Download print - National Taiwan University

SOME COMMON ISSUES
■ As observed from last “hands-on time”, there were some common
issues from some of you…
■ Issue #1: How to use the interpreter mode –– starting the
interpreter mode you can just type “python” in your terminal or
in your command line window:
In the slides of this
lecture, this is always
shown in a blue box.
% python
>>> 2+2
4
>>>
1
SOME COMMON ISSUES
(CONT.)
■ In the interpreter mode, the python interpreter can be used as a
calculator, ie. if you type in any math operations it will show the
answer on the screen directly.
■ If you type in any variable, the value of the variable will be shown.
■ If you type in any function, the return value of the function will be
shown.
% python
>>> 2+2
4
>>> x = 123
>>> x ⇐ this is a variable
123
>>> type(x) ⇐ this is a function
<type 'int'>
2
SOME COMMON ISSUES
(CONT.)
■ Issue #2: How to use the script mode –– put your code into a file
■
(mostly named with .py, any text editor can be used!), and execute
in the terminal command “python xxx.py”.
if you use the Unix/Linux “#!” trick in your code, then you do not
need to type in “python xxx.py”, but just “xxx.py”, if you have
added the executable permission to the file.
⇐ ask python to run your script
⇐ the #! trick
3
SOME COMMON ISSUES
(CONT.)
■ In the script mode, you cannot do what we did in interpreter
mode. You cannot just type the math calculations, or variables, or
functions to show the output. You have to use the “print” at least.
In the slides of this lecture, for the code in the
script mode, it is always shown in a yellow box!
2+2 # this will not show anything
type(123) # this will not show anything
print 2+2 # this will print 4 on the screen
print type(123) # this will print <type 'int'> on the screen
4
SOME COMMON ISSUES
(CONT.)
■ Issue #3: But I’m using an IDE from Anaconda / Canopy… if this
is the case, you have both “modes” available in a single screen:
script mode
If you put your code in script mode, you
have to follow the regular coding rule!
Press the “run” to execute your code in
the interpreter window!
5
interpreter mode
SOME COMMON ISSUES
(CONT.)
■ Issue #4: Why I double-clicked my xxx.py, it just shows
something quickly and disappeared quickly? –– You are likely
using a windows system. The default action when a .py file is
double-clicked will be just “run it”. Since your program runs very
fast, your program terminate and the command line window also
closed right away, so you cannot see the output.
- Solution #1: start a command line window by yourself and run
the program by typing the commands.
- Solution #2: always use your IDE to load your .py code and run.
- Solution #3: add the following two lines to the end of your code:
import msvcrt
msvcrt.getch() ⇐ this will wait for a key (Windows only!).
6
A STEP TOWARD
PROGRAMMING
■ A program is a sequence of instructions that specifies how to
perform a mathematical/symbolic computation.
■ This is a simple example of mathematical computation:
s = t = 1.
for n in range(1,21):
t /= n ⇐ t /= n is equivalent to t = t/n
s += t
print "1/0!+1/1!+1/2!+1/3!+...+1/20! =",s
1/0!+1/1!+1/2!+1/3!+...+1/20! = 2.71828182846
7
THE PRINT STATEMENT
■ As you already seen in the “hello world!” program, as well as the
previous example, the print statement simply present the output to
the screen (your terminal).
■ A couple of examples:
>>> print 'show it now!'
show it now!
>>> print 1,2,(10+5j),-0.999,‘STRING’ ⇐ connect w/ comma
1 2 (10+5j) -0.999 STRING
>>> print 1,2,3,4,5
1 2 3 4 5
>>> print str(1)+str(2)+str(3)+str(4)+str(5)
12345
Remark: for python 3, the print becomes a function, you
have to use something like print(‘hello world!’).
8
STRUCTURE OF
A PROGRAM
■ Input: ■
■
■
■
get data from the keyboard, a file, or some other device.
Output: display data on the screen or send data to a file or other device.
