CS31 Discussion 1E Spring 17: week 06 TA: Bo-Jhang Ho - PowerPoint PPT Presentation

CS31 Discussion 1E Spring 17’: week 06 TA: Bo-Jhang Ho bojhang@cs.ucla.edu Credit to former TA Chelsea Ju

Puzzle time: How many lines will this program print? #include <iostream> int main() { for (unsigned int i = 3; i >= 0; i--) cout << "Print one line" << endl; return 0; }

Puzzle time: How many lines will this program print? #include <iostream> int main() { for (unsigned int i = 3; i >= 0; i--) cout << "Print one line" << endl; return 0; } Print lines like crazy!!!!

Overflow issue

Puzzle time: How many lines will this program print? #include <iostream> int main() { for (unsigned int i = 3; i >= 0; i--) cout << "Print one line" << endl; return 0; } } Basic data types Type Bytes Bits Value range int 4 32 -2,147,483,648 to 2,147,483,647 unsigned int 4 32 0 to 4,294,967,295 double 8 64 -1.7 * 10^308 to 1.7 * 10^308

Puzzle time: How many lines will this program print? #include <iostream> int main() { for (unsigned int i = 3; i >= 0; i--) cout << "Print one line" << endl; return 0; } } Basic data types Type Bytes Bits Value range int 4 32 -2,147,483,648 to 2,147,483,647 unsigned int 4 32 0 to 4,294,967,295 double 8 64 -1.7 * 10^308 to 1.7 * 10^308 } Discussion: Will double variable overflow?

Today’s Agenda } Overflow } Good coding style part 3 } Project 4 discussion } C-style string } Hints for learning pointer (spoiler alert)

Project 3 call graph - planed main() executeCommands() C F@ B@ H00 V00 plotLine() draw() clearGrid() setChar() getChar()

Project 3 call graph of the sample solution main() executeCommands() executeOneCommand() C F@ B@ H00 V00 consumeNumber() plotLine() draw() clearGrid() setChar() getChar()

Good coding style guideline } (2) Indentation } One more tab inside a { } block } (6) If a function receives invalid parameters, reject the request ASAP! } And by rejecting, I mean “return false.” } (7) Use single quote character when you refer to a certain character, don’t use numbers! } ‘a’ or 97 ?

(2) Indentation – printing rectangle int main() { for (int r = 1; r <= 3; r++) { for (int c = 1; c <= 4; c++) { cout << "*"; } cout << endl; } return 0; } } Execution result:

(2) Indentation – printing rectangle int main() { for (int r = 1; r <= 3; r++) { for (int c = 1; c <= 4; c++) { cout << "*"; } cout << endl; } return 0; }

(2) Indentation – printing rectangle int main() int main() { { for (int r = 1; r <= 3; r++) { for (int r = 1; r <= 3; r++) for (int c = 1; c <= 4; c++) { { cout << "*"; for (int c = 1; c <= 4; c++) } { cout << endl; cout << "*"; } } cout << endl; return 0; } } return 0; }

(2) Indentation – printing rectangle int main() { for (int r = 1; r <= 3; r++) { for (int c = 1; c <= 4; c++) { cout << "*"; } cout << endl; } return 0; }

(2) Indentation – printing rectangle int main() { for (int r = 1; r <= 3; r++) { for (int c = 1; c <= 4; c++) cout << "*"; cout << endl; } return 0; }

(6) Be a mean person: Reject invalid request ASAP! bool plotLine(int r, int c, int dir, int distance, char plotChar, int fgbg) { // Validate arguments if (r < 1 || r > NROWS || c < 1 || c > NCOLS) return false; switch (dir) { case HORIZ: if (c + distance < 1 || c + distance > NCOLS) return false; break; case VERT: if (r + distance < 1 || r + distance > NROWS) return false; break; default: return false; } if (fgbg != FG && fgbg != BG) return false; if (!isprint(plotChar)) return false; ...

