It is really difficult to look at a few lines of code and know what will happen. The good news is, we don't have to guess! XCode comes with a built in visual debugger that will let us watch the execution of our programs step by step.

Start with the program you wrote for the format strings example that print out variables of several types. On the left side of the editor pane, you will see an empty grey column. Click in this column at the first line of code within the @autorelease block. You should see a dark blue arrow appear.

Try running the program again, this time, instead of running to completion, execution will halt at the blue arrow. At this point you can use the step controls to continue exeucting the program line by line. Note that the bottom left of the display now shows

Continue - Leave debugging and keep running the program

Step Over - Execute the current function and move to the next line

Step Into - Step through the current function

Step Out Of - Return back to the calling function