[FIXED] Change first row of a column if it meets a condition

Issue

This is a very basic problem. I have searched SO, but I didn't find the answer.

My DataFrame:

df = pd.DataFrame(
    {
       'a': [10, 50, 3],
       'b': [5, 4, 5],
    }
)

Now I want to change df.b.iloc[0] to 1 if df.a.iloc[0] is greater than 5.

Expected output:

    a    b
0  10    1
1  50    4
2   3    5

Attempts:

# attempt 1
df[df.a > 5].iloc[0]
# attempt 2
df.loc[df.a.iloc[0] > 5, 'b'] = 1

Solution

use if

if df.loc[0, 'a'] > 5:
    df.loc[0, 'b'] = 1

df

    a   b
0   10  1
1   50  4
2   3   5

---

if you don want if and inplace df, use following code

cond1 = df.loc[0, 'a'] > 5
cond2 = df.index == 0
out = df.assign(b=df['b'].mask(cond1 & cond2, 1))

out

    a   b
0   10  1
1   50  4
2   3   5

Answered By - Panda Kim