Previously I spent a lot of time to see answer on similar question but neither work with my problem.
I have QGIS 3.4 and a DEM where I've to substitute more accurate height in cells that are identified by a "river" coming from rasterization of a poly line (... aligned and same cell width to DEM grid ..).
Outside the grid cell of river there are nodata cells, this is the problem I suppose, because
ifelse function as 0/1 trick in QGIS doesn't work outside the river cells.
Also I'm not able to reclassify nodata in 0, neither of reclassification suggestion work.
When I was young ... I used the simple CON command in ArcView that worked in a very intuitive manner, to realiz this simple question "Where river raster exists pick height from this raster elsewhere maintain height of dem ..) My administration believe to save cash to use open software but do not realize that I spent a lot of time to solve problem, as this, that as I know from previous experience should be intuitive.