This function requests a string input and reverses it. For some reason, I just cannot wrap my head around the logic behind it.
def reverse(s):
new = ""
for i in s:
print(new)
new = i + new
return new
oldStr = input("String?")
newStr = reverse(oldStr)
print(newStr)
print(reverse("good bye"))
A friend suggested I print the variable new in the string which I added and it helped a little, but I just don't understand it.
It looks to me as if you are in a stage where you want to learn how to debug small programs.
For questions on StackOverflow, you want to provide a minimal reproducible example. What does that mean in your case?
"repoducible" means that you should not depend on user input. Replace user input by hardcoded values.
"minimal" means that you don't call the function twice. Once is enough.
For that reason, I'll not walk you through your original program, but through this one instead:
def reverse(s):
new = ""
for i in s:
print(new)
new = i + new
return new
print(reverse("abcdef"))
I'll use a free program called PyCharm Community Edition. You set a breakpoint where you want to understand how things work. Since you don't understand the reverse() method, put it right at the beginning of that method. You do that by clicking between the line number and the text:
Even if your code has no bug, go to the Run/Debug menu:
Execution will stop at the breakpoint and you'll now be able to see all the variables and step through your code line by line.
Look at all the variables after each single line. Compare the values to what you think the values should be. If there is a mismatch, it's likely a misunderstanding on your side, not by Python. Do that a few times and it'll be obvious why and how the string is reversed.
Let analysis it:
Iteration 1:
new = 'A', i = g
Perform new = i + new : new = gA
Iteration 2:
new = gA, i = o
Perform new = i + new: new = ogA
.
.
.
This happens because we add new i before we add the existing string from previous iteration.
the key is in "new = i + new" note that new is the previous iteration character and is in the right side of the current character, this cause the string to reverse
Related
beginer question:
I often have to repeat the same line of code 10 times for 10 different variables (i read you can't or should not create variable dynamically)
I thought to help me once i written it once I could iterate with a find replace (I am currently doing it in word)
I wrote this but get tons of error when i input code like if console trying to execute the code. what am i missing? is it because i paste it i the console does it need to be imported from a ext file ?
LineOfCode = input("Enter the code you want to iterate the iteration variable need be 8")
for i in range(10):
LineOfCode.replace("8",str(i))
print(LineOfCode)
Your code is almost correct:
String.replace() returns a new string with the changes, doesn't change the existing string. So the code should be like:
LineOfCode = input("Enter the code you want to iterate the iteration variable need be 8")
for i in range(10):
var = LineOfCode.replace("8", str(i))
print(var)
The question isn't clear enough, so if this isn't what you wanted, edit the question.
Besides this, you can use Visual Code as an IDE, it'll make things much, much easier than Word.
About the errors, I think you might be trying to run the code directly in console. Instead of typing only the file name, type python file.py, assuming the file name is file. The file must be a python file (ending with .py), also it shouldn't be saved as a word file. It must be plain text.
If you want to run the same code on multiple variables, you can create the code as a string, and then use the builtin exec to execute it. Example:
a = 1
b = 2
c = 3
variables = ["a", "b", "c"]
for var in variables:
exec("print(" + var + ")") # print the variable before modifying
exec(var + " *= 2") # multiply the current variable by 2
exec("print(" + var + ")") # print the variable after modifying
If this is not what you need, your question was not clear enough and i misunderstood it, you just have to provide more information on what excatly you need by adding a comment under your question, in which you first type #Programmer and then your text.
I want to import and insert word in sequence and NOT RANDOMLY, each registration attempt uses a single username and stop until the registration is completed. Then logout and begin a new registration with the next username in the list if the REGISTRATION is FAILED, and skip if the REGISTRATION is SUCCEDED.
I'm really confused because I have no clue. I've tried this code but it chooses randomly and I have no idea how to use the "for loop"
import random
Copy = driver.find_element_by_xpath('XPATH')
Copy.click()
names = [
"Noah" ,"Liam" ,"William" ,"Anthony"
]
idx = random.randint(0, len(names) - 1)
print(f"Picked name: {names[idx]}")
Copy.send_keys(names[idx])
How can I make it choose the next word in sequence and NOT RANDOMLY
Any Help Please
I am going to assume that you are happy with what the code does, with exception that the names it picks are random. This narrows everything down to one line, and namely the one that picks names randomly:
idx = random.randint(0, len(names) - 1)
Simple enough, you want "the next word in sequence and NOT RANDOMLY":
https://docs.python.org/3/tutorial/datastructures.html#more-on-lists
If you take a look at the link I've provided, you can see that lists have a pop() method, returning and removing some element from the list. We want the first one so we will provide 0 as the argument for the pop method.
