Develop Reference

Aerial firefighting kata python

I have built a code for a codewars problem. I think it is correct but it shows me an error I don't understand.
Can you tell me what am I doing wrong?
import math
def waterbombs(fire, w):
s=""
countx=0
for i in fire:
if i=="x":
countx+=1
elif i=="Y":
countx=0
return sum(math.ceil(countx/w))
waterbombs("xxYxx", 3)
This is the error:
Traceback (most recent call last):
File "D:\Curso Python Pildorasinformaticas\Ejercicios Codewars\Aerial Firefighting.py", line 16, in <module>
waterbombs("xxYxx", 3)
File "D:\Curso Python Pildorasinformaticas\Ejercicios Codewars\Aerial Firefighting.py", line 13, in waterbombs
return sum(math.ceil(countx/w))
TypeError: 'int' object is not iterable
[Finished in 0.2s]

Why are you doing sum(math.ceil(countx/w)) ?
What is the objective of the sum method here, since there is only value returned by math.ceil ?
The sum would throw that error if you pass a single value to it. You're supposed to pass a list of values of the sum method.
For eg: sum(5) would give you the same error you see above, but sum([5]) would return you 5.

recurse through json in python

I have a sample json:
I want to use the json module of python and recurse through to find the "MaxSize" in "pevcwebasg". Have the following code:
import json
param_file_handle = json.load(open("sample.json"))
print param_file_handle['Resources']['pevcwebasg']['Type']
resources = param_file_handle['Resources']
for asg in resources:
print asg["Type"]
The out put of which is :
> AWS::AutoScaling::AutoScalingGroup Traceback (most recent call last):
> File "test.py", line 8, in <module>
> print asg['Type'] TypeError: string indices must be integers
What I dont get is this line "print param_file_handle['Resources']['pevcwebasg']['Type']" works fine and gets me the output but when i recurse through and try and find asg["Type"], it fails. Any better way to do this ? I need to recurse through the tree and find the value.
Edit 1:
as I do recurse through values, I get stuck with error.
param_file_handle = json.load(open("sample.json"))
resources = param_file_handle['Resources']
for asg in resources.values():
if asg["Type"] == "AWS::AutoScaling::AutoScalingGroup":
for values in asg['Properties']:
print values["MaxSize"]
error :
Traceback (most recent call last):
File "test.py", line 9, in <module>
print values["MaxSize"]
TypeError: string indices must be integers

for asg in resources:
This iterates through the keys of resources, rather than the values. Try:
for asg in resources.values():

param_file_handle = json.load(open("sample.json"))
resources = param_file_handle['Resources']
for asg in resources.values():
if asg["Type"] == "AWS::AutoScaling::AutoScalingGroup":
print asg["Properties"]["MaxSize"]
try this.

you broke hirerchy, missed "pevcwebasg"
resources = param_file_handle['Resources']['pevcwebasg']
for asg in resources:
print asg["Type"]

Python: limit on the accuracy of float

The code gives an error because the value of "var" is very close to zero, less than 1e-80. I tried to fix this error using "Import decimal *", but it didn't really work. Is there a way to tell Python to round a number to zero when float number is very close to zero, i.e. < 1e-50? Or any other way to fix this issue?
Thank you
CODE:
import math
H=6.6260755e-27
K=1.3807e-16
C=2.9979E+10
T=100.0
x=3.07175e-05
cst=2.0*H*H*(C**3.0)/(K*T*T*(x**6.0))
a=H*C/(K*T*x)
var=cst*math.exp(a)/((math.exp(a)-1.0)**2.0)
print var
OUTPUT:
Traceback (most recent call last):
File "test.py", line 11, in <module>
var=cst*math.exp(a)/((math.exp(a)-1.0)**2.0)
OverflowError: (34, 'Numerical result out of range')
To Kevin:
The code was edited with following lines:
from decimal import *
getcontext().prec = 7
cst=Decimal(2.0*H*H*(C**3.0)/(K*T*T*(x**6.0)))
a=Decimal(H*C/(K*T*x))

