Python Help for Compsci 6/101
On this page we'll keep track of the Python types, functions, and operators that we've covered in class. You should also review the online Python References for more complete coverage.
| Mathematical Operators | Symbol | Meaning | Example |
|---|---|---|
| + | addition | 4 + 5 = 9 |
| - | subtraction | 9 - 5 = 4 |
| * | multiplication | 3*5 = 15 |
| / | division | 6/3 = 2 6/4 = 1 6.0/4 = 1.5 |
| % | mod/remainder | 5 % 3 = 2 |
| ** | exponentiation | 3**2 = 9, 2**3 = 8 |
| String Operators | + | concatenation | "ab"+"cd"="abcd" |
| * | repeat | "xo"*3 = "xoxoxo" |
| Comparison Operators | == | is equal to | 3 == 3 is True |
| != | is not equal to | 3 != 3 is False |
| >= | is greater than or equal to | 4 >= 3 is True |
| <= | is less than or equal to | 4 <= 3 is False |
| > | is strictly greater than | 4 > 3 is True |
| < | is strictly less than | 3 < 3 is False |
| Boolean Operators | ||
x=5 | not | flips the value of a bool | (not x == 5) is False |
| and | returns True only if both parts of it are True | (x > 3 and x < 7) is True (x > 3 and x > 7) is False |
| or | returns True if at least one part of it is True | (x < 3 or x > 7) is False (x < 3 or x < 7) is True |
| String Functions | ||
s="colorful" | Name | Returns | Example |
| .find(str) | index of first occurrence | s.find("o") == 1 |
| s.find("e") == -1 | ||
| .count(str) | number of occurrences | s.count("o") == 2 s.count("r") == 1 s.count("e") == 0 |
| .strip() | copy with leading/trailing whitespace removed | " big ".strip() == "big" |
| .split() | list of "words" in s | "big bad dog".split() == ["big","bad", "dog"] |
| .split(",") | list of "items " in s that are separated by a comma | "this,old,man".split(",") == ["this", "old", "man"] |
| In general can split on any string, not just a comma, e.g., s.split(":") will split on a colon and s.split("gat") will split on the string "gat". | ||
| .startswith(str) | boolean if starts with string | s.startswith("color") == True s.startswith("cool") == False |
| Type Conversion Functions | ||
| int(x) | turn x into an integer value | int("123") == 123 |
| int can fail, e.g., int("abc") raises an error | ||
| float(x) | turn x into an float value | float("2.46") == 2.46 |
| float can fail, e.g., float("abc") raises an error | ||
| str(x) | turn x into a string value | str(432) == "432" |
| type(x) | the type of x | type(1) ==
int type(1.2) == float |
| Miscellaneous Functions | ||
| len(x) | length of sequence x, e.g., String/List | len("duke") == 4 |
| sorted(x) | return list that is sorted version of sequence/iterable x, doen't change x | sorted("cat") == ['a','c','t'] |
| range(x) | a list of integers starting at 0 and going up to but not including x | range(5) == [0, 1, 2, 3, 4] |
| range(start, stop) | a list of integers starting at start and going up to but not including stop | range(3, 7) == [3, 4, 5, 6] |
| help(x) | documentation for module x | |
| min(x, y, z) | minimum value of all arguments | min(3, 1, 2) == 1
min("z", "b", "a") == "a" |
| max(x, y, z) | maximum value of all arguments | max(3, 1, 2) == 3
max("z", "b", "a") == "z" |
| abs(x) | absolute value of the int or float x | abs(-33) == 33
abs(-33.5) == 33.5 |
| List Functions | sum(lst) | returns sum of elements in list lst | sum([1,2,4]) == 7 |
| max(lst) | returns maximal element in lst | max([5,3,1,7,2]) == 7 |
| lst.append(...) | append an element to lst, changing lst | [1,2,3].append(8) == [1,2,3,8] |
| lst.extend(lst2) | append every element of lst2 to lst | [1,2,3].extend([8,9]) == [1,2,3,8,9] |
| lst.sort() | sorts the elements of lst | |
| lst.index(elt) | return index of elt in lst, error if elt not in lst | [1,5,3,8].index(5) == 1 |
| lst.count(elt) | return number of occurrences of elt in lst
| [1,2,1,2,3].count(1) == 2 |
| lst.pop(index) | remove and return element at position index in
lst, so has side-effect of altering list and returns
value. Default index is last value.
| |
| Set Functions | s.add(item) | adds the item into the set, and returns nothing. |
| s.union(t) | returns new set representing s UNION t, i.e., all elements in either s OR t, t can be any iterable (e.g., a list) | |
| s.intersection(t) | returns new set representing s INTERSECT t, i.e., only elements in both s AND t, t can be any iterable (e.g., a lsit) | |
| s | t | returns/evaluates to union of s and t, both must be sets. | |
| s & t | returns/evaluates to intersection of s and t, both must be sets. | |
| s - t | returns/evaluates to set with all elements in s that are not in t | |
| Dictionary Functions | d[key] |
returns the value associated with key, error
if key not in dictionary d
|
| d.get(key) |
returns value associated with key,
error if key not in dictionary d
| |
| d.get(key,default) |
returns value associated with key,
returns default if key
not in d
| |
| d.keys() | returns a list/view of the keys in dictionary | |
| d.items() | returns a list/view of tuples, (key,item) pairs from dictionary | |
| d.values() | returns a list/view of values in dictionary | |
| Random Functions | random.choice(list_of_choices) | returns a random element from list_of_choices. Gives an error if list_of_choices has length 0. |
| random.randint(start, end) | Returns a random integer between start and end. Unlike range() and list slicing, the largest value it can return is end, not end-1. | |
| random.random() | Returns a random float between 0 and 1. | |
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Last modified 17 May 2012 |
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