Why lambda does not change a variable's value

Question

I have some problem understanding lambda. My point is adding 1 to a variable in lambda until some point. Example;

x = 0
t = lambda y: y+1

while True:
    print ("Hello")
    t(x)
    if x==5: break

I thought it will stop after 5 times, but I realized that lambda is adding 1 only once. And this while loop is infinite. Why is that? Why lambda doesn't add 1 to that variable until while loop is finish like x += 1 ?

When you do `y+1` you create a new object which is returned by the lambda, but you do not collect the returned value. — cdarke, Feb 23 '16 at 21:18
The same way that `y + 1` *alone* won't change `y` *either*. — Martijn Pieters, Feb 23 '16 at 21:19
I believe that you're confusing about pass-by-reference vs pass-by-value. — emartinelli, Feb 23 '16 at 21:22
Imagine it did work the way you imagined, then should `1+y` change the meaning of `1` to become whatever the sum og the current value of `1` added to the value of `y`? — Sylwester, Feb 23 '16 at 22:38

score 10 · Accepted Answer · answered Feb 23 '16 at 21:14

10

You need to assign the output of t to x. What you did now is like doing:

def t(y):
    return y+1

x = 0
t(x)

Instead of

x = t(x)

You need to do:

x = 0
t = lambda y: y+1

while True:
    print ("Hello")
    x = t(x)
    if x==5: break

answered Feb 23 '16 at 21:14

olofom

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Why lambda does not change a variable's value

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