Computer Programming

Transcription

1 Computer Programming Dr. Deepak B Phatak Dr. Supratik Chakraborty Department of Computer Science and Engineering Session: for statement in C++ Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 1

2 Quick Recap of Relevant Topics Iteration idioms in programming Necessity and convenience of iteration while and do while statements in C++ 2 Dr. Deepak B. Phatak & Dr. Supratik Chakraborty,

3 Overview of This Lecture Iteration using for. statement in C++ break statement in for loops Comparison of different iteration constructs in C++ 3 Dr. Deepak B. Phatak & Dr. Supratik Chakraborty,

4 Recall Generic Iteration Construct Part of program before iteration Iteration initialization (setting up initial values, etc) Iterate/Repeat as long as a logical condition stays true { Block of statements Optional instructions to execute at end of every iteration Part of program after iteration Loop Body Loop Condition Loop Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 4

5 for Statement in C++ Semi-colons not to denote end of executable statements, but to separate three parts inside for (.. ) Part of program before iteration for (iteration initialization ; loop condition ; instructions to execute at end of every iteration) { Block of statements ( for loop body ) Part of program after iteration Note absence of semi-colon Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 5

6 Flowchart Representation of for Part of program before iteration Iteration Initialization Instructions at end of every iteration Logical expression (loop condition) TRUE for loop body FALSE Part of program after iteration Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 6

7 Points to Remember About for for (initialization; loop condition; instructions after every iteration) { for loop body Initialization code executed only once before first entry in loop Loop condition checked before executing for body Can lead to zero executions of for body Number of times loop condition is checked = Number of times for body executed + 1, if loop terminates Loop condition can be changed in for body or in instructions after every iteration Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 7

8 Revisiting The Quiz Marks Problem Read number of students in CS101, read quiz 1 marks of all CS101 students and print their sum, average, maximum and minimum Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 8

9 Flowchart Representation Read numstudents Initialize aggregates (sum, min, max) Initialize count to 1 Increment count count <= numstudents? FALSE Ask for marks Read marks Update aggregates TRUE Compute average Print aggregates Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 9

10 C++ Program with for int main() { int marks, sum = 0, min, max, numstudents; float average, count; // Variable declarations cout << Give number of students: ; cin >> numstudents; for (count = 0.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min average = sum/count; // Print average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 10

11 C++ Program with for int main() { int marks, sum = 0, min, max, numstudents; float average, count; // Variable declarations cout << Give number of students: ; cin >> numstudents; for (count = 0.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min average = sum/count; // Print average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 11

12 C++ Program with for int main() { int marks, sum = 0, min, max, numstudents; float average, count; // Variable declarations cout << Give number of students: ; cin >> numstudents; for (count = 1.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min average = sum/count; // Print average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 12

13 C++ Program with for int main() { int marks, Initialization sum = 0, min, codemax, numstudents; Loop condition Instruction to float average, count; // Variable declarations execute after cout << Give number of students: ; cin >> numstudents; every iteration for (count = 1.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min for loop body average = sum/count; // Print average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 13

14 C++ Program with for int main() { sum = sum + marks; int marks, sum = 0, min, max, numstudents; float average, count; // Variable declarations else { cout << Give number of students: ; cin >> numstudents; for (count = 1.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min average = sum/count; // Print average, sum, min, max return 0; if (count == 1) { min = marks; max = marks; min = (min > marks)? marks: min; max = (max < marks)? marks: max; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 14

15 C++ Program with for int main() { int marks, sum = 0, min, max, numstudents; float average, count; // Variable declarations cout << Give number of students: ; cin >> numstudents; for (count = 1.0; count <= numstudents; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; // Update sum, max, min average = sum/count; // Print average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 15

16 break Statement in for Loops Behaviour same as in while and do while loops Jump out of the loop immediately on executing break Recall our variant of the quiz 1 marks problem Read quiz 1 marks of CS101 students one at a time Stop reading if is entered as marks Print number of marks entered, sum, average, maximum and minimum Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 16

17 Our C++ Program with break int main() { int marks, sum = 0, min, max; float average, count; // Variable declarations Infinite loop!!! for (count = 1.0; true ; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; if (marks == -1000) { break; else { Update sum, min, max average = sum/(count 1); // Print count 1, average, sum, min, max return 0; Jump out of for loop on executing break Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 17

18 Our C++ Program with break int main() { int marks, sum = 0, min, max; float average, count; // Variable declarations for (count = 1.0; true ; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; if (marks == -1000) { break; else { Update sum, min, max average = sum/(count 1); // Print count 1, average, sum, min, max return 0; These instructions skipped after executing break Jump out of for loop on executing break Next statement executed after break Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 18

19 for Loops with Empty Parts int main() { Skipping loop condition in for int marks, sum = 0, min, max; equivalent to true loop condition float average, count; // Variable declarations for (count = 1.0; true ; count = count + 1) { cout << Give marks of student << count << : ; cin >> marks; if (marks == -1000) { break; else { Update sum, min, max average = sum/(count 1); // Print count 1, average, sum, min, max return 0; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 19

20 for Loops with Empty Parts ; for ( count = 1.0 ; true ; count = count + 11) { cout << Give marks of student << count << : ; cin >> marks; if (marks == -1000) { break; else { Update sum, min, max ; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 20

21 From for to while for (initialization; loop condition; instraftereveryiteration) { for Loop Body Initialization ; while (loop condition) { for Loop Body; instraftereveryiteration; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 21

22 From while to for for ( ; loop condition; ) { while Loop Body while (loop condition) { while Loop Body for to do while, and do while to for can be done by recalling the transformation of while to and from do while Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 22

23 for vs while for (count = 1.0; count <= numstudents; count = count + 1) { // Process marks Book-keeping cleanly isolated Real computation we want to do count = 1.0; while (count <= numstudents) { // Process marks count = count + 1; Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 23

24 for vs while for (count = 1.0; count <= numstudents; count = count + 1) { // Process marks count = 1.0; while (count <= numstudents) { // Process marks count = count + 1; Real computation we want to do Book-keeping mixed up Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 24

25 for vs while Often a programmer s choice dependent on the context In general, a good idea to separate book-keeping from real computation Prefer for loops However, loop condition may not simply be based on bookkeeping Real computation in loop body may determine loop condition Prefer while loops Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 25

26 Summary for statement in C++ Variants of for statement Use of break statement in for statements Comparison with while and do while statements Dr. Deepak B. Phatak & Dr. Supratik Chakraborty, 26