(6) Be a mean person: Reject invalid request ASAP! bool hasCorrectSyntax(string cmd, int& pos) { pos = 0; while (pos != cmd.size()) { switch (toupper(cmd[pos])) { default: return false; case 'C': pos++; break; case 'F': case 'B': // must be followd by a printable character pos++; if (pos == cmd.size() || !isprint(cmd[pos])) return false; pos++; break; case 'H': case 'V': pos++; if (pos == cmd.size()) return false; ... return true; }

(6) Be a mean person: Reject invalid request ASAP! int executeCommands(string cmd, char& plotChar, int& mode, int& badPos) { int pos; if ( ! hasCorrectSyntax(cmd, pos)) { badPos = pos; return SYNTAX_ERROR; } int r = 1; int c = 1; pos = 0; // At the start of each loop iteration, pos is the position of the // start of the next plotting command within the command string, or // the end of that string. while (pos != cmd.size()) { if (!executeOneCommand(cmd, pos, r, c, plotChar, mode)) { badPos = pos; return EXECUTION_ERROR; } } return COMMAND_OK; }

(6) Be a mean person: Reject invalid request ASAP! } Discussion: What are the advantages? } Tell the readers that I’m not going to address this cases } Cross out these impossible cases to make life easier } Make the logic clear } You don’t need to worry about the correctness of parameters anymore

(7) Don’t replace single quote as numbers bool hasCorrectSyntax(string cmd, int& pos) { pos = 0; while (pos != cmd.size()) { switch (toupper(cmd[pos])) { default: return false; case 'C': pos++; break; case 'F': case 'B': // must be followd by a printable character pos++; if (pos == cmd.size() || !isprint(cmd[pos])) return false; pos++; break; case 'H': case 'V': pos++; if (pos == cmd.size()) return false;

(7) Don’t replace single quote as numbers bool hasCorrectSyntax(string cmd, int& pos) { pos = 0; while (pos != cmd.size()) { switch (toupper(cmd[pos])) { default: return false; case 67: pos++; break; case 70: case 66: // must be followd by a printable character pos++; if (pos == cmd.size() || !isprint(cmd[pos])) return false; pos++; break; case 72: case 86: pos++; if (pos == cmd.size()) return false;

Today’s Agenda } Overflow } Good coding style part 3 } Project 4 discussion } C-style string } Hints for learning pointer (spoiler alert)

Project 4 demystify } 11 functions: } int enumerate(const string a[], int n, string target); } int locate(const string a[], int n, string target); } bool locateSequence(const string a[], int n, string target, int& begin, int& end); } int locationOfMin(const string a[], int n); } int moveToEnd(string a[], int n, int pos); } int moveToBeginning(string a[], int n, int pos); } int locateDifference(const string a1[], int n1, const string a2[], int n2); } int eliminateDups(string a[], int n); } bool subsequence(const string a1[], int n1, const string a2[], int n2); } int makeMerger(const string a1[], int n1, const string a2[], int n2, string result[], int max); } int divide(string a[], int n, string divider);

Project 4 demystify } 11 functions: } int enumerate(const string a[], int n, string target); } int locate(const string a[], int n, string target); } bool locateSequence(const string a[], int n, string target, int& begin, int& end); } int locationOfMin(const string a[], int n); } int moveToEnd(string a[], int n, int pos); } int moveToBeginning(string a[], int n, int pos); } int locateDifference(const string a1[], int n1, const string a2[], int n2); } int eliminateDups(string a[], int n); } bool subsequence(const string a1[], int n1, } 2 Observations: const string a2[], int n2); } int makeMerger(const string a1[], int n1, } An array always follows in integer n, indicating the size const string a2[], int n2, string result[], int max); } A const modifier in front of an array } int divide(string a[], int n, string divider);

Project 4 demystify } Observation 1: Why passing array size to a function? } Can we not get the array size from C++? } In C++, there is no standard way to get array size! } Not like string, you can call .size() method

Project 4 demystify } Observation 1: Why passing array size to a function? } But, can we get the array size from C++? } In C++, there is no standard way to get array size! } Not like string, you can call .size() method } This thread talks about some hacky ways to get the size: http://stackoverflow.com/questions/4108313/how-do-i-find-the-length-of-an-array } That’s why we should have the agreement with function users that please pass the array as well as the array size } That being said, providing correct parameters is the user’s duty. You shouldn’t worry about invalid parameters } If some users pass bad parameters, it’s their fault!!

Project 4 demystify } Observation 2: Why const?

Pass an array to a function } From caller: int arr[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; foo(arr); } In the function: void foo(int params[10]) { … } or void foo(int params[]) { … } or void foo(int *params) { … }

Pass an array to a function void fillInArray(int params[]) { for (int i = 0; i < 5; i++) params[i] = i; } int main() { int arr[5] = {100, 99, 98, 97, 96}; fillInArray(arr); for (int i = 0; i < 5; i++) cout << "Element " << i << ": " << arr[i] << endl; return 0; } } What’s the output?