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For a particular reason I am trying to find (not exhaustively) the worst hash algorithm. I found this post analyzing some, and I targeted the Loselose algorithm. This page states the algorithm in C (I think) as:
unsigned long hash(unsigned char *str)
{
unsigned int hash = 0;
int c;
while (c = *str++)
hash += c;
return hash;
}
I'm not a programmer, and C (or C++ ?) of this block is killing me. Could you please give me a help and provide the equivalent in python?
PS.: For those asking "why the worst hash?",I intend to create set2, an "equivalent" to an original set1, but reduced in elements due to high algorithm hash collisions. I don't need to go back to the original set1. I just need to know if a hash is present in set2.
The C++ code that you have provided simply iterates over each character of the given string and adds its ASCII value to the variable hash and returns the hash.
We can achieve the same in Python 2.7 by doing this:
def custom_hash(s):
hsh = 0
for c in s: hsh += ord(c)
return hsh
Or you can always try to be Pythonic and do the same in one line :)
hsh = reduce(lambda x, y: ord(x) + ord(y), list(s))
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In C, I can do
for(i=0, j=0; i < k; i++, j++) {
// do stuff
}
Is there an equivalent of that in Python?
If you mean exactly what you wrote, then not really, because Python does not have C-style loop.
If you actually mean something like "iterate on two collections at the same time" then yes: you compose the two collections into one iterator using zip or zip_longuest:
for a, b in zip(x, y):
...
That works with arbitrary collections so e.g.
for i, j in zip(range(k), itertools.count()):
...
will perform the same traversal of i and j your C example does, assuming the loop body does not actually update either. Which obviously makes your example not very useful, because i and j always have the same value.
If i and j need to be updated during the loop, then your only recourse is to use a fully desugared while loop and manual increment, possibly an offset rather than two counters. Because, again, Python doesn't have C-style loops.
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I've coded a check for integers which returns a boolean value:
def NoZerosFives(n: str):
if '0' in n or '5' in n: #It checks if any of the digits is 0 or 5
return False
return True
which I then put in a script:
for n in range(10**20000, 10**100000000):
d = str(n)
if NoZerosFives(d):
pass
else:
print('Failed the check.')
n is obviously extremely large, and the else:print('Failed the check.') is here just to tell me if the script's going or not, but it's not printing anything.
My first thought is that the problem is not with Python but with my hardware (I have a pretty powerful laptop, but still a laptop). Is my computer not powerful enough? Should I have done something to my PC that I didn't do?
Or is the code not fit for the objective and I just need to do something different?
As #jordanm points out in a comment, NoZerosFives will not return True (which is the only way your code produces any output at all) for a very long time. Think about how long it's going to take, starting from 10**20000 (shown below), to reach a value with no zeros in it by just adding 1 at each step.
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0
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While programming in python 3.6, I came across with some problem in list :
l = [2,2,3,3,3]
for n in range(len(l)):
print(l[n+1]) #increasing value of n by 1
This code will obviously give an error :
IndexError: list index out of range
But in case of c language :
#include<stdio.h>
int main() {
int i =0;
int arr[5] = {1,2,3,4,5};
for(i = 0 ; i<=4;i++){
printf("%d " , arr[i+1]);
}
return(0);
}
Gives the correct output(including garbage value):
2 3 4 5 32764
I wanted to know why are we not getting the value of elements of list in python by using the future values of n whereas in c it is just reverse?
In C, an "array" is mainly a pointer to a memory location (nb: this is an obvious simplification, you can read this for more details), and arr[x] will return N bytes from address_of_arr + (N * x) (where N is the size of the array data type). Like all memory access in C, there's absolutely NO check so you can read (and write) just any arbitrary memory location.
Note that, as Daniel Pryden mentions in a comment, the behaviour you observe with your C example is called an "undefined behaviour", which means the result is not defined by the C langage specs, depends on the implementation (your C compiler, platform and whatnots), and could actually yield just any result (including a core dump, erasing your hard drive or launching a nuclear missile). Luckily the implementation you tested it with just returns garbage. Now if you really feel lucky, for a bit of fun try writing at this memory location and find out what happens ;)
In Python, a list is an object which knows it's length and won't let you try to access items at indexes that don't exist. Where and how the effective list contents are stored in memory is handled by the implementation and is totally opaque to the Python code.
Disclaimer : this answer is based on C implementations most commonly found on today's operating systems - the C language spec doesn't specify anything about how an array should be implemented (just how it's supposed to work) so an implementation could as well choose to store array datas on clay tablets disposed on a rubber band and move that rubber band by N positions left or right, in which case my answer would be totally irrelevant...
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How to do loop like: for(i = 0; i < 100; i++)?
This code is in PHP, i want create this loop like in Python 3x.
Thanks my brothers for any what you do for me.
Use the built-in range function which takes an optional step and an interval [a, b) like so:
for i in range(0, 100, 1):
# do something
A range based for loop in python is not equivalent to C like for loop that is prevalent in javascript, php or languages as such which supports them. Particularly, the distinction comes in
when the need comes to change the iterator variable inside the loop block
Condition is more complex beyond a limiting condition
Ideally the best alternative is to re-write the for loop to a while loop in the similar way as you would have written in languages that supports both. For example
for(<initializer>; <cond>; <increment>) {
// Loop Body
}
should equivalently be written as
<initializer>
while cond:
<Loop Body>
<increment>
so in your particular case, the equivalent of the php for(i = 0; i < 100; i++) construct in python would be
i = 0
while i < 100:
# Loop Body
i += 1
Using any other construct might have some surprising consequences.
for i in range(100):
# code here
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How can I write following while loop in python?
int v,t;
while (scanf("%d %d",&v,&t)==2)
{
//body
}
Python being a higher level language than C will normally use different patterns for this kind of situation. There is a deleted question that in fact had the exact same behavior than your C snippet - and thus would be "correct", but it became so ugly a piece of code in Python it was downvoted pretty fast.
So, first things first - it is 2015, and people can't simply look at a C's "scanf" on the prompt and divine they should type two white space separated integer numbers - you'd better give then a message. Anther big difference is that in Python variable assignments are considered statements, and can't be done inside an expression (the while expression in this case). So you have to have a while expression that is always true, and decide whether to break later.
Thus you can go with a pattern like this.
while True:
values = input("Type your values separated by space, any other thing to exit:").split()
try:
v = int(values[0])
t = int(values[1])
except IndexError, ValueError:
break
<body>
This replaces the behavior of your code:
import re
while True:
m = re.match("(\d) (\d)", input()):
if m is None: #The input did not match
break #replace it with an error message and continue to let the user try again
u, v = [int(e) for e in m.groups()]
pass #body