Replace value per row with value in first column

My question is very simple. I have a data frame with various numbers in each row, more than 100 columns. First column is always a non zero number. What I want to do is replace each nonzero number in each row (excluding the first column) with the first number in the row (the value of the first column)

I would think in the lines of an ifelse and a for loop that iterates through rows but there must be a simpler vectorised way to do it...