高手帮我解释下我写的代码逻辑错误在哪里

import pandas as pd

df = pd.DataFrame({'a': [5, 6, 7], 'b': [3, 5, 8]})

def func(i):
    if i == 0:
        return df.iloc[i].a + df.iloc[i].b
    else:
        return df.at[i-1, 'c'] + df.iloc[i].a

df['c'] = df.index.map(func)

print(df)

# !/usr/bin/python3

import pandas as pd

df = pd.DataFrame({'a': [5, 6, 7], 'b': [3, 5, 8]})

c = []


def func(i):
    if i == 0:
        number = df.iloc[i].a + df.iloc[i].b
    else:
        number = c[i - 1] + df.iloc[i].a
    c.append(number)
    return number


df['c'] = df.index.map(func)