Replacing all negative values in certain columns by another value in Pandas

Question

Suppose I have four successively arranged columns as a part of a data frame and I want to replace all the negative values in these 4 columns by another value (-5 let's say), how do I do it?

T1   T2  T3  T4
20   -5  4   3
85  -78  34  21
-45  22  31  75
-6   5   7  -28

Logically, I was hoping this would work. But, it doesn't.

for i in df.iloc[:,df.columns.get_loc("T1"):df.columns.get_loc("T1")+4]<0:
    for j in df[i]:
        if j<0:
            j=-5

Answer 1

You can just use indexing by applying a condition statement.

cols = ['T1','T2','T3','T4']
df[df[cols] < 0] = -5

Output

In [35]: df
Out[35]:
     T1  T2  T3  T4
  0  20  -5   4   3
  1  85  -5  34  21
  2  -5  22  31  75
  3  -5   5   7  -5

In your example you're just replacing the value of variable. You need to replace one cell's value using at method.

for i in df.iloc[:,df.columns.get_loc("T1"):df.columns.get_loc("T1")+4]<0:
for index, j in enumerate(df[i]):
    if j<0:
        df.at[index, i] = -5

Answer 2

You can use indexing:

c = ['T1','T2','T3','T4']
df[df[c] < 0] = -5

Or clip:

In [47]: df[c].clip(lower=-5)
Out[47]:
   T1  T2  T3  T4
0  20  -5   4   3
1  85  -5  34  21
2  -5  22  31  75
3  -5   5   7  -5