comparison of standard deviation between various features

Question

hi i'm kinda new on python and i'm trying to create a code that compares standard deviation of different feature in order to extract the smallest one, i'm not sure about the code i've made

minstd=0;
for feature in range(13):
var = np.var(trainx[trainy==1,feature])
std = np.sqrt(var) # deviazione standard
a = std(feature)-std(feature+1)
if a>0
 minstd=std(feature);
else 
 minstd=std(feature+1);
minstd

i also got this error

File "<ipython-input-18-44801ce3407e>", line 6
if a>0
      ^SyntaxError: invalid syntax

can someone explain the problem?

Answer 1

Sorry, but I have to say what you need is some basic python grammar.

1. In python, you need not add a ; at the end of the line
2. In python, the function definition with a : behind the bracket (as well as for some control flow keywords like for, if...;
3. In python, the layer of the code are represented by indent (rather than {} in C or C++) Your code should be like this:

minstd=0
for feature in range(13):
    var = np.var(trainx[trainy==1,feature])
    std = np.sqrt(var) # deviazione standard
    a = std(feature)-std(feature+1)
    if a>0:
        minstd=std(feature);
    else:
        minstd=std(feature+1);
minstd
``