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CS 170 – Intro to Scientific and engineering Programming

CS 170 – Intro to Scientific and engineering Programming . Software Engineering. Goals: Understand difficulties in solving large programming problems Develop effective methods for writing large programs How do we define a good program? What is a bug? Why is SW Engineering important?.

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CS 170 – Intro to Scientific and engineering Programming

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  1. CS 170 – Intro to Scientific and engineering Programming

  2. Software Engineering • Goals: • Understand difficulties in solving large programming problems • Develop effective methods for writing large programs • How do we define a good program? • What is a bug? • Why is SW Engineering important?

  3. Top-Down Design • Should we start solving a problem by coding the solution immediately? • CASE: Computer Aided SW Engineering • Decomposition: divide problem to sub problems • Stepwise refinement: repeated application of decomposition

  4. Top-Down Design • State the problem • Define the inputs and outputs • Design the algorithm • Decompose into subproblems • Perform stepwise refinement • Convert the algorithm into a Matlab program • Test the program Which step is the most difficult and why? What is an iterative process?

  5. Script Files • Script files • text files (created with a text editor) • contain many lines of MATLAB expressions and assignments. • You can execute all of the lines of the file (sequentially) by typing the filename at the MATLAB prompt. • If filename is computetraj.m, then >> computetraj • causes the instructions (ie., the MATLAB commands) in the file to be run.

  6. Debugging your programs • Is it possible to have errors in your program? • Three different types of errors are • Syntax errors: you simply have typed an illegal expression, independent of the values of the variables in the expression. • Run-time errors: logic errors that result in an illegal expression for specific values of the data (harder to fix). • Logic errors that result in the program executing completely, but the answer that they return is incorrect (hardest to fix).

  7. Finding run-time errors (bugs) The most primitive way to debug a program is to • Work out (by “hand”) • Remove the trailing semicolons ; • Call your program, and compare the printouts (your program) to your hand-calculations

  8. Finding run-time errors (bugs) A better (but ultimately the same) way is to • Work out (by “hand”) • Insert the command keyboard at some point in your program. This is called “setting a breakpoint” • Run the program • When execution gets to the keyboard line • program execution is temporarily stopped • the Command window appears, the prompt is K>> • control it at the keyboard (ie., you can type) • Type expressions at the K>> prompt • You are a detective at this point. • Type K>> return (resume execution) or K>> dbquit (exit)

  9. Single-line stepping through an m-file Use dbstep to step through an M-file a line-at-a-time To execute the next line of code, type K>> dbstep To execute the next 6 lines of code, type K>> dbstep 6 To execute code until another m-file function is called, and then to “stop inside” that function at its first line, type K>> dbstep in To execute code until the m-file function exits, and then to stop inside the m-file function that called it, type K>> dbstep out After any of these, the “instruction pointer” moves to the next line of code which has not executed.

  10. Stopping in an M-file Use dbstop to stop in an M-file at a certain line if a condition is true. To stop in loancalc at line 11 if the variable G is less than 1 K>> dbstop in loancalc at 11 if G<1 This sets a conditional breakpoint.

  11. Clearing breakpoints All breakpoints can be cleared with the command >> dbclear all You can also selectively clear breakpoints that were created with dbstop. See >> help dbstop >> help dbclear for more information.

  12. Using dbup and dbdown Normally at a breakpoint • execution is temporarily stopped • the K>> prompt appears in the command window • the variables in the function’s workspace can be examined. The command dbup changes the scope of visible variables to the caller’s workspace. K>> % examine variables in function’s workspace K>> dbup K>> % examine variables in caller’s workspace K>> dbdown K>> % examine variables in function’s workspace

  13. Seeing all of the function instances Use dbstack to see the list of all function instances that are running. • The top line gives the function instance where the dbstack command occurred. • The (n+1)’th line is the function instance of the function which called the function instance listed on the n’th line.

  14. Profiling your code When running a program, Matlab can keep track of • How many times a single line is executed • How many times a function is called • How much total time is spent executing a specific line of code • How much time is spent running a specific function Why is this information useful? >> profile on >> myprogram >> profile report

  15. Questions??

  16. Resources • “Introduction to Programming with Matlab”, J. Michael Fitzpatrick and John D. Crocetti • Lecture slides E77, Andy Packard, http://jagger.me.berkeley.edu/~pack/e77

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