boolean expressions are important for controlling the flow of programsboth in while loops and in other constructs that we will introduce in their evaluation and use should be fairly familiar from mathematicsbut it is always good idea to explore fundamental concepts such as this by typing in few examples in an interactive python shell using lists to store sequences of data so farwe have used one variable to refer to one number (or stringsometimes we naturally have collection of numberssuch as the -values (years created in the example above in some casessuch as the one abovewe are simply interested in writing all the values to the screenin which case using single variable that is updated and printed for each pass of the loop works fine howeversometimes we want to store sequence of such variablesfor instanceto process them further elsewhere in the program we couldof courseuse separate variable for each value of nas followsn howeverthis is another example of programming that becomes extremely repetitive and boringand there is obviously better solution in pythonthe most flexible way to store such sequence of variables is to use listn [ notice the square brackets and the commas separating the valueswhich is how we tell python that is list variable now we have single variable that can hold all the values we want python lists are not reserved just for numbers and can hold any kind of objectand even different kinds of objects they also have great deal of convenient built-in functionalitywhich makes them very flexible and useful and extremely popular in python programs we will not cover all the aspects of lists and list operations in this bookbut we will use some of the more basic ones we have already seen how to initialize list using square brackets and comma-separated valuessuch as [- ' string' to retrieve individual elements from the listwe can use an indexfor instance [ will pick out the element with index that isthe fourth element (having value of zeroin the listsince the numbering starts at zero list indices start at zero and run to the where is the number of elements in the list