I am learning python and I have some question:
What is the difference between
a,b = 0,1
for x in range(100):
print(a)
a=b
b=a+b
and
a,b = 0,1
for x in range(100):
print(a)
a,b = b, a+b
First one gives bad result, but why?
1 Answer
Because you first set a = b, your new b will have the value of twice the old b. You overwrite a to early. A correct implementation should use a temporary variable t:
a,b = 0,1
for x in range(100):
print(a)
t=a
a=b
b=t+bThis is basically what you do by using sequence assignment.
In your original code you have:
a,b = 0,1
for x in range(100):
print(a)
a=b # a now is the old b
b=a+b # b is now twice the old a so b'=2*b instead of b'=a+b
So this would result in jumping by multiplying with two each time (after first loading in 1 into a the first step).
An equivalent problem is the swap of variables. If you want a to take the value of b and vice versa, you cannot write:
#wrong swap
a = b
b = a
Because you lose the value of a after the first assignment. You can use a temporary t:
t = a
a = b
b = t
or use sequence assignment in Python:
a,b = b,a
where a tuple t = (b,a) is first created, and then assigned to a,b.
answered Jan 23, 2017 at 12:05
willeM_ Van Onsem
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8 Comments
Mohammad Yusuf
You on Windows or Linux?
willeM_ Van Onsem
@MYGz: Linux of course, why?
Mohammad Yusuf
Why do you write
<code><pre> There is a chrome extension called text area formatter, You can select the code part and indent it with tab.willeM_ Van Onsem
@MYGz: well usually it is as fast as writing code itself (the formatting part) when using an advantage kinesis :) Every swap between keyboard and mouse is a waste of time.
Mohammad Yusuf
Are you telling me select code and press a tab can sometimes be slower for you than writing
<pre><code></code></pre>. I can never reach those typing speeds :D |
ahas changed and thusa + bwill be different. Assignment calculates the results of the right hand side before applying the results to the left.