Select a column based on another column's value in Spark Dataframe using Scala

Question

I have a dataframe with 5 columns - sourceId, score_1, score_3, score_4 and score_7. The values of sourceId column can be [1, 3, 4, 7]. I want to convert this into another dataframe that has the columns sourceId and score, where score depends on the value of the sourceId column.

sourceId	score_1	score_3	score_4	score_7
1	0.3	0.7	0.45	0.21
4	0.15	0.66	0.73	0.47
7	0.34	0.41	0.78	0.16
3	0.77	0.1	0.93	0.67

So if sourceId = 1, we select value of score_1 for that record, if sourceId = 3, we select value of score_3, and so on...

Result would be

sourceId	score
1	0.3
4	0.73
7	0.16
3	0.1

What would be the best way to do this in Spark?