Types of Errors and Examples
IndexError |
The index of a sequence is out of range. |
(1,)[1] |
KeyError |
Key is not found in the dictionary. |
cs = {10:10}; cs[s] |
SyntaxError |
Invalid syntaxes found in code. |
print(Hello World) (no quote marks) or for i in range(3) (no colon) |
TypeError |
A function/operation is applied to objects of incorrect types. |
(1,2) + [3,4] or "cs" + 1010 |
ValueError |
Function gets an argument of a correct type but improper value. |
int("3.14") |
NameError |
The variable with the name is not found in the local & global scope. |
del cs1010s |
AttributeError |
An attribute reference or assignment fails due to incorrect data type. |
(1,2).append(3) |
ZeroDivisionError |
Second operand (denominator) of a division/module operation is zero. |
1/0 |
RecursionError |
An operation/runs out of memory. |
def f(x): return x + f(x-1) |
UnboundLocalError |
A reference is made to a local variable in a function/method, but no value has been bound to that variable. |
def f(x): ;z = y + x; y = 0 |
RuntimeError |
Can be due to many reasons. One of them being list/dictionary size changing during iteration. |
a = [1, 2]; for ele in a: a.append(ele) |
Exception Handling
try: |
The code inside the try block will run until it reaches an error. Once it does, it will be handled accordingly by the respective exception statements. |
except Error: |
The code inside this block will only run if the encountered error is the error to be handled by this except statement. |
except Exception: |
The code inside this block will only run if the error has not been handled by the previous except statements. |
else: |
The code inside the else block will only run if no errors were encountered in the try statement. |
finally: |
The code inside the finally block will run no matter what. |
For/While Loop Statements
break |
Terminates the loop (once) |
continue |
Stops the current iteration of the loop, and goes on to the next iteration of the current loop |
pass |
Does nothing and continues the rest of the code inside the current iteration of the loop |
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List shallow copies (list.copy(), list[:])
List and dictionary functions
string.split(sep) |
Splits the string into a list by the seperator, which by default is "". |
"CS1010S".split("1") == ["CS", "0", "0S] |
dict.get(key, value) |
Returns the value that is paired to the key in the dictionary. If the key does not exist in the dictionary, then it will return the value instead. |
d = {"CS":10, 10: "S"} ; d.get("CS", "NOT FUN") == 10; d.get("10", "GG CS") == "GG CS" |
del dict[key] |
Removes the key-value pair with the indicated key in the dictionary. Returns a KeyError if key is not found in the dict |
d = {"CS":10, 10: "S"}; del d[10]; d = {"CS": 10} |
sep.join(iterable) |
Joins all the items in the iterable into a string, with the sep inbetween each item. |
d = {"CS":10, "10":"S"}; " - ".join(d); "CS - 10" |
Passcode locking
def lock(obj, passcode):
o = obj.copy()
o.clear()
o["locked"] = lambda x: o if x == passcode else False
def unlock(obj, passcode):
o = obj["locked"] (passcode)
if o != False:
obj.clear()
obj.update(o) |
Orders of Growth (OOG)
O(1) < O(log n) < O(n) < O(n log n) < O(n2) < O(2n) < O(nn)
O(1): Indexing, replacing variable name
O(log n): Constantly halving/doubling a number (depending on direction)
O(n): Going through the whole tuple/string (for loop/recursion)
O(n2): Going through the whole tuple once for each element (Usually nested for loop)
O(2n): The tree splits into 2/x number of branches for each level (Usually for recursion tree)
Sample Answer:
Time: O(n), there is a total of n recursive calls.
Space: O(n), there is a total of n recursive calls, and each call will take up space on the stack.
Time: O(n), the loop will iterate n times.
Space: O(1), no extra memory is needed because the variables are overwritten with the new values.
String slicing and concatenation takes O(n) time as well |
Boolean Values
False evaluates to 0; int(False) == 0, while True evaluates to 1; int(True) = 1
On the other hand, any empty string, tuple, list, dict etc ("", (), [], {}), value 0 and None all evaluates to False; bool(0/None/""/()/[]/{}) = False, and any other expression will evaluate to True; bool(1/-95/"CS1010S is fun"/("C", "S", "S", "U", "C", "K", "S") = True |
Checking data type
1. type(value) == Type |
2. isinstance(value, Type) |
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