I have a hard time understanding for loops in Python. I have the following code in Javascript (Modolus 11 test) I want to convert to Python:
var secNumber = this.dob + lastDigits;
var factors = [4, 3, 2, 7, 6, 5, 4, 3, 2, 1];
for (var i = 0, sum = 0; i < factors.length; i++) {
sum += secNumber.substring(i, i + 1) * factors[i];
}
// modulus 11 test
return sum % 11 !== 0 ? false : true;
What would the Python equivalent be ?
Python uses for each loops, so iterating over numbers would require you to produce a sequence of numbers first. Python uses the range() type for this:
sum = 0
for i in range(len(factors)):
pass
Note that sum is assigned 0 explicitly, separately.
Usually however, you'd just loop over objects directly, rather than generate indices:
sum = 0
for element in factors:
pass
But if you are summing the factors, just use the sum() function:
total = sum(factors)
Note that I avoided using the name sum there to avoid masking the built-in function.
To multiply bytes from a sequence of bytes, just zip() the bytes and factors together:
sum = 0
for byte, factor in zip(string, factors):
sum += byte * factor
which can be collapsed with sum() and a generator expression:
total = sum(byte * factor for byte, factor in zip(string, factors))
This only works if string is a bytes object, not a Unicode str string object; bytes objects are really sequences of integers in the range 0-255.
If you wanted to interpret the characters as digits, use a Unicode string and use int() to convert each to an integer:
total = sum(int(digit) * factor for digit, factor in zip(string, factors))
Demo of the latter with the sample input you provided:
>>> sec_number = '0411994545'
>>> factors = [4, 3, 2, 7, 6, 5, 4, 3, 2, 1]
>>> sum(int(digit) * factor for digit, factor in zip(string, factors))
164
>>> total = sum(int(digit) * factor for digit, factor in zip(string, factors))
>>> total % 11 == 0
False
Note that the == test already produces a boolean; there is no need to use a conditional expression in Python to produce a boolean result.
ssum = sum(int(x) * y for (x, y) in zip(S, factors))
read about documentation of for in python:
for i in range(len(factors)):
# do stuff
Note: indexing start from 0 and end one less
demo:
>>> [x for x in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
I think this is what you want:
>>> s="0411994545"
>>> f="1234567894"
>>> sum([int(x)*int(y) for x,y in zip(s,f)])
238
>>> sum([int(s[i])*int(f[i]) for i in range(len(s))])
238
In Python there is no "traditional" for-loop, but there are many constructs which make loops in Python very powerful:
for i in range(0, 10):
print(str(i))
This prints integers from 0 to 9, range creates an iterator with the interval [low, high)
for i in range(0, 10, 2):
print(str(i))
Prints 0, 2, 4, 6, 8 because the steps (third optional parameter are set to 2).
Also you can enumerate through your factors:
for key, val in enumerate(factors):
pass
This gets you a key and the corresponding value of each element in factors.
If you do not need keys, than this simply does the job for you:
for val in factors:
# some stuff to do
This iterates over every element in factors.
If you need a special condition to be checked, you can use break or continue:
for val in factors:
if val == 0:
continue # skips one iteration
elif val == 1:
break # cancels the whole loop
else
# do some stuff
Python doesn't strictly have C/Javascript style for loops but prefers a simpler syntax.
The C/JS style for has:
var i=0, sum=0i< factors.lengthi++sum += string.substring(i, i + 1) * factors[i]A pythonic for loop has only a variable and a list or iterable and a task, and looks like this:
mysum = 0
for i in range(length):
mysum += int(strings[i])*factors[i]
where range(length) is a built-in function that expands to [0,1,2,3,.., length-1] and the task consists of the indented block after the colon
In JS strings and numeric types are automatically converted (even when you dont want it), but in Python that is not true. int() converts to integer and float() converts to floating point number.
iandsum?factorsand expected output.