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Python comands for ESO II
Basics
Spaces and indentation (tabs) are relevant parts of the code |
Instructions in a block of code will have the same indentation |
A block of code contains one or more lines of code inside it. The contained lines will be indented one more level than the container one. |
A colon : opens a new block of code in the following line. |
# at the beginning of a line marks this line as non-executable. For example |
# This is a single line comment |
To create multiple line comments, use three apostrophees in a row |
''' |
This is a |
Multiline comment |
''' |
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Variables
There's no declaration of variables. When you use a new name in an assign sentence, that becomes a variable of the type of the value assigned to it |
a = 10 creates a variable called a that stores an integer value |
The decimal separator is the point |
a = 3.14 creates a variable called a that stores a decimal (float) value |
To transform a numerical value into a string you cast it using the function str() |
str(10) will create the string value "10" |
To transform a string value into an integer you cast it using the function int() |
int("10") will create the integer value 10 |
A variable contains a literal value of a certain type (integer, alphanumeric, decimal, boolean, etc) and it can be used to perform different operations or construct logical expressions
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Basic input / output
print(a) |
Prints the content of the variable called a and opens a new line |
print("Helo world")
Hello word
and opens a new line |
print(a,end=",") |
Prints the content of the variable called a and then a comma without opening a new line |
print(name,end=",")
My name, |
input() |
Reads a string value form the keyboard |
a = input() will store in a variable called a the value entered by the user as a string |
int(input()) |
Reads a string value form the keyboard, ant casts it into an integer |
a = int(input()) will store in a variable called a the value entered by the user casted as an integer |
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Arithmetic operators
+ |
add |
12 + 5 returns 17 |
- |
subtract |
12 - 5 returns 7 |
* |
product |
12 * 5 returns 60 |
/ |
decimal division |
12 / 5 returns 2.4 |
// |
division (whole numbers) |
12 // 5 returns 2 |
% |
remainder of the division |
12 % 5 returns 2 |
** |
exponentiation |
12 ** 5 returns 248832 |
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Comparison operators (logical)
< |
less than |
12 < 5 evaluates as False |
<= |
less than or equal to |
12 <= 5 evaluates as False |
== |
equal to |
12 == 5 evaluates as False |
>= |
greater than or equal to |
12 >= 5 evaluates as True |
> |
greater than |
12 > 5 evaluates as True |
!= |
not equal to |
12 != 5 evaluates as True |
In a comparison, the sign of equality (=) can never be alone as it would be confused with the assignment of values (a = 10). This is why the logical equality operator is a double sign of equality. Therefore a=10 means assign the value 10 to the variable a and a==10 means is the content of the variable a a number 10?
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Logical operators
<expr1> and <expr2> |
True if and only if the two expressions are True |
(a>0) and (a<5) |
<expr1> or <expr2> |
True if and only if at least one of the two expressions is True |
(a<0) or (a>=5) |
not <expr> |
True if and only if <expression> is False |
not (a==0) |
By logical we understand an expression or operation that can only take two different values: True or False
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Lists
len |
Calculates the number of elements in the list. |
while (position < len(myList)): |
update |
Modifies the content of a position of the list. |
myList[position] = newValue |
append |
Add a new position at the end of the list, and stores there a value. |
myList.append(newValue) |
insert |
Creates a new position in the middle of the list, and stores there a value. |
myList.insert(position, newValue) |
pop |
Retrieves the value stored at a given position, and then, removes the position. |
value = myList.pop(position) |
delete |
Erases a position of the list. |
delete(myList[position]) |
remove |
Locates the first occurrence of a value in the list, and then erases its position. |
myList.remove(value) |
in |
Membership operation. Check for the existence of a value in the list. |
if (value in myList): |
index |
Locates the first occurrence of a value in the list, and returns it's position. In case the valoue is not found, it will generate a run-time error. |
position = myList.index(value) |
count |
Counts the number of occurrencies of a value in the list. |
occ = myList.count(value) |
reverse |
Flips all the values in the list, so the first will become the last and viceversa. |
myList.remove() |
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Maths
abs(arg) |
receives an integer number as an argument and returns the integer absolute value |
abs(-12) returns 12 |
math.fabs(arg) |
receives a float as an argument and returns the float absolute value |
math.fabs(-12.34) returns 12.34
math.fabs(-12) returns 12.0 |
math.floor(arg) |
receives a float as an argument and rounds it down to the nearest integer |
math.floor(2.5) returns 2
math.floor(-3.4) returns -4 |
math.ceil(arg) |
receives a float as an argument and rounds it up to the nearest integer |
math.ceil(2.5) returns 3
math.ceil(-3.4) returns -3 |
math.pi |
returns the value of Pi |
math.pi returns 3.141592653589793 |
You will need to import math
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Flow control (more to be added along the course)
if (<expr>) : |
Forks the execution stream according to the logical value of the expression <expr> |
if (a == b):
<do something> |
else: |
As part of an if .. else block, starts the block code to be executed if the expresion was False |
else:
<do something esle> |
elif (<expr2>): |
Compound an else: statement with a new if statement |
elif (a<b) : <and another> |
while (<expr>): |
Generates a loop that will run as long as the expression <expr> is True |
while (a<10):
<do something>
<update a> |
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