Chapter 4 Loops Write code that prints out the numbers Very often, we want to repeat a (group of) statement(s). In C++, we have 3 major ways of doing this: while loops, do-while loops, and for loops. while Loop - Syntax while (condition) { stmt;//Loop body } Symantics: 1. Evaluate the condition. 2. If condition is true execute body of loop go to step 1 3. If condition is false the loop terminates and the next statement after the while loop is executed

int a = 1; while (a <= 100) { cout << a << endl; a = a + 1; } //while cout << “Finished”; What will happen if the statement a = a + 1 is deleted? There are several different ways to increment a value. a = a + 1; a += 1; a++; ++a; For looping  a++;

Incrementing values num = num + 1; num++; //postincrement ++num; //preincrement See Page 48 for the difference between postincrement and preincrement. Decrementing values num = num – 1; num--; --num; Specialized Assignment equivalent statement num += 4;num = num + 4; num -= 4;num = num – 4; num *= 4;num = num * 4; num /= 4;num = num / 4; num %= 4;num = num % 4;

int p = 1; while (p < 50) { cout << p << endl; p = p * 2; } //while cout << “p = “ << p; Write a while loop to print out ALL the odd numbers from 15 to 1 Write a while loop that adds up the numbers from 1 to 100, inclusive, then prints out only the answer. int sum = 0; int count = 1; while (count <= 100) { sum = sum + count; count++; }// while cout << “Sum = “ << sum; cout << “Count = “ << count;

cond can be any legal Boolean expression variables in the cond must have a value prior to entering the loop BE SURE TO MODIFY THE LOOP CONTROL VALUE INSIDE THE LOOP BODY!!!! a = 1; while (a < 10) cout << a; cout << “done”; ============================== a = 1; while (a < 10) { cout << a; a--; } //while cout << “done”; Be sure that the loop control variable does eventually reach the terminating condition!!!

n = 1; m = 10; while (n < m) { cout << “m =“ << m << “ n =“ << n; m--; n++; } //while ================================ int votes = 0; bool go = true; while(go) { cout << "Enter number of votes: "; int n; cin >> n; go = (n > 0); votes = votes + n; } cout << "Total votes = " + votes;

Checking for valid input! cout << “Input a number between 1 and 10”; cin >> num; while ((num 10)) { cout << “Invalid. Try again”; cin >> num ; }//while cout << “Thank You!”;

do-while loop The do-while loop is very similar to the while loop, except that the test is made at the bottom of the loop. do { statement; } while (cond); nextStatement; Execute the statement/body. Check cond – if true, execute body and recheck otherwise, go on to nextStatement int i = 1; do { cout << “i = “ << i << endl; i++; } //do while (i < 10);

Checking for invalid input with a do-while do { cout << “Enter a number between 0 and 10”; cin >> num; if (num 10) cout << “Invalid Input << endl; } while (num 10); The loop will execute at least one time, but will continue to execute as long as the input does not meet the condition.

For Loops int i; for(i = 1; i <= 100; i = i + 1) cout << i; Syntax: for (init_action; condition; update_action) stmt; next_stmt; where stmt can be a compound statement. Symantics: 1. Execute initializing action. 2. Evaluate the condition 3. If the condition is true then execute body of loop; Else exit loop 4. Execute update action 5. Repeat from step 2.

int sum = 0; for (y=1; y <=10; y++) sum += y; cout << sum; =================================== int count = 0; for (int index = 3; index < 8; index++) { cout << index << “ “ << index * index; count++; } //for index cout << count; =================================== What will happen if we add the following statement at the end of the above code fragment? cout << index; =================================== cout << “before loop”; int x = 5; for (j=x; j<=4; j++) cout << j; cout << “After loop ”;

for (x = 1; x <= 5; x++) cout << “hello”; cout << “bye”; ==================================== int fact = 1; cout << “Enter a number”; cin >> n; for (f = 1; f <= n; f++) fact = fact * f; cout << fact; ==================================== /* BE CAREFUL */ for (x = 1; x <= 10; x++); cout << “x = “ << x << endl; ==================================== for (m = 10; m > 0; m--) cout << m;

Nested Loops for (a = 1; a < 4; a++) for (b = 1; b < 3; b++) cout << “a= “ << a << “ b= “ << b << endl; for (a = 1; a < 4; a++) for (b = 1; b < a; b++) cout << “a= “ << a << “ b= “ << b << endl; for (x = 1; x < 9; x += 2) { cout << “x = “ + x); for (y = x; y < 10; y += x) cout << “ y = “ << y << endl; } // for x

Write a loop that prints out the odd numbers between 1 and 30, inclusive. (Do it using all 3 constructs; while loop, do-while loop and for loop) total = 1; for (j=1; j <= 5; j++) { total *= 2; cout << j << “ “; cout << total; } //for Rewrite it using while. Rewrite it using do-while.

What are the differences among for loops, while loops and do-while loops? What’s the minimum number of times the body of each type of loop can be executed? SKIP the notes at the end of section 4.4 SKIP sections 4.8 and 4.9 Review Questions: