Python: While and For loops

Question

Please help me in understanding why my code is not working as I want it to be. I cannot match the result when I run the code in my head vs the actual result in the console. It is very simple:

Given List "num_list", I want to take the odd numbers, sum them up and show the result, with the condition that it has to take and sum the first 5 odd numbers from the list.

i.e.

result= 85 + 719 + 85 + 17 + 191

num_list = [422, 136, 524, 85, 96, 719, 85, 92, 10, 17, 312, 542, 87, 23, 86, 191, 116, 35, 173, 45, 149, 59, 84, 69, 113, 166]
result=0
odd_num=[]
count_odd=0
while count_odd < 5:
 for i in range(len(num_list)):
 if num_list[i]%2!=0:
 odd_num.append(num_list[i])
 count_odd+=1 
 result+=sum(odd_num)
print(result)

The thing is that the while loop does not stop at count_odd < 5.

I am learning the very basics and I want to understand what is going on here.

Thank you

• anon01: result is wrong. This would be correct: 85 + 719 + 85 + 17 + 87

Answer 1

I see a couple things wrong with your current code and some things that are unessaccary.

Your while loop is not needed, because you use a `for range`.

You don't want to use a `for range` loop when iterating through a list

you can simply just append odd numbers to a list in you loop. Then outside of your loop find the sum

Below I have fixed these mistakes and cleaned up the code.

num_list = [422, 136, 524, 85, 96, 719, 85, 92, 10, 17, 312, 542, 87, 23, 86, 191, 116, 35, 173, 45, 149, 59, 84, 69, 113, 166]
odd_num=[]
count_odd=0
for num in num_list:
 if count_odd == 5:
 break
 elif num%2!=0:
 odd_num.append(num)
 count_odd+=1 
print(sum(odd_num))

output

993

Answer 2

Is it required for you to use a while loop? Otherwise a simple solution would be to add a if condition inside your for loop for example

num_list = [422, 136, 524, 85, 96, 719, 85, 92, 10, 17, 312, 542, 87, 23, 86, 191, 116, 35, 173, 45, 149, 59, 84, 69, 113, 166]
result=0
odd_num=[]
count_odd=0
for i in range(len(num_list)):
 if count_odd < 5:
 if num_list[i]%2!=0:
 print(num_list[i])
 odd_num.append(num_list[i])
 count_odd+=1 
 result+=sum(odd_num)
print(result)

What is happening in your while loop is that your while loop runs, it runs 5 times, meaning your for loop runs 5 times so essentially the result= 85 + 719 + 85 + 17 + 191 calculation is ran 5 times which is something you dont want. You want to check your count inside your for loop, not have the for loop run 5 times. Hope that makes sense

Answer 3

It's because the for loop finishes completely each iteration of the while loop even if 5 odds have been found. You could just check the count at the end of each loop.

while count_odd < 5:
 for i in range(len(num_list)):
 if num_list[i] % 2 != 0:
 odd_num.append(num_list[i])
 count_odd += 1 
 result+=sum(odd_num)
 if count_odd >= 5:
 break

Or you could just remove the for loop entirely.

i = 0
while count_odd < 5:
 if num_list[i] % 2 != 0:
 odd_num.append(num_list[i])
 count_odd += 1 
 result += sum(odd_num)
 print(count_odd)
 i += 1

Answer 4

You only need one loop, the while loop.

num_list = [422, 136, 524, 85, 96, 719, 85, 92, 10, 17, 312, 542, 87, 23, 86, 191, 116, 35, 173, 45, 149, 59, 84, 69, 113, 166]
odd_num=[]
count_odd=0
i = 0
while count_odd < 5:
 print(num_list[i])
 if num_list[i]%2!=0:
 odd_num.append(num_list[i])
 count_odd+=1 
 i += 1
result = sum(odd_num)
print(result)

Answer 5

You can use a list comprehension to iterate through the list, filter out all the even numbers, use list slicing to select the first 5 odd numbers, then sum them.

n = sum(list(filter(None, [i if i%2==1 else None for i in num_list]))[:5])

print(n) gives:

993

or 85 +たす 719 +たす 85 +たす 17 +たす 87 =わ 993 (the first 5 odd numbers)

• chepner: filter(lambda x: x%2 == 1, num_list) would be a lot simpler.
• chepner: Or (i for i in num_list if i%2 == 1)
• chepner: You also don't need to convert the filter instance to a list first; sum can add up arbitrary iterable values.
• user9321739: still need to draw it in a sum function sum(list(filter(lambda x: x%2 != 0, num_list))[:5])
• user9321739: TypeError: 'filter' object is not subscriptable you do need to convert it to a list, how else are you going to use list slicing to select only the first 5 items?