Tutorial Python untuk bukan pengatur cara/Contoh Fungsi Lanjutan: Perbezaan antara semakan
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→Contoh: terjemah contoh |
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=== Contoh === |
=== Contoh === |
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''' |
'''faktorial.py''' |
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<source lang="python"> |
<source lang="python"> |
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# mentakrif fungsi yang mengira faktorial |
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#defines a function that calculates the factorial |
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def |
def faktorial(n): |
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if n <= 1: |
if n <= 1: |
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return 1 |
return 1 |
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return n * |
return n * faktorial(n - 1) |
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print "2! =", |
print "2! =", faktorial(2) |
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print "3! =", |
print "3! =", faktorial(3) |
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print "4! =", |
print "4! =", faktorial(4) |
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print "5! =", |
print "5! =", faktorial(5) |
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</source> |
</source> |
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5! = 120 |
5! = 120 |
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''' |
'''kira_detik.py''' |
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<source lang="python"> |
<source lang="python"> |
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def |
def kira_detik(n): |
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print n |
print n |
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if n > 0: |
if n > 0: |
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return |
return kira_detik(n-1) |
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kira_detik(5) |
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</source> |
</source> |
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0 |
0 |
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Ulasan fungsi_menarik.py |
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Commented function_interesting.py |
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<source lang="python"> |
<source lang="python"> |
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# Ulasan di bawah diberi nombor tingkat agar |
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# The comments below have been numbered as steps, to make explanation |
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# penjelasan kod lebih terang. Sila baca mengikut tingakatan. |
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# of the code easier. Please read according to those steps. |
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# ( |
# (tingkat 1, contohnya, ada di bawah) |
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def mult(a, b): # (2.) |
def mult(a, b): # (2.) Fungsi ini akan berulang kerana ... |
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if b == 0: |
if b == 0: |
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return 0 |
return 0 |
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baki = mult(a, b - 1) # (3.) ... Apabila sampai ke SINI, jujukan bermula kembali dari atas! |
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value = a + |
value = a + baki |
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return value # (4.) |
return value # (4.) justeru, "return value" tidak akan berlaku |
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# sehingga tingkat 3 dilewati program. |
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print "3 * 2 = ", mult(3, 2) # (1.) |
print "3 * 2 = ", mult(3, 2) # (1.) Fungsi "mult" berasal di sini |
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# |
# Peristiwa "return value" di penghujung hanya boleh berlaku |
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# sebaik sahaja b sama dengan sifar (b mengurang dengan bilangan 1 setiap kali tingkat 3 berlaku). |
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# once b equals zero (b decreases by 1 everytime step 3 happens). |
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# Dan hanya pada masa itu baharu perintah "print" dipaparkan. |
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# And only then can the print command at the bottom be displayed. |
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# Lihatnya sebagai kesan "lompatan". Amnya apa yang anda perlu faham |
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# See it as kind of a "jump-around" effect. Basically, all you |
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# adalah fungsi dimulakan semual |
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# should really understand is that the function is reinitiated |
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# |
# DALAM DIRINYA pada tingkat 3. Justeru, jujukan "melompat" |
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# |
# ke atas. |
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</source> |
</source> |
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Semakan pada 01:42, 17 Julai 2013
Ada pembaca yang mendapati bahagian ini berguna dan sebilangan pula akan mendapatinya mengelirukan. Jika anda mendapatinya mengelirukan, tinggalkan (ataupun lihat contoh-contohnya) dahulu. sekarang kita akan membincangkan program berikut satu persatu:
def mult(a, b):
if b == 0:
return 0
baki = mult(a, b - 1)
nilai = a + baki
return nilai
print "3 * 2 = ", mult(3, 2)
3 * 2 = 6
Secara amnya program ini mewujudkan satu fungsi darab integer positif (yang lebih perlahan daripada fungsi darab bina dalam) dan kemudian menunjukkan fungsi ini melalui penggunaannya. Atur cara ini menunjukkan penggunaan rekursi, sebuah bentuk lelaran (ulangan) yang menyebabakan fungsi ini memanggil dirinya sendiri sehinggalah sebuah syarat tamat dilunaskan. Ia mengguna tambahan berulang bagi memulangkan keputusan yang sama dengan darab: contoh, 3 + 3 (tambah) memberikan keputusan yang sama dengan 3 * 2 (darab)
PELAKSANAAN 1
- Soalan: Apakah perkara pertama yang dilakukan atur cara?
