-1.0) * float(i)/max(values) for i in values if i < 0]

Which means it's everything else following that doesn't work because I get an empty list for norm_BIC when running after the or .

How do I fix this condition?

Answer 1

It seems that you are simply filtering out the negative values and turning them positive. The abs function takes the absolute value, turning your inputs positive.

would

norm_BIC = [ abs(float(i))/max(values) for i in values]

not fix your problem?

Answer 2

您应该使用else子句：

norm_BIC = [(-1.0) * float(i)/max(values) if i < 0 else float(i)/max(values) for i in values]

Answer 3

To achieve "this or that" in a list comprehension you need to use an if expression. This allows you to add an else clause as expected —

values = [
    -97476.70633454417,
-93170.30642401175,
-89901.82679086612,
-87187.62533194348,
-87269.09594982903,
-85513.31676486236,
-83545.26529198853,
-82411.91255123452,
-81620.01849452594,

]

norm_BIC = [(-1.0) * float(i)/max(values) if i < 0 else float(i)/max(values) for i in values if i > 0]

The syntax here isn't list-comprehension specific, compare to this if expression —

a = 10 if b < 10 else 20

Running your example values without that final if ie

norm_BIC = [(-1.0) * float(i)/max(values) if i < 0 else float(i)/max(values) for i in values]

I get the following output.

[-1.1942744945724522,
 -1.1415129295794078,
 -1.1014678561595233,
 -1.0682137414339221,
 -1.0692119109931586,
 -1.0477002865491576,
 -1.0235879240531134,
 -1.0097022038381638,
 -1.0]

If you just want to ensure the result of your calculation is positive you could use the built-in abs() function.

norm_BIC = [abs(float(i)/max(values)) for i in values]