python - How to fix .index() method returning the wrong value?

Question

I'm trying to get the index of the values higher than 70 from the following list:

temperatures = [33, 66, 65, 62, 59, 60, 62, 64, 70, 76, 80, 69, 80, 83, 68, 79, 61, 53, 50, 49, 53, 48, 45, 39]

But something is going wrong when the loop finds equal values:

hour_ex = []
for i in temperatures:
    if i > 70:
        hour_ex.append(temperatures.index(i))

print(hour_ex)

The code above is printing:

[9, 10, 10, 13, 15]

When the loop reach the index 12, it prints again the index 10 because it has the same value. I don't know what's going on. How can I fix it?

Answer 1

index is a list-searching function that performs a linear walk through the list to find the first position of a given element. This explains your confusing output--in the case of duplicates like 80, index() will always give you the first index it can find for that element, which is 10.

Use enumerate() if you're interested in obtaining the indices as a tuple for each element of the list.

Additionally, the variable i suggests index, but actually represents a given temperature in the list; it's a misleading variable name.

temperatures = [33, 66, 65, 62, 59, 60, 62, 64, 70, 76, 80, 69, 80, 83, 68, 79, 61, 53, 50, 49, 53, 48, 45, 39]    
hour_ex = []

for i, temperature in enumerate(temperatures):
    if temperature > 70:
        hour_ex.append(i)

print(hour_ex) # => [9, 10, 12, 13, 15]

Consider using a list comprehension, which performs a filtering operation on the enumerated list:

hour_ex = [i for i, temp in enumerate(temperatures) if temp > 70]