Say we have 10x10 pixels(as coordinates) and we want to plot the data.
Before working with 10x10 instead i started off with 3x3. The point of that project is say we have coordinate(1,2) we put a start at that point. I figured to set my coordinates as dictionary. Then i used two loops. My codes are below
i get the error keyerror:0
File "/home/caglar/temp.py", line 6, in
if dict[i]!=j:
KeyError: 0
dict={1:3,2:1,3:1}
xarr=[]
yarr=[]
for i in range(0,3):
for j in range(0,3):
if dict[i]!=j:
xarr.append("") and yarr.append("")
j+=1
else:
xarr[i].append("*") and yarr[j].append("*")
i+=1
print(xarr)
Your dictionary, dict={1:3,2:1,3:1}, has keys 1, 2, 3. However, you access it with dict[i] != j. i in range(0,3) means "loop through a series of i values starting at 0 and ending before 3". So it will go through 0, 1, 2. This means that when it tries to access the key 0 in your dictionary it can't find it and so raises a KeyError saying that it can't find 0.
Unrelated to your question, you also have some rather odd lines of code there which don't quite make sense:
for i in range(0,3) iterates through values of i, so you don't have to do i += 1 each time. In your code i += 1 and j += 1 don't actually do anything.
and is a boolean operator. It evaluates the boolean statements on either side and tells you if both are true. Pretty much anything can be interpreted as a boolean statement so it doesn't throw an error. But when you're doing xarr.append("") and yarr.append("") you're telling it to evaluate each of the appends, interpret them as booleans, and return the the boolean value of xarr.append("") and yarr.append("). To simply do both things you put each expression on its own line:
xarr.append("")
yarr.append("")
xarr.append("") appends an empty string to the xarr array. If you start with xarr = [] and then do xarr.append("") you'd end up with [""]. If you did it again you'd get ["", ""] and so on. When you do xarr[i].append("*") you're saying "access the i-th item in xarr and then append "*" to it". However, since the items in xarr aren't arrays, you can't append to them so it will error.
Related
I'm writing a program to check if a given user input is a palindrome or not. if it is the program should print "Yes", if not "no". I realize that this program is entirely too complex since I actually only needed to check the whole word using the reversed() function, but I ended up making it quite complex by splitting the word into two lists and then checking the lists against each other.
Despite that, I'm not clear why the last conditional isn't returning the expected "Yes" when I pass it "racecar" as an input. When I print the lists in line 23 and 24, I get two lists that are identical, but then when I compare them in the conditional, I always get "No" meaning they are not equal to each other. can anyone explain why this is? I've tried to convert the lists to strings but no luck.
def odd_or_even(a): # function for determining if odd or even
if len(a) % 2 == 0:
return True
else:
return False
the_string = input("How about a word?\n")
x = int(len(the_string))
odd_or_even(the_string) # find out if the word has an odd or an even number of characters
if odd_or_even(the_string) == True: # if even
for i in range(x):
first_half = the_string[0:int((x/2))] #create a list with part 1
second_half = the_string[(x-(int((x/2)))):x] #create a list with part 2
else: #if odd
for i in range(x):
first_half = the_string[:(int((x-1)/2))] #create a list with part 1 without the middle index
second_half = the_string[int(int(x-1)/2)+1:] #create a list with part 2 without the middle index
print(list(reversed(second_half)))
print(list(first_half))
if first_half == reversed(second_half): ##### NOT WORKING BUT DONT KNOW WHY #####
print("Yes")
else:
print("No")
Despite your comments first_half and second_half are substrings of your input, not lists. When you print them out, you're converting them to lists, but in the comparison, you do not convert first_half or reversed(second_half). Thus you are comparing a string to an iterator (returned by reversed), which will always be false.
So a basic fix is to do the conversion for the if, just like you did when printing the lists out:
if list(first_half) == list(reversed(second_half)):
A better fix might be to compare as strings, by making one of the slices use a step of -1, so you don't need to use reversed. Try second_half = the_string[-1:x//2:-1] (or similar, you probably need to tweak either the even or odd case by one). Or you could use the "alien smiley" slice to reverse the string after you slice it out of the input: second_half = second_half[::-1].
There are a few other oddities in your code, like your for i in range(x) loop that overwrites all of its results except the last one. Just use x - 1 in the slicing code and you don't need that loop at all. You're also calling int a lot more often than you need to (if you used // instead of /, you could get rid of literally all of the int calls).