We modify the line to look something like this
name = names.pop(0)
Now you still want to have the for-loop that will loop over all of the actions including name picking so you encapsulate all of the code in a for-loop:
names = [
"Noah" ,"Liam" ,"William" ,"Anthony"
]
for i in range(len(names)):
# ...
Copy = driver.find_element_by_xpath('XPATH')
Copy.click()
name = names.pop(0)
print(f"Picked name: {name}")
Copy.send_keys(name)
# ...
You might notice that the names list is not inside the for-loop. That is because we don't want to reassign the list every time we try to use a new name.
If you're completely unsure how for-loops work or how to implement one yourself, you should probably start by reading about how they work.
https://docs.python.org/3/tutorial/controlflow.html?highlight=loop#for-statements
Last but not least you can see some # ... comments in my example indicating where the logic will probably go for the other part of your question: "Then logout and begin a new registration with the next username in the list if the REGISTRATION is FAILED, and skip if the REGISTRATION is SUCCEDED." I don't think we I can help you with that since there is simply not enough context or examples in your question.
Refer to this guide explaining how to ask a well formulated question so we can help you more next time.
I currently have the below syntax -
BEGIN PROGRAM.
import spss,spssdata
varlist = [element[0] for element in spssdata.spssdata('CARD_2_Q2_1_a').fetchall()]
varstring = " ".join(str(int(i)) for i in varlist)
spss.submit("if (Q4_2 = 2 AND CARD_2_Q2_1_a = %(varstring)s) Q4_2_FULL = %(varstring)s." %locals())
END PROGRAM.
I thought this would just loop through the values in my variable CARD_2_Q2_1_a and populate Q4_2_FULL where appropriate. It worked in long hand without Python use, but the code above doesn't change the input file at all. Any reason why this might not be working or an alternative way of doing this?
varstring will be a string of integers joined by blanks. Therefore, your test condition in the IF will never be satisfied. Hence Q4_2_FULL will never be populated. You can print out the command you are submitting to see this.
I'm not sure exactly what your desired result is, but remember that the IF command you are submitting will execute over the entire dataset.
As part of an assignment I've been given some code written in python that was used to encrypt a message, and I have to try and understand the code and decrypt the ciphertext. I've never used python before and am somewhat out of my depth.
I understand most of it and the overall gist of what the code is trying to accomplish, however there are a few lines near the end tripping me up. Here's the entire thing (the &&& denotes sections of code which are supposed to be "damaged", while testing the code I've set secret to "test" and count to 3):
import string
import random
from base64 import b64encode, b64decode
secret = '&&&&&&&&&&&&&&' # We don't know the original message or length
secret_encoding = ['step1', 'step2', 'step3']
def step1(s):
_step1 = string.maketrans("zyxwvutsrqponZYXWVUTSRQPONmlkjihgfedcbaMLKJIHGFEDCBA","mlkjihgfedcbaMLKJIHGFEDCBAzyxwvutsrqponZYXWVUTSRQPON")
return string.translate(s, _step1)
def step2(s): return b64encode(s)
def step3(plaintext, shift=4):
loweralpha = string.ascii_lowercase
shifted_string = loweralpha[shift:] + loweralpha[:shift]
converted = string.maketrans(loweralpha, shifted_string)
return plaintext.translate(converted)
def make_secret(plain, count):
a = '2{}'.format(b64encode(plain))
for count in xrange(count):
r = random.choice(secret_encoding)
si = secret_encoding.index(r) + 1
_a = globals()[r](a)
a = '{}{}'.format(si, _a)
return a
if __name__ == '__main__':
print make_secret(secret, count=&&&)
Essentially, I assume the code is meant to choose randomly from the three encryption methods step1, step2 and step3, then apply them to the cleartext a number or times as governed by whatever the value of "count" is.