Math: perform basic mathematical operations like addition and
multiplication.
Conditional execution: check for certain conditions and execute the appropriate code.
Repetition: perform some action repeatedly, usually with some variation.
Every program you’ve ever used, no matter how complicated, is
made up of instructions that look pretty much like these.
9
GIVE ME A FLYSWATTER
■ Programming is error-prone. Programming errors are called bugs
and the process of tracking them down is called debugging.
■ Three kinds of errors can occur in a program:
▫ Syntax errors
▫ Runtime errors
▫ Semantic errors
10
SYNTAX ERRORS
■ Syntax refers to the structure of a program and the rules about that
structure.
■ Python can only execute a program if the syntax is correct;
otherwise, the interpreter displays an error message.
■ An example:
>>> y = 1 + 4x
File "<stdin>", line 1
y = 1 + 4x
^
SyntaxError: invalid syntax
>>>
11
You probably already see a
couple of syntax errors in
your previous trials.
RUNTIME ERRORS
■ A runtime error does not appear until after the program has
started running.
■ These errors are also called exceptions because they usually
indicate that something exceptional (and bad) has happened.
■ The following “ZeroDivisionError” only occur when it runs.
>>> a = b = 2
>>> r = ((a + b)/(a - b))**0.5 * (a - b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>>
12
SEMANTIC ERRORS
■ If there is a semantic error in your program, it will run successfully,
but it will not do the right thing.
■ It does something else.
■ The problem is that the program you wrote is not the exactly
program you wanted to write.
■ The meaning of the program (its semantics) is wrong.
■ Identifying semantic errors can be tricky because it requires you to
work backward by looking at the output of the program and trying
to figure out what it is doing.
■ (Much) more difficult to debug.
13
SEMANTIC ERRORS
■ An example:
Just one “dot” gives you a
completely different answer.
s = t = 1.
s = t = 1
for n in range(1,21):
t /= n
s += t
print "1/0!+1/1!+1/2!+1/3!+...+1/20! =",s
1/0!+1/1!+1/2!+1/3!+...+1/20! = 2
2.71828182846
14
DEBUGGING
■ It is very difficult to write a bug free program at the first shot.
■ Programming and debugging can be the same thing. Programming is the process of debugging a program until it does
exactly what you want.
■ Having a good programming/coding habit sufficiently helps.
■ Re-run a buggy program never helps!
15
Kai-Feng Chen
National Taiwan University
PROGRAMMING &
NUMERICAL ANALYSIS 16
0
2
Lecture 02: Control flow
16
CONTROL FLOW
■ You are not always on the
same route, do you?
17
CONDITIONAL
EXECUTION
■ One definitely needs the ability to check some certain conditions
and change the behavior of the program accordingly.
■ Conditional statements give us this ability.
■ The simplest form is the if statement:
if x > 0:
print 'x is positive!'
■ The boolean expression (x>0) after if is called the condition.
■ If it is true, then the indented statement (print) gets executed.
Otherwise nothing will happen.
18
THE STRUCTURE
The condition,
which must be a
boolean expression.
header is ending with a colon (:)
if x > 0 :
print 'x is positive!'
HEADER
BODY
The body can
contain any number
of statements.
Body has to be indented.
Indentation is important. The lines started with the
same indentation is treated as a group. The
convention is four spaces, but this is not restricted.
19
BOOLEAN EXPRESSIONS
■ A boolean expression is either true or false.
■ For example, the basic operator “==” compares two operands and
produces True if they are equal:
>>> 5==5
True
>>> 5==6
False
■ The True and False (T,F must be capital) belong to bool type:
>>> type(True)
<type 'bool'>
20
RELATIONAL OPERATORS
■ The == operator is one of the relational operators; the others are:
x
x
x
x
x
!= y
> y
< y
>= y
<= y
#
#
#
#
#
x
x
x
x
x
is
is
is
is
is
not equal to
greater than
less than y
greater than
less than or
y
y
or equal to y
equal to y
■ A common error is to use a single equal sign “=” instead of a
double equal sign “==”.
