Unit 11 Nested Loops 2 What Can Go Inside? What kind of code can - PowerPoint PPT Presentation

1 Unit 11 Nested Loops

2 What Can Go Inside? • What kind of code can we put in the body of a loop? • ANYTHING…even other loops while ( condition ) for( init; condition; update ) { { // What can go here? // What can go here? } } init False condition False condition True True What code can we put in the body of a while Block For Block loop? Statements Statements Following Update statements Following

3 Nested Loops • Loops can contain other loops in their body False cond1 while (cond1) { True // code1 code1 while(cond2) { False // code 2 } cond2 // code3 True } code2 code3 Following statements

4 Nested Loop Sequencing • Key Idea : The inner loop runs in its entirety for each iteration of the outer loop False T T Cond1: F cond1 while (cond1) { 1 9 15 True 2 // code1 10 Cond2: T T F code1 while(cond2) { 5 3 7 13 11 Cond2: T F // code 2 12 4 6 False } cond2 // code3 8 14 True } code2 16 code3 Following statements

5 Nested Loops Example 1 • When you write loops int main() { consider what the body int secret, guess; char again = 'y'; of each loop means in // outer loop while(again == 'y') an abstract sense { // Choose secret num. 0-19 secret = rand() % 20; – The body of the outer guess = -1; // inner loop loop represents 1 game while(guess != secret) 1 game (and we repeat that { 1 turn cout << "Enter guess: "; over and over) cin >> guess; } – The body of the inner cout << "Win!" << endl; cout << "Play again (y/n): "; loop represents 1 turn cin >> again; (and we repeat turn } return 0; after turn) }

6 Nested Loops Example 2 • Key idea : Perform all int main() { iterations of the inner loop for(int i=0; i < 2; i++){ for(int j=0; j < 3; j++){ before starting the next cout << i << " " << j << endl; } iteration of the outer loop } } – Said another way: The inner loop executes completely for i j each single iteration of the outer loop • Trace through the execution of this code and show what will be printed

7 Nested Loops Example 3 • Trace through the execution int main() { of this code and show what int x = 0; cin >> x; will be printed if the user while( x%2 == 0 ){ for(int i=x; i >= 0; i -= 2){ types in: 8 4 7 6 cout << i << " "; } cout << endl; cin >> x; } cout << "Done" << endl; return 0; } Program Output:

8 break Statement with Nested Loops • break will only exit the inner- char again = 'y'; while(again == 'y' ) most loop, not all the nested { /* Give the user 10 turns loops. but stop if guess right */ • This can be exactly what you int i, guess, secretNum = /*..*/ for(i=0; i < 10; i++) want in some cases { cin >> guess; • In other cases, you may want to if(guess == secretNum){ break ; break out of all loops, but } realize a single 'break' } if( i == 10 ) statement cannot do that. cout << "You lose!" << endl; else – Instead must change a variable so cout << "You win!" << endl; that the outer loop condition will cin >> again; fail }

9 Tips • Nested loops often help us represent and process multi-dimensional data – 2 loops allow us to process data that corresponds to 2 dimension (i.e. rows/columns) – 3 loops allow us to process data that corresponds to 3 dimensions (i.e. rows/columns/planes)

10 More Practice • cpp/nestedloops/rectangle • cpp/nestedloops/flag • cpp/nestedloops/etox-range • cpp/nestedloops/sphere

11 SOLUTIONS

12 Nested Loops Example 2 • Trace through the execution int main() { of this code and show what for(int i=0; i < 2; i++){ for(int j=0; j < 3; j++){ will be printed cout << i << " " << j << endl; } } } i j 0 0 1 2 Program Output: 1 0 0 0 1 0 1 2 0 2 1 0 1 1 1 2

13 Nested Loops Example 3 • Trace through the execution int main() { of this code and show what int x = 0; cin >> x; will be printed if the user while(x%2 == 0){ for(int i=x; i >= 0; i -= 2){ types in: 8 4 7 6 cout << i << " "; } cout << endl; cin >> x; } cout << "Done" << endl; return 0; } Program Output: 8 6 4 2 0 4 2 0 Done