The exercise I am doing was given by a book that takes a dictionary argument and asks for me to give a return value of True or False. I am new to Python 3 and as a personal exercise for learning I want to convert all the conditions of a "valid dictionary as a chessboard into" a single return value. I haven't actually tested this code for errors as it isn't finished, but I did run it through an online validator I found https://extendsclass.com/python-tester.html.
I want to know how I can convert the following 2 code blocks into simple expressions to be used in the return statement in my function below, You can see below that I've converted most expressions into the return value already with "and" because "ALL expressions must == True"
for pieces in dictionary.values():
if all(pieces.startswith('b')) or \
all(pieces.startswith('w')):
return True
else:
return False
The above code block loops through the dictionary keys passed to function as "pieces",
and compares each key individually to determine if it starts with a value of 'b' or 'w'. So if any key does not start with 'b' or 'w' the dictionary "chessboard" is false as it contains an improper piece. Ok I think I see an error in this I'm going to look into it and try to figure it out. Ok I noticed some errors in the above code that need to be addressed I am currently researching how to properly execute the above code.
for square in dictionary:
try:
if int(square[:0]) <= 8 and \
square[-1] <= 'h':
return True
else:
return False
except ValueError:
return False
I worked on the above code block a very long time and am still not sure that's the "best" implementation of what I want it to do. But I am still new and did my best.
Anyway it slices the dictionary key and compares the first char in the key to make sure it isn't over 8 which is the maximum "valid range" if over valid range it returns false and anything not int is obviously automatically False and returned as such by the "exception".
Then it slices the dictionary key to get the last char of the dictionary key and compares it to <= 'h' as that is the valid range and anything over 'h' or not a valid type value will return as False.
And then it compares the results of True/False "and" True/False with "and" because both conditions must be True.
Here is the function as it currently is with a test dictionary at the end:
def cBV(dic): # (c)hess(B)oard(V)alidator
Err = 'Error: Invalid Board -'
if not isinstance(dic, type({})):
raise TypeError('Object passed to cBV is not of type <class dict>')
chess_pieces = {'bk pieces': 0, 'wh pieces': 0,
'bk pawns': 0, 'wh pawns': 0}
# converts dictionary values into keys and assigns those keys "counn of values"
for squares, pieces in dic.items:
if pieces.startswith('b'): # alt if pieces[:0] == 'b':
if pieces.startswith('bpawn'): # alt if pieces == 'bpawn':
chess_pieces['bk pawns'] += 1
chess_pieces['bk pieces'] += 1
elif pieces.startswith('w'):
if pieces.startswith('wpawn'):
chess_pieces['wh pawns'] += 1
chess_pieces['wh pieces'] += 1
return 'wking' in dic.values() and \
'bking' in dic.values() and \
chess_pieces['bk pieces'] <= 16 and \
chess_pieces['wh pieces'] <= 16 and \
chess_pieces['bk pawns'] <= 8 and \
chess_pieces['wh pawns'] <= 8 and \
dict = {'8h': 'wking', '2c': 'bking', '3a': 'wpawn', '3b': 'wpawn', '3c': 'wpawn',
'3d': 'wpawn', '3e': 'wpawn', '3f': 'wpawn', '3g': 'wpawn', '3h': 'wpawn', '4b': 'wpawn'}
test = cBV(dict)
print('Is this a valid chessboard? ' + str(test))
What you have now is good, and you should feel proud - there are more fancy techniques for making things more concise, and you'll get more used to them as you wrap your head around how the various data structures work.
for pieces in dictionary.values()
if pieces.startswith('b') or \
pieces.startswith('w'):
return True
else:
return False
can be converted to the one-liner
return all(
piece.startswith('b') or piece.startswith('w')
for piece in dictionary.values()
)
which does a few things.
The all() function takes any iterable object, and returns True if all of the values in that iterable are truthy. If even one of them is not, then it 'short-circuits' and returns False instead.
As our argument to all(), we give a "comprehension". A comprehension is essentially a one-line for loop, of the form f(element) for element in iterable: it performs whatever f(element) is, for every element in the iterable.
In our case, the iterable is dictionary.values(), which returns all of the values (but not the keys) in dictionary (which, here, is 'wking', 'wpawn', ...). In this case, these are strings.
piece is what we assign each element of, for each 'iteration' of the comprehension. It'll run for piece = 'wking', then for piece = 'wpawn', etc.
piece.startswith('b') or piece.startswith('w') is the function that we perform for each piece. This outputs either True or False, depending on whether the conditions are met.
