If & ifelse statement in R

As the new user in R, I met few problems when I tried to valuate a.

Code: // time2 are numbers // a=d3$Time2
b=c(...) for (i in 1:65){ for (j in 1:1762){ if ((a[j]>=1474161415+900*(i-1))&(a[j]<1474161415+900*i)) a[j]=b[j] } }

Results: Error in if ((a[j] >= 1474161415 + 900 * (i - 1)) & (a[j] < 1474161415 + : missing value where TRUE/FALSE needed

Also I have tried: ifelse( ((a[j]>=1474161415+900*(i-1)) & (a[j]<1474161415+900*i)) , a[j]=b[j])

Results: -unexpected '=' in -ifelse( ((a[j]-=1474161415+900-(i-1)) & (a[j]-1474161415+900-i)) , a[j]=--

Regards