- Jawapan: Perkara pertama yang dilakukan adalah fungsi "mult" diberi definisi dengan semua baris melainkan baris yang terakhir.
def mult(a, b):
if b == 0:
return 0
baki = mult(a, b - 1)
value = a + baki
return value
- Ini mewujudkan sebuah fungsi yang mengambil dua parameter dan memulangkan nilai apabila ia selesai. Fungsi ini dapat dilaksanakan kemudian.
- Apa yang jadi kemudian?
- Baris selepas fungsi,
print "3 * 2 = ", mult(3, 2)
dilaksanakan.
- Dan apa yang dilakukannya?
- Ia mencetak
3 * 2 =
dan memulangkan nilaimult(3, 2)
- Dan apa yang dipulangkan
mult(3, 2)
? - Kita perlu melihat fungsi
mult
satu persatu jika kita ingin tahu.
RUN 2
- What happens next?
- The variable
a
gets the value 3 assigned to it and the variableb
gets the value 2 assigned to it.
- And then?
- The line
if b == 0:
is run. Sinceb
has the value 2 this is false so the linereturn 0
is skipped.
- And what then?
- The line
rest = mult(a, b - 1)
is run. This line sets the local variablerest
to the value ofmult(a, b - 1)
. The value ofa
is 3 and the value ofb
is 2 so the function call ismult(3,1)
- So what is the value of
mult(3, 1)
? - We will need to run the function
mult
with the parameters 3 and 1.
RUN 3
- So what happens next?
- The local variables in the new run of the function are set so that
a
has the value 3 andb
has the value 1. Since these are local values these do not affect the previous values ofa
andb
.
- And then?
- Since
b
has the value 1 the if statement is false, so the next line becomesrest = mult(a, b - 1)
.
- What does this line do?
- This line will assign the value of
mult(3, 0)
to rest.
- So what is that value?
- We will have to run the function one more time to find that out. This time
a
has the value 3 andb
has the value 0.
- So what happens next?
- The first line in the function to run is
if b == 0:
.b
has the value 0 so the next line to run isreturn 0
- And what does the line
return 0
do? - This line returns the value 0 out of the function.
- So?
- So now we know that
mult(3, 0)
has the value 0. Now we know what the linerest = mult(a, b - 1)
did since we have run the functionmult
with the parameters 3 and 0. We have finished runningmult(3, 0)
and are now back to runningmult(3, 1)
. The variablerest
gets assigned the value 0.
- What line is run next?
- The line
value = a + rest
is run next. In this run of the function,a = 3
andrest = 0
so nowvalue = 3
.
- What happens next?
- The line
return value
is run. This returns 3 from the function. This also exits from the run of the functionmult(3, 1)
. Afterreturn
is called, we go back to runningmult(3, 2)
.
- Where were we in
mult(3, 2)
? - We had the variables
a = 3
andb = 2
and were examining the linerest = mult(a, b - 1)
.
- So what happens now?
- The variable
rest
get 3 assigned to it. The next linevalue = a + rest
setsvalue
to3 + 3
or 6.
- So now what happens?
- The next line runs, this returns 6 from the function. We are now back to running the line
print "3 * 2 = ", mult(3, 2)
which can now print out the 6.
- What is happening overall?
- Basically we used two facts to calculate the multiple of the two numbers. The first is that any number times 0 is 0 (
x * 0 = 0
). The second is that a number times another number is equal to the first number plus the first number times one less than the second number (x * y = x + x * (y - 1)
). So what happens is3 * 2
is first converted into3 + 3 * 1
. Then3 * 1
is converted into3 + 3 * 0
. Then we know that any number times 0 is 0 so3 * 0
is 0. Then we can calculate that3 + 3 * 0
is3 + 0
which is3
. Now we know what3 * 1
is so we can calculate that3 + 3 * 1
is3 + 3
which is6
.