You can wrap a comprehension in square-brackets [] to have it output as a regular list. However, if you give a comprehension as an argument to a function like all(), which is what we're doing here, then it will end up as a "generator", a slightly more efficient object that only calculates one object at a time. For our purposes, this isn't important.
The comprehension, overall, produces a series containing either True or False, that all() will consume.
Similarly with your second code snippet. You have the basics down, but can be more concise. Your code:
def allSquaresAreInRange(dictionary):
for square in dictionary:
try:
if int(square[:0]) <= 8 and \
pieces[-1] <= 'h':
return True
else:
return False
except ValueError:
return False
can be turned into
def allSquaresAreInRange(dictionary):
try:
return all(
(1 <= int(row) <= 8) and ('a' <= col <= 'h')
for (row, col) in dictionary
)
except ValueError:
return False
Here we make use of a few things:
As before, we use all(), and as before, we use a comprehension. But this time, we iterate through dictionary directly
Iterating through a dict is functionally identical to iterating through dict.keys(). So, we're iterating through the keys '8h', '2c', ...
Each key is a two-character string. Strings are iterable, just like lists are, and most iterables have an interesting property called "multiple assignment": if we assign exactly as many variables as the iterable has elements, then those elements get split up.
(row, col) = '8h', for example, is functionally identical to row = '8h'[0] and col = '8h'[1]. In both cases, we're left with row = '8' and col = 'h'.
This produces a ValueError if the number of elements on either side is mismatched - for example, if the key has only one character, or only three characters. A byproduct of this is that row and col are guaranteed to be exactly one-character long strings, if that error doesn't happen.
Our condition checks if the row is between 1 and 8, and whether the col is between A and H, using greater than/less than signs. This returns True or False, once again.
As you seemed to discover, using int() on something that doesn't represent an integer will also throw a ValueError.
This new snippet keeps the try/except blocks you came up with in yours, because they work just fine.
Python has a bit of a culture surrounding it that prides 'efficiently-written' code. Which is to say, code that looks as fancy as possible, and follows Functional Programming paradigms. Comprehensions, as well as all() and any(), are a big part of that, and so they're probably the 'correct' solution for any problem which they are a solution for (if they can be written concisely).
Similarly, the snippet
if condition:
return True
else:
return False
can almost always be condensed to
return condition
(or return bool(condition), in the case that condition deals with a value that has truthiness but isn't a boolean, such as None or an empty list or string). If this is applicable, it's good practice (but again, it's not always applicable).
The most important thing, though, is that your code works the way you want it to, and that it's clear enough for you to come back to it a few months down the line and figure out what you were doing, and why you were doing it that way. There are some cases where things can be written as comprehensions but that makes them extremely complicated and unreadable - and in those cases, it's sometimes a good idea to not write them as comprehensions, and do it the more verbose way. Just keep that in mind as you're continuing to develop, and you'll do fine.
I am very new to Programming and only started learning Python 3 about 2 wks ago.
Doing an exercise that I found rather difficult, that is designed to create a function that accepts a dictionary as an argument and is supposed to determine if the dictionary represents a "valid" chessboard. Plz note the following codes only address a single aspect of the function. The part I had the greatest struggle with.
I spent quite a bit of time working on this particular project and trying to insure that both options are "valid" code so afaik there are no errors in either?
Imagine a grid (I will print the list) that is supposed to represent the squares on a chessboard. Could someone tell me which code would be deemed as more acceptable? and Why? Or if there is a simpler way I could have done this? I will only post what I feel is "relevant" to my question if more is needed plz lmk.
checks that dictionary keys are valid Chessboard Squares
# acceptable range is columns 1 - 8 rows a - h
for board_squares in dic:
try: # this will accept any value as int
if (int(board_squares[0:-1]) <= 8 # as slice up to last key char
and board_squares[-1] <= 'h') \
is False:
print((Err) + ' Square outside range')
return False
except ValueError as e:
print((Err) + ' Improper dictionary')
return False # when testing function this can be replaced with return False
Important note: In this occurrence I am referring to "board_squares" as the dictionary keys. This is the first code I came up with after a lot of effort. It slices the dictionary key and compares it to what is supposed to be represent a "valid" chessboard square. I got a bit of negative feedback on it so I went back to the drawing board and came up with this code:
def char_range(c1, c2):
"""Generates the characters from `c1` to `c2`, inclusive."""