The problem is that (math.exp(a)-1.0)**2.0 is too large to hold as an intermediate result.
>>> (math.exp(a) - 1.0)**2.0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
OverflowError: (34, 'Result too large')
However, for the value of a you are using,
>>> math.exp(a)/(math.exp(a)-1.0) == 1.0
True
so you can essentially cancel that part of the fraction, leaving
var = cst/(math.exp(a)-1.0)
which evaluates nicely to
>>> cst/(math.exp(a)-1.0)
7.932672271698049e-186
If you aren't comfortable rewriting the formula to that extent, use the associativity of the operations to avoid the large intermediate value. The resulting product is the same.
>>> cst/(math.exp(a)-1.0)*math.exp(a)/(math.exp(a)-1.0)
7.932672271698049e-186

I solved this issue but that will work only for this particular problem, not in general. The main issue is the nature of this function:
math.exp(a)/(math.exp(a)-1.0)**2.0
which decays very rapidly.
Problem can be easily solved restricting the value of "a" (which won't make any significant change in calculation). i.e.
if a>200:
var=0.0
else:
var=cst*math.exp(a)/((math.exp(a)-1.0)**2.0)

Trouble with for loops

We just learned for loops in class for about five minutes and we were already given a lab. I am trying but still not getting what I need to get. What I am trying to do is take a list of integers, and then only take the odd integers and add them up and then return them so if the list of integers was [3,2,4,7,2,4,1,3,2] the returned value would be 14
def f(ls):
ct=0
for x in (f(ls)):
if x%2==1:
ct+=x
return(ct)
print(f[2,5,4,6,7,8,2])
the error code reads
Traceback (most recent call last):
File "C:/Users/Ian/Documents/Python/Labs/lab8.py", line 10, in <module>
print(f[2,5,4,6,7,8,2])
TypeError: 'function' object is not subscriptable

Just a couple of minor mistakes:
def f(ls):
ct = 0
for x in ls:
# ^ Do not call the method, but just parse through the list
if x % 2 == 1:
ct += x
return(ct)
# ^ ^ parenthesis are not necessary
print(f([2,5,4,6,7,8,2]))
# ^ ^ Missing paranthesis

You're missing the parenthesis in the function call
print(f([2,5,4,6,7,8,2]))
rather than
print(f[2,5,4,6,7,8,2])

Return the average mark givin the respective lecture

Each line represents a single student and consists of a student number, a name, a section code and a midterm grade, all separated by whitespace.
The first parameter is already done and the file is open and
The second parameter is a section code
this is the link http://www.cdf.toronto.edu/~csc108h/fall/exercises/e3/grade_file.txt
My code:
def average_by_section(the_file, section_code):
'''(io.TextIOWrapper, str) -> float
Return the average midtermmark for all students in that section
'''
score = 0
n = 0
for element in the_file:
line = element.split()
if section_code == line[-2]:
mark = mark + float(line[-1])
n += 1
lecture_avg = mark / n
return lecture_avg
I'm getting an index out of range. Is this correct? Or am I just opening up the wrong file?
can someone test this code and download that file? I'm pretty sure it should work, but not for me.

Well, you can troubleshoot the index out of range error with a print line or print(line) to explore the number of items in "line" (i.e. the effect of split()). I'd suggest looking closer at your split() statement...

It looks like you are omitting parts of your code where you define some of those variables (section_code, mark, etc.), but adjusting for some of those things seems to work properly. Assuming that the error you got was IndexError: list index out of range, that happens when you try to access an element of a list by index where that index doesn't exist. For instance:
>>> l = ['one']
>>> l[0]
'one'
>>> l[1]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
>>> l[-1]
'one'
>>> l[-2]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
Therefore in your code, you will get that error if line is ever fewer than two items. I would check and see what you are actually getting for line to make sure it is what you expect.