This is how the whole thing works:
3 * 2 3 + 3 * 1 3 + 3 + 3 * 0 3 + 3 + 0 3 + 3 6
Should you still have problems with this example, look at the process backwards. What is the last
step that happens? We can easily make out that the result of mult(3, 0)
is
0
. Since b
is 0
, the function mult(3, 0)
will return 0
and stop.
So what does the previous step do? mult(3, 1)
does not return 0
because b
is not 0
. So the next lines are executed:
rest = mult (a, b - 1)
, which is rest = mult (3, 0)
,
which is 0
as we just worked out. So now the variable rest
is set to 0
.
The next line adds the value of rest
to a
, and since a
is 3
and rest
is 0
, the result is 3
.
Now we know that the function mult(3, 1)
returns 3
. But we want to
know the result of mult(3,2)
. Therefore, we need to jump back to the
start of the program and execute it one more round:
mult(3, 2)
sets rest
to the result of mult(3, 1)
. We know
from the last round that this result is 3
. Then value
calculates as a + rest
,
i. e. 3 + 3
. Then the result of 3 * 2 is printed as 6.
The point of this example is that the function mult(a, b)
starts itself inside
itself. It does this until b
reaches 0
and then calculates the result as explained above.
Recursion
Programming constructs of this kind are called recursive and probably the most intuitive definition of recursion is:
- Recursion
- If you still don't get it, see recursion.
These last two sections were recently written. If you have any comments, found any errors or think I need more/clearer explanations please email. I have been known in the past to make simple things incomprehensible. If the rest of the tutorial has made sense, but this section didn't, it is probably my fault and I would like to know. Thanks.
Contoh
faktorial.py
# mentakrif fungsi yang mengira faktorial
def faktorial(n):
if n <= 1:
return 1
return n * faktorial(n - 1)
print "2! =", faktorial(2)
print "3! =", faktorial(3)
print "4! =", faktorial(4)
print "5! =", faktorial(5)
Output:
2! = 2 3! = 6 4! = 24 5! = 120
kira_detik.py
def kira_detik(n):
print n
if n > 0:
return kira_detik(n-1)
kira_detik(5)
Output:
5 4 3 2 1 0
Ulasan fungsi_menarik.py
# Ulasan di bawah diberi nombor tingkat agar
# penjelasan kod lebih terang. Sila baca mengikut tingakatan.
# (tingkat 1, contohnya, ada di bawah)
def mult(a, b): # (2.) Fungsi ini akan berulang kerana ...
if b == 0:
return 0
baki = mult(a, b - 1) # (3.) ... Apabila sampai ke SINI, jujukan bermula kembali dari atas!
value = a + baki
return value # (4.) justeru, "return value" tidak akan berlaku
# sehingga tingkat 3 dilewati program.
print "3 * 2 = ", mult(3, 2) # (1.) Fungsi "mult" berasal di sini
# Peristiwa "return value" di penghujung hanya boleh berlaku
# sebaik sahaja b sama dengan sifar (b mengurang dengan bilangan 1 setiap kali tingkat 3 berlaku).
# Dan hanya pada masa itu baharu perintah "print" dipaparkan.
# Lihatnya sebagai kesan "lompatan". Amnya apa yang anda perlu faham
# adalah fungsi dimulakan semual
# DALAM DIRINYA pada tingkat 3. Justeru, jujukan "melompat"
# ke atas.
Commented factorial.py
# Another "jump-around" function example:
def factorial(n): # (2.) So once again, this function will REPEAT itself....
if n <= 1:
return 1
return n * factorial(n - 1) # (3.) Because it RE-initiates HERE, and goes back to the top.
print "2! =", factorial(2) # (1.) The "factorial" function is initiated with this line
print "3! =", factorial(3)
print "4! =", factorial(4)
print "5! =", factorial(5)
Commented countdown.py
# Another "jump-around", nice and easy:
def count_down(n): # (2.) Once again, this sequence will repeat itself....
print n
if n > 0:
return count_down(n-1) # (3.) Because it restarts here, and goes back to the top
count_down(5) # (1.) The "count_down" function initiates here