for c in range(ord(c1), ord(c2)+1):
yield chr(c)
chessboard_squares = []
for chr1 in range(1, 9):
for chr2 in char_range('a', 'h'):
chessboard_squares.append(str(chr1) + chr2)
print(chessboard_squares) # this is simply to print list so I have a visual representation
for key in dic:
if key in list \
is False:
print((Err) + ' Square outside range')
return False
Important note: In this occurrence I am referring to chessboard_squares as values in the list that the dictionary keys are compared to. This second code requires the function at the top to range over letters. I tried to insure it was very readable by using clearly defined variable labels. It creates a list of what the "valid dictionary keys should be" to represent Chessboard Squares. And lastly here is the printed list of what the valid dictionary keys "should be". Post is in the format of chessboard squares for clarity.
['1a', '1b', '1c', '1d', '1e', '1f', '1g', '1h',
'2a', '2b', '2c', '2d', '2e', '2f', '2g', '2h',
'3a', '3b', '3c', '3d', '3e', '3f', '3g', '3h',
'4a', '4b', '4c', '4d', '4e', '4f', '4g', '4h',
'5a', '5b', '5c', '5d', '5e', '5f', '5g', '5h',
'6a', '6b', '6c', '6d', '6e', '6f', '6g', '6h',
'7a', '7b', '7c', '7d', '7e', '7f', '7g', '7h',
'8a', '8b', '8c', '8d', '8e', '8f', '8g', '8h']
Since I posted this question I've learned a lot of new things and decided to answer my own question. Or if there is a simpler way I could have done this? Here is a much cleaner, and should be considered the "best", option.
try:
if all (
(1 <= int(row) <= 8) and ('a' <= col <= 'h')
for row, col in dict
):
return True
except ValueError:
return False
First we use the all() function that takes ALL the arguments passed to it and returns True if all are True. Empty strings count as a special exception of True.
All our dictionary keys are (supposed to be) 2 character strings which are in themselves iterable, and I can use multiple assignment(aka tuple unpacking) here if I assign exactly as many characters as are in the dictionary key to variables. In this case we assign the 1st char of the dictionary key to row and the 2nd char of the dictionary key to col(umn). I can still use try/except ValueError because if the dictionary key isn't exactly 2 characters it will raise the same error and I am checking for specific keys.
A simple understanding short version of a list or generator "comprehension" is doSomething for variable in iterable this is a "generator comprehension". What we end up with is:
Do something: cmp int(row) to 1 - 8 and col 'a' - 'h'
for: row(1st char of dict key), col(2nd char of dict key)
in: dictionary keys.
Because this is a "generator comprehension" it will create a set of values based off each loop iteration. and as an example might look something like this: True, False, False, True etc.
These values will in turn be passed to all() that will consume them and return True if ALL are True else False.
here are several resources to help understand the code should anyone wish to look further:
the all function:
https://docs.python.org/3/library/functions.html#all
understanding list comprehension:
https://medium.com/swlh/list-comprehensions-in-python-3-for-beginners-8c2b18966d93
this is great in that it explains "yield" which is vital in understanding generator comprehension:
What does the "yield" keyword do?
Multiple Assignment:
https://treyhunner.com/2018/03/tuple-unpacking-improves-python-code-readability/
I want to reverse the string using the Loop & Function. But when I use the following code, it is output the exact same string again. But it suppose to reverse the string. I can't figure out why.
def reversed_word(word):
x=''
for i in range(len(word)):
x+=word[i-len(word)]
print(i-len(word))
return x
a=reversed_word('APPLE')
print(a)
If you look at the output of your debug statement (the print in the function), you'll see you're using the indexes -5 through -1.
Since negative indexes specify the distance from the end of the string, -5 is the A, -4 is the first P, and so on. And, since you're appending these in turn to an originally empty string, you're just adding the letters in the same order they appear in the original.
To add them in the other order, you can simply use len(word) - i - 1 as the index, giving the sequence (len-1) .. 0 (rather than -len .. -1, which equates to 0 .. (len-1)):
def reversed_word(word):
result = ""
for i in range(len(word)):
result += word[len(word) - i - 1]
return result
Another alternative is to realise you don't need to use an index at all since iterating over a string gives it to you one character at a time. However, since it gives you those characters in order, you need to adjust how you build the reversed string, by prefixing each character rather than appending:
def reverse_string(word):
result = ""
for char in word:
result = char + result
return result
This builds up the reversed string (from APPLE) as A, PA, PPA, LPPA and ELPPA.
