Change values in a list using a for loop (python)

Question

I currently have some code that reads like this:

letters = {
10 : "A",
11 : "B",
12 : "C",
13 : "D",
14 : "E",
15 : "F"
}
vallist = [rd1, rd2, gd1, gd2, bd1, bd2]
for i in vallist:
    if i >= 10:
        i = letters[i]

What I want to happen is the for loop to iterate through vallist and replace any value that is greater than 10 with its corresponding letter. However, my current code just changes i and not the original value in the list. For example, if rd1 is set to 15, the code runs through and i is set to "F", but rd1 does not change to "F", and instead just stays as 15. How can I fix this?

ruohola · Accepted Answer · 2019-03-03 22:59:31Z

13

For each iteration of the for loop the variable i is assigned with just a copy of the value of an item in vallist, so changes made to i won't be reflected in i.

You should update the items of i via index, which you can generate with the enumerate function:

for index, value in enumerate(vallist):
    if value >= 10:
        vallist[index] = letters[value]

edited Mar 3, 2019 at 22:59

ruohola

24.8k7 gold badges76 silver badges118 bronze badges

answered Mar 3, 2019 at 22:52

blhsing

109k9 gold badges88 silver badges132 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Splines Over a year ago

I think this statement is not correct: "[...] the variable i is assigned with just a copy of the value of an item in vallist, [...]". According to Ned Batchelder's guide "Assignment never copies data". A for-loop is an assignment as well; in this case it makes i refer to a value already referenced by an item in vallist. It's the line i = letters[i] where now i refers to another value, in this case the value of letters[i], e.g. the string "A". There is no copying involved in the author's code example.

Georges Lorré · Accepted Answer · 2019-03-03 22:56:51Z

2

rd1, rd2, gd1, gd2, bd1, bd2 = 10, 11, 12, 13, 14, 9
letters = {
10 : "A",
11 : "B",
12 : "C",
13 : "D",
14 : "E",
15 : "F"
}
vallist = [rd1, rd2, gd1, gd2, bd1, bd2]
for index, value in enumerate(vallist):
    if value >= 10 and value <= 15:
        vallist[index] = letters[value]
print(vallist)

As mentioned in the other comment you need both the index and the value while looping over your vallist. so you can replace the value on the index with the value in your dictionary.

answered Mar 3, 2019 at 22:56

Georges Lorré

4431 gold badge3 silver badges14 bronze badges

Comments

Kristóf Varga · Accepted Answer · 2019-03-03 22:58:01Z

0

for i in range(len(vallist)):
    if vallist[i] >= 10:
        vallist[i] = letters[i]

Because in your case i is just a copy of the current item in vallist. But this way i will be an index, so you can modify your original list throught it.

answered Mar 3, 2019 at 22:58

Kristóf Varga

16810 bronze badges

Comments

Madfish · Accepted Answer · 2019-03-03 23:01:17Z

0

actually i is the reference of that element of array and the value changes will not be affected to that element , instead try this,

# iterating through every index of array

for i in range(len(vallist)):
 if vallist[i] >= 10 and vallist[i]<=15:
      vallist[i] = letters[vallist[i]]

answered Mar 3, 2019 at 23:01

Madfish

857 bronze badges

Collectives™ on Stack Overflow

Change values in a list using a for loop (python)

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related