when I run the code, it returns 9 but the answer should be 10
list = [9, 6, 4, 10, 13, 2, 3, 5]
max = list[0]
second_last = list[0]
for x in list:
if x > max:
max = x
# Here is an issue with this statement
if x > second_last and x != max:
second_last = x
print(second_last)
Once you have a new max, just push the old max down to the second max
list = [9, 6, 4, 10, 13, 2, 3, 5]
max = float('-inf')
second_last = float('-inf')
for x in list:
if x > max:
second_last = max
max = x
elif x > second_last and x != max:
second_last = x
print(second_last)
x: max and second_last
9: 9 and 9
6: 9 and 9
4: 9 and 9
10: 10 and 9
13: 13 and 9 <<<
2: 13 and 9
3: 13 and 9
5: 13 and 9
You are essentially losing information about second_last, which is held by max until it is replaced by x when x > max. Whenever max is updated, technically second_last should also be updated, as the old max is the new second_last.
Notice that if the first if statement is satisfied, the second cannot be satisfied, so max and second_last are never updated simultaneously.
So, unless there is some value y further down in the iteration such that second_last < y != max - Eg: list = [9, 12, 10], y = 10 -, your code will produce incorrect outputs. So, your code never updates second_last when the sequence is strictly increasing.
Here is a fix for your code:
list = [9, 6, 4, 10, 13, 2, 3, 5]
max = list[0]
second_last = list[0]
for x in list:
if x > max:
second_last = max # add this line
max = x
elif x > second_last and x != max:
second_last = x
print(second_last)
Besides, it is a good idea, not to assume the list is non-empty.
Try this:
l = [9, 6, 4, 10, 13, 2, 3, 5]
_max = max(l)
_second = min(l)
for num in l:
if _second < num < _max:
_second = num
print(_second)
if _second < num < _max:._max > num is very likely true, causing two comparisons most of the time, while _second < num is very likely false, so if you test that first, most of the time you need just one comparison.list = [9, 6, 4, 10, 13, 2, 3, 5]
max = list[0]
second_last = list[0]
for x in list[1:]:
if x > max:
second_last = max
max = x
elif x < max and x > second_last:
second_last = x
print(second_last)
You could build a new list containing 2 elements (max and second_last):
lst = [9, 6, 4, 10, 13, 2, 3, 5]
# results[0] = max; results[1] = second_last
results = [float("-inf"), float("-inf")] # initialise negative infinite!
for item in lst:
if item > results[0]:
results = [item, results[0]]
print(results)
# OR
max_value, second_last = results
print(max_value)
print(second_last)
Out:
[13, 10]
13
10
Easiest approach would be using built-in sort:
results = sorted(lst, reverse=True)
print(results[:2])
>>> [13, 10]
Node: list and max are Python built-ins, do not use them as variable names.
listandmaxas names for your own variables. You overwrite the built-ins (list,max) with the same name.