CODEWITHSUNDEEP
pythonvariable-assignmentfibonacci
user9654395
user9654395

Reputation: 109

What is difference between x,y = y,y+x and x=y , y=x+y?

I've just written these codes but outputs are different. second code's output is correct that I expected, but the first code's output is incorrect. but why?

def fib(n):
    x = 0
    y = 1
    print x
    for i in range(n):
        x = y
        y = x+y
        print x
    return x

output is; when n = 5

0
1
2
4
8
16


def fib(n):
    x,y = 0,1
    print x
    for i in range(n):
        x,y = y,x+y
        print x
    return x

output is; when n = 5

0
1
1
2
3
5

second code is correct but, x,y = y,x+y and x = y , y = x+y they look like same but outputs are different why?

Upvotes: 1

Views: 1896

Answers (3)

Andrey Donchev
Andrey Donchev

Reputation: 1

If you want to go without tuple, you'll have to assign the value of x to a new variable z before updating it with the value of y and use this new variable to calculate the value of y. The code is from a different problem, but the output is a Fibonacci sequence.

    x = 0
    y = 1
    print(x)
    while y < 50:
        print(y)
        z = x
        x = y
        y = z+y

Upvotes: 0

Petar Velev
Petar Velev

Reputation: 2355

The difference is that you will update x and then use it in the first example and you will directly use it in the second example.

x, y = 0, 1
x = y     # after this line x will be 1
y = x + y   # y = 1 + 1

Second example

x,y = 0, 1
x,y = y,x+y  # x, y = (1, 0 + 1) the old value of x will be used

This is because it will first generate the tuple on the right-hand side and after that, the tuple in your case (1, 1) will be assigned to x and y

Upvotes: 1

Lincoln Fuller
Lincoln Fuller

Reputation: 36

They give different outputs because with x=y, then y=x+y, you are setting x to the value of y. Then you take x, once its value has been updated and add it to y to find the y variable. With the one line variable declaration (x,y=y,y+x) in the y = y+x part of the equation, it will use a previous value of x instead of what you set it to in that line.

If you had:

x=0
y=1

Then you tried the one line declaration, it would look like this:

x,y=1,0+1
x,y=y,x+y

I hope this helped :)

Upvotes: 1

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