■ There is no “=<” or “=>”.
21
LOGICAL OPERATORS
■ There are three logical operators: “and”, “or”, “not”.
■ The operands of the logical operators should be boolean
expressions, but Python is not very strict – any nonzero number is
interpreted as True.
➦True
0
<
x
<
10
x > 0 and x < 10
➥False otherwise
➦True if n is multiple of 2 or 3
n % 2 == 0 or n % 3 == 0
➥False otherwise
not (x > y) ➦True if x is less or equal to y
➥False x > y
22
ALTERNATIVE
EXECUTION
■ Alternative execution –– with two possibilities and the condition
determines which one gets executed.
■ The syntax:
if x%2 == 0:
print 'x is even'
else:
print 'x is odd'
■ Since the condition must be true or false, exactly one of the
branches will be executed.
23
CHAINED
CONDITIONALS
■ Sometimes there are more than two possibilities –– one way to
express a computation like that is a chained conditional:
if x < y:
print 'x is less than y'
elif x > y:
print 'x is greater than y'
else:
print 'x and y are equal'
■ Exactly one branch will be executed. There is no limit on the
number of elif (“else if”) statements.
■ If there is an else, it has to be at the end.
24
NESTED
CONDITIONALS
■ One conditional can also be nested within another. For example:
If-else block #1
if x == y:
print 'x and y are equal'
else:
If-else block #2
if x < y:
print 'x is less than y'
else:
print 'x is greater than y'
indentation of the
statements define
the blocks.
The first branch contains a simple statement.
The second branch contains another if statement,
which has two branches of its own.
25
NESTED
CONDITIONALS (II)
■ Indentation makes the structure apparent. But nested conditionals
become difficult to read very quickly. Good to avoid them.
■ Logical operators often provide a way to simplify nested
conditional statements.
if 0 < x:
if x < 10:
print 'x is a positive single-digit number.'
Simplified with “and”:
if 0 < x and x < 10:
print 'x is a positive single-digit number.'
26
COMMENT: INDENTATION
■ Leading whitespace at the beginning of lines is used to define the
indentation level of the line, which is used to determine the
grouping of statements.
■ Due to the nature of various text editors, it is unwise to use a
mixture of spaces and tabs for the indentation.
Example of a correctly (but confusingly?) indented code:
def perm(l):
if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
27
INTERMISSION
■ Try the following logic operations:
>>>
a = 1.0/3.0
>>>
b = 1.0 - 2.0/3.0
>>> a == b
Do you see True or False? Why?
not (((True and (True or False) and True) and
>>>
False)
or False) and False or True
Do you see True or False? Why?
28
INTERMISSION (II)
■ Try this kind of “weird-indented” programs?
print
"a"
print "aa"
print "aaa"
print
"aaaa"
or
x = 3
if x < 4:
print "x is smaller than 4."
if x > 1:
print "x is greater than 1."
29
Which line do you
expect to see the
(syntax) error?
INTERMISSION (III)
■ Try this kind of “weird-indented” programs?
x = y = 2
if x > 1:
if y < 4:
x += y
⇐ x += y is the same as x = x + y
y += 4 is the same as y = y + 4
y += 4
elif x + y > 1:
y = (x + y)*2
else:
x -= (y / 2)
y == x % 2:
elif
y = x**2
else:
x -= y**2
print x,y
30
What do you
expect to see in the
end (x,y)?
ITERATION
■ Computers are often used to automate repetitive tasks: repeating
identical or similar tasks.
■ Because iteration is so common, Python provides several language
features to make it easier:
▫ The while statement, which can be used as general loops.
▫ The for statement, which is usually used to run through a block
of code for each item in a list sequentially.
■ An example (printing 1 to 10 on the screen):
n = 1
while n <= 10:
print n
n += 1
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THE “WHILE” STATEMENT
Setting the initial
value to n
The condition
n = 1
while n <= 10 :
print n
n += 1
header is also ending with a colon
HEADER
BODY
n += 1 is the same as n = n + 1,
means n should be updated with n+1
■ The flow of execution for a while statement:
▫ Evaluate the condition, yielding True or False.
▫ If the condition is false, exit the while statement.
▫ If the condition is true, execute the body and then go back to step 1.
32
TERMINATION OF THE
LOOP
■ The loop should change some of the variables so that eventually
the condition becomes false and ends the loop (e.g. the n+=1
statement).
■ Otherwise the loop will repeat forever as an infinite loop.
■ Or use the break statement to jump out of the loop:
n = 1
while True: ⇐ an infinite loop actually!
print n
if n>=10:
break ⇐ jump out of the loop
n += 1
33
CONTINUE THE LOOP
■ Break versus Continue (can be confusing for beginners):
▫ The break statement, which should jump out of the loop can
continue to execute the next statement.
▫ The continue statement, which should jump back to the head of
the loop, check the condition, and continue to the next iteration.
n = 0
while True:
n += 1
if n > 10:
break
if n % 2 == 0:
continue
print n
print 'the end!'
continue the loop
break the loop
34
LOOP – ELSE
STATEMENT
■ Similar to the if statement, else can be attached to the end of loop.
■ The code block after else will be executed if the loop ended
normally (without calling the break statement):
n = 0
m = 5 ⇐ if m is not between 1–10, the ‘ended normally!’ will be printed.
while n<10:
n += 1
print n
if n==m:
print 'n ==',m,'hit. Break the loop.'
break
else:
print 'ended normally!'
35
THE “PASS”
STATEMENT
■ The pass statement does nothing. It can be used when a statement
is required syntactically but the program requires no action.
■ For example:
while True:
pass # this is a do-nothing, infinite loop
n = 3
if
n == 1:
print 'One!'
elif n == 2:
print 'Two!'
elif n == 3:
print 'Three!'
else:
pass ⇐ do nothing, simply a placeholder for now
36
THE “FOR” STATEMENT
■ The for statement in Python differs a bit from what you may be
used to in C – it iterates over the items of any sequence (a list or a
string), in the order that they appear in the sequence.
■ An example (iterating the letters in a string):
letters = 'bhJlmprsty'
for initial in letters: ⇐ loop over the letters
print initial + 'ack',
Output:
back hack Jack lack mack pack rack sack tack yack
37
ACCESSING A LIST
■ The for statement can access to the elements in a list. For example:
animals = ['cat','dog','horse','rabbit','mouse']
for a in animals:
print a
odd_numbers = [1,3,5,7,9]
for n in odd_numbers:
print n
The list “odd_number” can be actually replaced by
a simple call to the built-in function range()
38
THE RANGE() FUNCTION
■ If you do need to iterate over a sequence of numbers, the built-in
function range() comes in handy. It generates lists containing
arithmetic progressions:
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(5, 10)
[5, 6, 7, 8, 9]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(-10, -100, -30)
[-10, -40, -70]
The given end point (e.g. 10) is never part of the generated list.
39
RANGE() + FOR
■ Get a list of odd numbers:
for n in range(1,10,2):
print n
■ To iterate over the indices of a sequence, you can combine range()
and len() as follows:
animals = ['cat','dog','horse','rabbit','mouse']
for i in range(len(animals)):
print i,'=>',animals[i]
The function len() return the number of elements in
a list or how many characters in a string.
40
A LITTLE BIT MORE
ON THE LIST
■ Python has a number of data types, used to group together other
values. The most versatile is the list. For example:
>>> a = ['spam','eggs',100,1234]
>>> a
['spam', 'eggs', 100, 1234]
>>> a[0]
'spam'
>>> a[3]
1234
>>> a[-2] ⇐ Access to the elements in a reversed order
100
>>> a[1:3] ⇐ Get a sub list
['eggs', 100]
41
A LITTLE BIT MORE
ON THE LIST (CONT.)
■ Few more operations with python list:
>>> b = ['spam','eggs',[1,2,[3,4,5]]] ⇐ anything can be in a list;
including another list!
>>> b
['spam', 'eggs', [1, 2, [3, 4, 5]]]
>>> b[0] = 'foo' ⇐ list is mutable!
>>> b
['foo', 'eggs', [1, 2, [3, 4, 5]]]
>>> b.append(1+3j)
>>> b
['foo', 'eggs', [1, 2, [3, 4, 5]], (1+3j)]
>>> len(b) ⇐ there are 4 items!
4
42
A LITTLE BIT MORE
ON THE STRING
■ The basic operations on the string are quite similar to that of list.
■ A similar way can be used to access individual characters, or a sub-string.
>>> word = 'Help' + 'Me'
>>> word
'HelpMe'
>>> word[4]
'M'
>>> word[0:2] ⇐ Get a sub string
'He'
>>> word[0:-2]
'Help'
>>> len(word) ⇐ character counting
6
43
A LITTLE BIT MORE
ON THE STRING (CONT.)
■ Few more operations with python string:
>>> fruit = 'banana'
>>> fruit
Python string/list features are
actually very powerful!
'banana'
We will come back to discuss them
>>> fruit.upper()
in details at a upcoming lecture.
'BANANA' ⇐ return the upper case
>>> fruit[0]
'b'
>>> fruit[0] = ‘c’
⇐ you cannot replace one of the characters!
TypeError: 'str' object does python strings are immutable!
not support item assignment
>>> 'c'+fruit[1:] ⇐ you can only do this
'canana'
44
PUT IT ALL TOGETHER
■ Testing prime numbers:
numbers = [17,59,83,129,187]
for m in numbers:
for n in range(m): ⇐ Surely you can do range(2,m) if n<2:
instead of the ‘continue’ statement.
continue
if m%n == 0:
print m,'is a multiple of',n
break
else:
print m,'is a prime number'
17 is a prime number
...
187 is a multiple of 11
45
PUT IT ALL TOGETHER (II)
■ Or one can get the testing number with the raw_input function:
input = raw_input('Please enter a number: ')
m = int(input)
for n in range(m):
if n<2:
continue
if m%n == 0:
print m,'is a multiple of',n
break
else:
print m,'is a prime number'
Please enter a number: 127 ⇐ input by keyboard
127 is a prime number
46
INTERMISSION
■ Please try to find out:
▫ Is 1237 a prime number? How about 12347, 123457, and 1234567?
▫ Print out all of the factors of 12345678.
47
HANDS-ON SESSION
■ Up to now we have gone through:
▫ The basic structure and syntax of python
▫ Variables and operators
▫ Branching, conditionals, and iterations
■ You should be able to write a meaningful program and carry out
some interesting calculations already!
■ Let’s start our 2nd round of hands-on session now!
48
HANDS-ON SESSION
■ Practice 1: Print a multiplication table up to 12x12 on your screen:
2x1 =
2x2 =
2x3 =
2x4 =
2x5 =
2x6 =
...
12x12
2
4
6
8
10
12
= 144
49
HANDS-ON SESSION
■ Practice 2: Find out all of the prime numbers which are smaller than 10000.
2 3 5 7 11 13 17 19 23 29 31 37 41
43 47 53 59 61 67 71 73 79 83 89
97 ... 9973
50
HANDS-ON SESSION
■ Practice 3: A score and grade mapping is given below:
90–100: A+
85–89: A 80–84: A–
77–79: B+
73–76: B 70–72: B–
67–69: C+
63–66: C 60–62: C–
50–59: D
40–49: E 0–39: F
Please write a small program which can convert the score to grade
levels, e.g.:
Please enter a score: 95
⇐ input by keyboard
Your grade is “A+”. ⇐ output by your code
51