I am writing a simple program that coverts a string into integers depending upon there there occurences

I am writing a program that codifies the string such as "Hello world this is Hello STOP" it stops after STOP keyword and the output come like 0 1 2 3 0 as word "Hello" is assigned a code 0. My problem is that it has to count Hello and hello as same. Means it should be case insensitive. for HelLo it is same as hELLo. how do i do that? Please help me.