Of course, you could also go fully Pythonic:
def reverse_string(word):
return "".join([word[i] for i in range(len(word), -1, -1)])
This uses list comprehension to create a list of characters in the original string (in reverse order) then just joins that list into a single string (with an empty separator).
Probably not something I'd hand in for classwork (unless I wanted to annoy the marker) but you should be aware that that's how professional Pythonistas usually tackle the problem.
Let's say your word is python.
You loop will then iterate over the values 0 through 5, since len(word) == 6.
When i is 0, i-len(word) is -6 (note carefully that this value is negative). You'll note that word[-6] is the character six places to the left from the end of the string, which is p.
Similarly, when i is 1, i-len(word) is -5, and word[i-len(word)] is y.
This pattern continues for each iteration of your loop.
It looks like you intend to use positive indices to step backward through the string with each iteration. To obtain this behavior, try using the expression len(word)-i-1 to index your string.
def reversed_word(word):
reversed = ''
for i in range(len(word)-1, -1, -1):
reversed += word[i]
return reversed
print(reversed_word("apple"))
I am trying to write a function which will return True or False if the given number is not greater than 2.
So simple, but the if condition is returning different outputs for same value '2'. The code I used is:
The code I used is:
ele_list = [1,2,3,2]
for i in ele_list:
if not i>2:
print(i,False)
ele_list.remove(i)
print(ele_list)
The ouput I am receiving is:
1 False
[2, 3, 2]
2 False
[3, 2]
I am confused to see that the first 2 in the list is passing through the if condition but the second 2 in the list is not passing through the condition. Please help me figure out this..
Removing elements from the list you're looping over is generally a bad idea.
What's happening here is that when you're removing an element, you're changing the length of the array, and therefor changing what elements are located at what indexes as well as changing the "goal" of the forloop.
Lets have a look at the following example:
ele_list = [4,3,2,1]
for elem in ele_list:
print(elem)
ele_list.remove(elem)
In the first iteration of the loop elem is the value 4 which is located at index 0. Then you're removing from the array the first value equal to elem. In other words the value 4 at index 0 is now removed. This shifts which element is stored at what index. Before the removal ele_list[0] would be equal to 4, however after the removal ele_list[0] will equal 3, since 3 is the value that prior to the removal was stored at index 1.
Now when the loop continues to the second iteration the index that the loop "looks at" is incremented by 1. So the variable elem will now be the value of ele_list[1] which in the updated list (after the removal of the value 4 in the previous iteration) is equal to 2. Then you're (same as before) removing the value at index 1 from the list, so now the length of the list just 2 elements.
When the loops is about to start the third iteration it checks to see if the new index (in this case 2) is smaller than the length of the list. Which its not, since 2 is not smaller than 2. So the loop ends.
The simplest solutions is to create a new copy of the array and loop over the copy instead. This can easily be done using the slice syntax: ele_list[:]
ele_list = [1,2,3,2]
for elem in ele_list[:]:
if not elem > 2:
print(elem, False)
ele_list.remove(elem)
print(ele_list)
the problem is that you're modifying your list as you're iterating over it, as mentioned in #Olian04's answer.
it sounds like what you really want to do, however, is only keep values that are > 2. this is really easy using a list comprehension:
filtereds_vals = [v for v in ele_list if v > 2]
if you merely want a function that gives you True for numbers greater than 2 and False for others, you can do something like this:
def gt_2(lst):
return [v > 2 for v in lst]
or, finally, if you want to find out if any of the values is > 2 just do:
def any_gt_2(lst):
return any(v > 2 for v in lst)
I think the problem here is how the remove function interacts with the for function.
See the documentation, read the "note" part:
https://docs.python.org/3.7/reference/compound_stmts.html?highlight=while#grammar-token-for-stmt
This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence
A possible solution, as suggested into the documentation:
ele_list = [1,2,3,2]
for i in ele_list[:]:
if not i>2:
print(i,False)
ele_list.remove(i)
print(ele_list)
"""
1 False
[2, 3, 2]
2 False
[3, 2]
2 False
[3]
"""