R - If-else statements within a for loop is populating entire column at a time rather than each index within that column [duplicate]

I'm new to R and trying to populate a column bmi_cat in my data frame dat based on the numerical value provided in the bmi column. However, it populates all of bmi_cat with "Normal" even when that is incorrect. The last bmi value in the dataframe is within the Normal range, so I suspect it is continuously updating the entirety of bmi_cat with the most recent result. However, I'm not sure why. Can anyone point out the fault in my approach?

for (num in 1:nrow(dat)){
  if (dat$bmi[num] <= 18.5) {
      dat$bmi_cat[num] <- "Underweight"
  }  else if (dat$bmi[num] > 18.5 & dat$bmi[num] <= 25) {
      dat$bmi_cat[num] <- "Normal" 
  }  else if (dat$bmi[num] > 25 & dat$bmi[num] < 30) {
      dat$bmi_cat[num] <- "Overweight"    
  }  else if (dat$bmi[num] >= 30){
      dat$bmi_cat[num] <- "Obese"
  }
}

I hope this was enough information. Thank you in advance.