How to update a variable inside the while loop and see the results?

Question

I want to use a for loop to define a range and I would like to use a while loop to check a condition for every value inside this for loop and give me the results for different values of c, but unfortunately, my algorithm doesn't work and I don't know what my mistake is.

j=0
jl=[]
c=np.linspace(0,20,num=20)
for a in range(0,len(c)):
    while j<5:
        j=c[a]+2
    jl.append(j)

The result I am looking for is it puts different values of c inside the while loop and calculate j and check if it is bigger than 5 or not. if yes, it appends it to jl. Totally, I want to define a range with for loop with index and also check each value of this range inside while loop and get the results for j. so expected results are j with values smaller than 5 (c[a]+2<5) and store the values of j in jl

You start with j=0 and your while condition is j>5 so it never runs. — wwii
– wwii, Commented Feb 24, 2019 at 19:37
Your example has jl.append(j) outside/after the for loop - is that intended? - it will only run once after the for loop is done. — wwii
– wwii, Commented Feb 24, 2019 at 19:42
@wwii actually I am new in programming, especially python, that's why I am not aware of my mistakes, this task is very important for me because I am going to write a program in the university. Actually, I don't know where should i put this append right now. I didn't get any results. — Pouyi
– Pouyi, Commented Feb 24, 2019 at 19:44
You don't actually need the range here, because you never try to assign to c[a] or use a for anything other than indexing c. Just iterate over c directly: for item in c: while j < 5: j=item + 2; .... — chepner
– chepner, Commented Feb 24, 2019 at 19:52

Adam Bellaïche · Accepted Answer · 2019-02-24 19:39:46Z

1

There is one problem in your code:

j=0

"While loop" never runs because his condition is j>5. Correct that and tell us if it works.

answered Feb 24, 2019 at 19:39

Adam Bellaïche

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user10210717 · Accepted Answer · 2019-02-24 21:32:09Z

1

Please double check if the following suggested solution covers all of your requirements. In any case I think List Comprehensions (e.g. compare section 5.1.3.) can help you.

c=np.linspace(0,20,num=20)
ls = [x+2 for x in c if((x+2)<5)]
print(ls)

Will result in the following output:

[2.0, 3.052631578947368, 4.105263157894736]

If you want to do more complex data manipulation, you can also do this with the help of functions, e.g. like:

def someManipulation(x):
    return x+2

ls = [someManipulation(x) for x in c if(someManipulation(x)<5)]

answered Feb 24, 2019 at 21:32

user10210717

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Comments

Kit. · Accepted Answer · 2019-02-24 21:47:30Z

1

Your algorithm doesn't work because

while j<5:
    j=c[a]+2

is an infinite loop for j = 0, a = 0.

What you probably wanted to write was:

for x in c:
    j = x + 2
    if j < 5:
        jl.append(j)

Still, the list comprehension version in the other answer does exactly the same, but better.

answered Feb 24, 2019 at 21:47

Kit.

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