The "make_secret" method is the part that's bothering me, as I'm having difficulty working out how it ties everything together and what the overall purpose of it is. I'll go through it line by line and give my reasons on each part, so someone can correct me if I'm mistaken.
a = '2{}'.format(b64encode(plain))
This takes the base64 encoding of whatever the "plain" variable corresponds to and appends a 2 to the start of it, resulting in something like "2VGhpcyBpcyBhIHNlY3JldA==" using "this is a secret" for plain as a test. I'm not sure what the 2 is for.
r = random.choice(secret_encoding)
si = secret_encoding.index(r) + 1
r is a random selection from the secret_encoding array, while si corresponds to the next array element after r.
_a = globals()[r](a)
This is one of the parts that has me stumped. From researching global() it seems that the intention here is to turn "r" into a global dictionary consisting of the characters found in "a", ie somewhere later in the code a's characters will be used as a limited character set to choose from. Is this correct or am I way off base?
I've tried printing _a, which gives me what appears to be the letters and numbers found in the final output of the code.
a = '{}{}'.format(si, _a)
It seems as if this is creating a string which is a concatenation of the si and _a variables, however I'll admit I don't understand the purpose of doing this.
I realize this is a long question, but I thought it would be best to put the parts that are bothering me into context.
I will refrain from commenting on the readability of the code. I daresay
it was all intentional, anyway, for purposes of obfuscation. Your
professor is an evil bastard and I want to take his or her course :)
r = random.choice(secret_encoding)
...
_a = globals()[r](a)
You're way off base. This is essentially an ugly and hard-to-read way to
randomly choose one of the three functions and run it on a. The
function globals() returns a dict that maps names to identifiers; it
includes the three functions and other things. globals()[r] looks up
one of the three functions based on the name r. Putting (a) after
that runs the function with a as the argument.
a = '{}{}'.format(si, _a)
The idea here is to prepend each interim result with the number of the
function that encrypted it, so you know which function you need to
reverse to decrypt that step. They all accumulate at the beginning, and
get encrypted and re-encrypted with each step, except for the last one.
a = '2{}'.format(b64encode(plain))
Essentially, this is applying step2 first. Each encryption with
step2 prepends a 2.
So, the program applies count encryptions to the plaintext, with each
step using a randomly-chosen transformation, and the choice appears in
plaintext before the ciphertext. Your task is to read each prepended
number and apply the inverse transformation to the rest of the message.
You stop when the first character is not in "123".
One problem I see is that if the plaintext begins with a digit in
"123", it will look like we should perform another decryption step. In
practice, however, I feel sure that the professor's choice of plaintext
does not begin with such a digit (unless they're really evil).
I'm fairly new to python, but I'm making a script and I want one of the functions to update a variable from another file. It works, but when I exit the script and reload it, the changes aren't there anymore. For example (this isn't my script):
#File: changeFile.txt
number = 0
#File: changerFile.py
def changeNumber():
number += 1
If I retrieve number during that session, it will return 1, but if I exit out and go back in again and retrieve number without calling changeNumber, it returns 0.
How can I get the script to actually save the number edited in changeNumber to changeFile.txt? As I said, I'm fairly new to python, but I've looked just about everywhere on the Internet and couldn't really find an answer that worked.
EDIT: Sorry, I forgot to include that in the actual script, there are other values.
So I want to change number and have it save without deleting the other 10 values stored in that file.
Assuming, as you show, that changeFile.txt has no other content whatever, then just change the function to:
def changeNumber():
global number # will not possibly work w/o this, the way you posted!
number += 1
with open('changeFile.txt', 'w') as f:
f.write('number = {}\n'.format(number))
ADDED: the OP edited the Q to mention (originally omitted!-) the crucial fact that changefile.txt has other lines that need to be preserved as well as the one that needs to be changed.
That, of course, changes everything -- but, Python can cope!-)
Just add import fileinput at the start of this module, and change the last two lines of the above snippet (starting with with) to:
for line in fileinput.input(['changefile.txt'], inplace=True):
if line.startswith('number ');
line = 'number = {}\n'.format(number)'
print line,
This is the Python 2 solution (the OP didn't bother to tell us if using Py2 or Py3, a crucial bit of info -- hey, who cares about making it easy rather than very hard for willing volunteers to help you, right?!-). If Python 3, change the last statement from print line, to
print(line, end='')
to get exactly the same desired effect.