Can we look just at the other features when we have a missing vaue?

For my classification task i have 3 features a,b,c. Feature c is missing for some datas. I can have already a good score for my classifier training with the two other features a,b and even better if I don't look at the missing data and I train with a,b,c.

Is it possible to train my classifier with features a,b,c on all the data which don't have missing values, and in my prediction phase, if the feature c is missing, to predict the class like if I had train my classifier with a and b?

• A tedious way could be to train two different classifiers. One using a, b, and c as features by removing the instances that don't have values for c, and the other classifier using a and b only on the entire data. Then during the prediction phase, depending on whether the data has feature c missing or not, choose the appropriate classifier. – tomar__ Oct 25 '17 at 9:40
• Please clarify the question: are you suggesting training two distinct classifiers (as addressed in the comment by @tomar__) or are you asking if your classification algorithm would be robust enough to generalize well to cases (not seen in training) where c is missing (assuming you can represent that). In the latter case, please provide information about your classification algorithm. – mjul Oct 25 '17 at 9:54
• I would like to do why @tomar__ said, but is it possible to do it with just one classifier: Train on a,b,c and if in my test phase, c is missing, I rely just on feature a,b like If I had an other classifier train on a,b. – thovex Oct 25 '17 at 10:02

1. Fill in the mean/median from known $c$'s in the $a,b,c$ model
2. Create a linear regression model to predict $c$ from $a,b$, fill the estimate $c$ from this model in the $a,b,c$ model
Personally, I think 2. will work best in most cases, but 1. can already be sufficient when $c$ is not correlated to $a$ or $b$.