The Art of Design Ying Wu Electrical Engineering & Computer Science Northwestern University ECE230 Lectures Series

Outline Data flow Top-down vs. bottom-up Coarse to fine Two examples –MP#2 –MP#4

Data Flow Data –data representation  describe your inputs/outputs E.g. 1: an inputted sentence? E.g. 2: a segment of a command line? Inputs and outputs of the building blocks –what are given? –what are expected? Data flow –blocks are connected through data flow

Flow chart Coding is a very very small part of programming –design  40% –coding  10% –debugging  50% The starting point of design –always draw flow charts –organize your thoughts by flow charts –use testing cases to validate your flow charts

Top-down “Incremental” programming –Divide-and-conquer Task decomposition –No matter how small a task is, you can always divide it into a set of small sequential subtasks –You need to understand your tasks Focus –Make the structure clear and neat –Always use the basic controls Sequential Selection repetitive

Coarse to Fine Coarse  Fine Coarse design –determines the structure of the program –tells the basic idea –details are all ignored –concentrate on data flow –Organize it by subtasks Refinement –A rough “block” can always by replaced by a refined flowchart –It is much easier, because the tasks are simpler

Bottom-up “Brain-storming” programming Bottom design –creating a set of small “tools” –thinking based on “basic operations” Bottom  Up –putting tools together –this needs more experiences

“Tools” Extracting those basic operations –E.g. 1: bypass all “white spaces” –E.g. 2: check digits Forming functions –inputs –outputs –error checking

Before you start … Data representation? –How do you represent a “segment”? st/ed Your objectives? –Find a segment  determine st/ed

A Coarse Design Command line  buffer[500] Find the first element of a seg. Find the last element of the seg. Copy the segment to piece[100] piece  screen End of the string? Start the next segment Y N done

Code it int main() { cout << “Welcome bla bla bla” << endl; char buffer[500]; cout << "\nInput a command line: "; cin.getline(buffer, 500); char piece[500]; int st = 0, ed = 0; while( buffer[st]!=0 ){ st = _find_the_first_element( ); ed = _find_the_last_element( st ); _copy_segment(piece, st, ed); _display_segment(piece); st = ed + 1; } return 0; }

Checking after actions Command line  buffer[500] Find the first element of a seg. Successful? Find the last element of the seg. Successful? Copy the segment to piece[100] piece  screen End of the string? Start the next segment Error Y N N Y Y N

int main() { cout << “Welcome bla bla bla” << endl; char buffer[500]; cout << "\nInput a command line: "; cin.getline(buffer, 500); char piece[500]; int st = 0, ed = 0, err_code = 1; while( buffer[st]!=0 ){ st = _find_the_first_element( ); if (buffer[st] == 0) err_code = 0; ed = _find_the_last_element( st ); // err_code can be set inside if (err_code == 0) break; else { _copy_segment(piece, st, ed); _display_segment(piece); st = ed + 1; } return err_code; }

Coarse-to-fine Find the first element of a seg. st ‘ ‘v ‘‘ a‘‘ r‘‘ _‘‘ 1‘‘ ‘ a‘‘ ‘ +‘‘ ‘3‘‘.‘‘ 1‘‘ 4‘‘ 0 buffer[0] buffer[st] == ‘ ‘ && buffer[st] != 0 st ++ Y N st

Coarse-to-fine Find the last element of a seg. sted Scan to find ed init ed = st Is buffer[st] a letter? Is buffer[st] a digit? Is buffer[st] an operator? Scan to find ed err_code = 0 YN Y N Y N Scan to find ed Is buffer[st] a ‘:’? Is buffer[st] a ‘[‘? Scan to find ed Y Y ed and err_code

Go even finer Scan to find ed buffer[st] = ‘[‘ ed init ed = st+1 Conditions for buffer[ed] ed ++ err_code = 0 break buffer[ed] != ‘]’

Put things together From coarse-to-fine –always draw flowcharts –keep all your flowchart Adding things little by little –don’t put together all at once Validate the new components –whenever you refine a block, always debug it and make sure the whole thing is correct –keep the structure of the program clear