Different results by 1

Q&A
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#1

My Code: 

decade_frequency = {}
for decade in decades:
    if decade not in decade_frequency:
        decade_frequency[decade] = 1
    if decade in decade_frequency:
        decade_frequency[decade] += 1
        
print(decade_frequency)

Result:

{'30s': 4723, '60s': 1358, '70s': 560, '40s': 4082, '50s': 2435, '20s': 1857, 'Unknown': 1094, '90s': 254, '80s': 365, '100s': 4, '110s': 4}

Correct Answer:

decade_frequency1 = {}
for d in decades:
    if d not in decade_frequency1:
        decade_frequency1[d] = 1
    else:
        decade_frequency1[d] += 1
print(decade_frequency1)

Result:

{'30s': 4722, '60s': 1357, '70s': 559, '40s': 4081, '50s': 2434, '20s': 1856, 'Unknown': 1093, '90s': 253, '80s': 364, '100s': 3, '110s': 3}

Dear Community,
I have no idea why the codes make difference. The only difference is “if… in” in my code & “else” in the correct answer, but not sure why it would make different results. Why does the correct one have one less in each values?

#2

Let’s say your for loop has been running for a while. And right now, your decade_frequency dictionary contains -

decade_frequency = {"30s": 4, "40s": 5}

Now, consider the next iteration of your loop where decade is "30s".

For first part of your loop -

if decade not in decade_frequency:
        decade_frequency[decade] = 1

After the above code runs, what would decade_frequency contain?

Now, let’s go to the second part of your loop -

if decade in decade_frequency:
        decade_frequency[decade] += 1

After the above code runs, what would decade_frequency contain?

Now, move forward by one iteration. This time, decade is "50s".

Repeat the process above for both the parts I mentioned with this value for decade. After both of those parts, what would decade_frequency contain? Write it down and see whether or not that looks correct to you.

If you get stuck or confused, feel free to ask more questions.

#3

ahhhh i got it, after operating

if decade not in decade_frequency:
        decade_frequency[decade] = 1

the decade that was not in the decade_frequency will be added to the library, so then it’ll make

if decade in decade_frequency

true, so it’ll add one more. Thank you so much! well-understood :slight_smile:

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