Calculator using grid

The most difficult part was to figure it out how I can make my operations to execute in math order. For example 3 + 5 * 2 = 13, not 16. When I met "/" "*" signs, I execute the operations, and for the rest of operations and numbers, I build 2 arrays, which will be executed at the end. I saw a function "eval()", but I wanted to make all from 0. If you have any sugestions/feedback's feel free to write.