Getting wrong answer from data length calculation

Question

I have a list of 41 data from a functioned I defined, but I only want to access the first 40 of them. So, the index positions I am looking for are from 0 - 39

forecast_price(10)[:len(forecast_price(10)) - 1]

I also made it a variable for easier reference further on in my codes:

forecast_w10 = forecast_price(10)[:len(forecast_price(10)) - 1]

However, when I try printing the length of data in the variable, I still get 41. I feel like the mistake is right under my nose but can't seem to figure it out. What am I doing wrong?

In[46]: print(len(forecast_w10))
Out[46]: 41

Just do `forecast_price(10)[:-2]`. – enzo May 23 '21 at 14:59 — enzo, May 23 '21 at 14:59
Re-check the length of `forecast_price(10)` once. – Ank May 23 '21 at 15:09 — Ank, May 23 '21 at 15:09
@enzo Tried to do so, but I still get 41 as the output :( – Ejin May 23 '21 at 15:20 — Ejin, May 23 '21 at 15:20
@Ank done so too, the length is 41 – Ejin May 23 '21 at 15:21 — Ejin, May 23 '21 at 15:21

score 0 · Accepted Answer · answered May 23 '21 at 15:27

For reproducibility, let's assume forecast_price is a function that returns a list of 41 elements, starting at a given initial value x:

def forecast_price(x):
    return [i + x for i in range(41)]

print(forecast_price(1))          # [1, 2, 3, ..., 39, 40, 41]
print(len(forecast_price(1)))     # 41

print(forecast_price(10))         # [10, 11, 12, ..., 48, 49, 50]
print(len(forecast_price(10)))    # 41

Since you want to access only the 40 first values, use the slicing operator:

forecast_w10 = forecast_price(10)[:40] 
print(forecast_w10)               # [10, 11, 12, ..., 47, 48, 49]
print(len(forecast_w10))          # 40

Thank you! I seemed to have complicated my own codes hahaha – Ejin May 23 '21 at 15:43 — Ejin, May 23 '21 at 15:43

Getting wrong answer from data length calculation

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