What would you do now if you want to change the variable i into a k? You can't just use an instruction like

:254,267s/i/k/g

because that will change the letter i into k even in other words like int and list.

The solution to this is to add a c (for check) after the s command. For example,

:s/pattern/replacement/gc

When you do this, vi checks with you before making every substitution. Before each possible change, vi prints the line at the bottom of your screen and puts a ^ under the string that might be changed. If you want the change to happen, press the <Y> key followed by <Enter>. If you do not want the change to happen, press the <N> key followed by <Enter>.