This post is my study note when learning Python programming based on online tutorial provided by University of Waterloo.
0 Hello World
print(“Hello, World!”)
1 Variables & Errors
You don’t need keywords like var or int to decare variables.
The following code will results to a TypeError: Can't convert 'int' object to str implicitly.
print(“you cannot add text and numbers” + 12)
Q: What is the technical difference between syntax and run-time errors?
A: The program with the run-time error created some output, but the one with the syntax error did not. This is because Python runs in two steps:
- Python checks if your program has correct syntax, in order to determine its structure and parts.
- If no syntax errors were encountered in step 1, then the program is executed. So, a program with a syntax error will execute no steps at all, but a program with a run-time error will execute the steps that happened before the error occured.
The extra spaces in most situations had no effect on the output. However, be careful that extra space at the beginning of a line, called indenting, has a special meaning that can cause errors if used incorrectly.
2 Functions
Q: Code scramble: make the program sort the three numbers x, y and z into increasing order, so that x has the smallest value, y has the next smallest value, and z has the largest value. (only use max(a,b) and min(a,b)).
A: tmp = max(x,y) x = min(x,y) y = tmp tmp = max(y,z) y=min(y,z) z = tmp tmp = max(x,y) x = min(x,y) y = tmp
3 Comments and Quotes
Any line of instructions containing the # symbol
print(1) # here is a comment
print(2)
print(3) # another comment @!#!@$
If a pound sign # appears in a string, then it does not get treated as a comment.
Use character " inside of a string:
print(‘I said “Wow!” to him.’) print(“This is an "escape" of a double-quote”)
4 Types
print(type(“Hello, World!”)) # <class ‘str’> print(type(34)) # <class ‘int’> print(type(1.234)) # <class ‘float’>
typecast function:
x = float(“3.4”) # changes the string “3.4” to the float 3.4
converting a float to an int loses the information after the decimal point, e.g. int(1.234) gives 1, and int(-34.7) gives -34.
converting a str to an int causes an error if the string is not formatted exactly like an integer, e.g. int(“1.234”) causes an error.
converting a str to a float causes an error if the string is not a number, e.g. float(“sandwich”) causes an error.
5 Input
By calling input() multiple times, you can access multiple lines of input. The first call to input() gets the first line, the second gets the second line, et cetera. The string given by input() can be converted to an int or a float.
6 If
if «condition»:
the «condition» which must be a True/False expression
Then, the body consisting of one or more indented lines. The number of spaces (the amount of indentation) doesn’t matter, but being inconsistent will cause an error since Python uses the amount of indentation to determine where you mean the body to start or stop.
In Python, the bool type is used to represent Boolean values; only two bool values exist, True and False.
7 Design, Debugging and Donuts
// for integer division instead of / which does decimal division
8 String, Math & Loop
String
For a string S and an integer index, the notation S[index]. In many other programming languages, there is a separate type for characters. In Python, characters are the same as length-1 strings
Cutting strings: S[firstIndex:tailIndex] to get the substring starting at index firstIndex and ending at tailIndex-1.
length of a string: len(S)
Pasting strings: S+T is to create a new string that starts with S and has T immediately afterwards
If you want to concatenate numbers, you need to convert them to str first. such as print("high " + str(5))
S * n is short for S + S + ... + S.
ord: convert a character into its corresponding numerical code
chr: convert a numerical code into its corresponding character
To be continued…
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