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7/17/2018, , C/C++ Programming for Engineers:, Loops, John T. Bell, Department of Computer Science, University of Illinois, Chicago, , Review, What would be the best data type to use to record, the concentration of a solution in a chemistry, laboratory experiment?, A., B., C., D., E., , bool, char, double, int, long int, 2, , 1

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7/17/2018, , The Power of Loops, • Computers are dumb machines, that only do, what they are told., • Their power lies in that they follow instructions, very quickly, and don’t mind repeating the, same instruction millions of times per second., Hence the power of the loop., • The programmers job is then to tell the, computer how to loop, when to stop looping,, and what to do each pass through the loop., 3, , While Loops, • The simplest loops to understand are while, loops, which continue looping as long as some, condition remains true., • The condition is evaluated BEFORE each, iteration of the loop, and the loop is executed, only if the condition is true., • ( If the condition is initially false, then the loop, will execute zero times, i.e. not at all. ), 4, , 2

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7/17/2018, , While Loop Syntax, while( condition ) {, // code in body of loop, }, , condition False, ?, , • If braces are omitted, then a, single statement comprises, the body of the loop., , Following, code, , True, , body, , 5, , While Example, int guess = -1, answer = rand( ) % 10 + 1;, Must be initialized to ensure loop entry, , while( guess != answer ) {, cout << “Enter a guess from 1 to 10: “;, cin >> guess;, }, cout << “That’s right! Congratulations!\n”;, 6, , 3

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7/17/2018, , Looping With a Counter, • Although a while loop could be used with a, counter to loop a specified number of times,, that is not good practice., • The best approach when a loop must execute, a specified number of times is with a for loop,, which initializes the counter, tests it on each, iteration, and ( typically ) increments it on, each iteration., 7, , For Loop Syntax, for( init; condition; incr ) {, // code in body of loop, }, • If braces are omitted, then a, single statement comprises, the body of the loop., • Note “incr” always happens, after executing the body., , init, condition False, ?, True, , incr, , body, , Following, code, , 8, , 4

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7/17/2018, , The for incrementation component, most commonly uses auto-increment:, • “i++;”, as a stand-alone statement, is, equivalent to “i += 1;” or “i = i + 1;”, • “i--;”, as a stand-alone statement, is equivalent, to “i -= 1;” or “i = i - 1;”, • There is more to auto-increment ( and autodecrement ), which we cover elsewhere., , 9, , For Loop Example, cout << “ i, i^2, i^3” << endl;, int i;, for( i = 1; i <= 10; i++ ) {, cout << i << “ “ << i * i << “ “ << i * i * i;, cout << endl;, }, cout << “After the loop, i = “ << i << endl;, 10, , 5

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7/17/2018, , What will be printed after the loop ends?, for( i = 1; i <= 10; i++ ) {, // Assume loop body does not change i, }, cout << “After the loop, i = “ << i << endl;, A. 0, B. 1, C. 10, D. 11, E. Undefined. It may depend on the compiler., 11, , Do-While Loops, • Do-while loops are nearly identical to whiles., • The condition is evaluated AFTER each, iteration of the loop, and the loop is repeated, only if the condition is true., • A do-while loop will always execute at least, once. After that it functions identically to a, while loop., , 12, , 6

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7/17/2018, , Do-While Loop Syntax, do {, // code in body of loop, } while( condition );, • Note required semi-colon, • If braces are omitted, then a, single statement comprises, the body of the loop. ( Very, rarely omitted. ), , body, condition, ?, , False, , True, , Following, code, , 13, , Do-While Example, int guess, answer = rand( ) % 10 + 1;, No initialization needed, , do {, cout << “Enter a guess from 1 to 10: “;, cin >> guess;, } while( guess != answer ) ;, cout << “That’s right! Congratulations!\n”;, 14, , 7

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7/17/2018, , Break and Continue, • “break” causes a loop to finish immediately,, continuing execution with the code following, the loop body. ( Also used in switches. ), • “continue” causes the current iteration of a, loop to finish, starting the next iteration., – While or do-while loops will jump to the, evaluation of the loop condition., – For loops will execute the “incrementation”, and, then jump to the evaluation of the loop condition., 15, , For Loop Flowchart Illustrating, break and continue, for( init; condition; incr ) {, // code 1, if( test_C ), continue;, // code 2, if( test_B ), break;, // code 3, }, , init, False, , condition?, code 1, incr, , True, , test_C, , code 2, test_B, , True, , code 3, Following code, 16, , 8

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7/17/2018, , Input Checking With a While Loop, int age = -1; // Initialized to guarantee loop entry, while( age < 0 || age > 120 ) {, cout << “Please enter your age: “;, cin >> age;, if( age < 0 || age > 120 ), cout << age << “ is invalid. Try again.\n”;, } // Continue while input is bad, 17, , Input Checking With a Do-While Loop, int age; // No initialization needed, do { cout << “Please enter your age: “;, cin >> age;, if( age < 0 || age > 120 ), cout << age << “ is invalid. Try again.\n”;, } while( age < 0 || age > 120 ); //Continue while bad, , 18, , 9

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7/17/2018, , Input Checking With an Infinite Loop, int age; // No initialization needed, while( true ) {, cout << “Please enter your age: “;, cin >> age;, if( age > 0 && age <= 120 ) // Exit loop if good, break;, cout << age << “ is invalid. Try again.\n”;, } // Loops infinitely while input is bad, 19, , Review, In the following statement, which of the following is the, correct order in which the operators will be evaluated?, A *= B + C / D – E++;, A., *=, +, /, -, ++, B., /, +, -, *=, ++, C., ++, /, +, -, *=, D., ++, +, /, -, *=, E., None of the above. The correct order of, operations is not listed here., , 20, , 10

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7/17/2018, , When to Use What Kind of Loop, • If you can count ( or calculate ) how many, times the loop must execute, use a for loop,, always with an INTEGER counter., • Else if you need to ensure the loop executes at, least once, ( and can’t rig a while to do so ),, use a do-while loop. ( A comment at the top, of the loop improves readability. ), • Else use a while loop., 21, , Review, What type of loop is guaranteed to execute at least, once, and then continue to repeat until a condition, becomes false?, A., B., C., D., E., , for, until, do until, while, do while, 22, , 11

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7/17/2018, , Nested Loops, • The body of a loop can contain any valid code,, including other loops, termed nested loops., – The internal loop does not have to be the same type, as the enclosing outer loop., – A nested “for” loop is a common way to iterate over, two variables and/or produce a table:, for( i = 0; i < iMax; i++ ) {, for( j = 0; j < jMax; j++ ), cout << i * j << “ “;, cout << endl;, , // Prints numbers on a line, // New line when j loop exits., , }, 23, , Empty Loops, • Loops can also be empty., – This is very rarely done intentionally., – Comments and braces are needed if this is done, on purpose., – Error example: An infinite empty loop:, int i = 0;, while( i < 5 ) ;, i = i + 1;, , Error here. The, body of the loop, is between the, closing ) and the ;, , “i = i + 1;” is after, the loop, not, inside it, and will, never be executed., , 24, , 12

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7/17/2018, , Always Use an INTEGER Loop Counter, int i, nLoops; // ( Assume all vars are given values. ), double x, xMin, xMax, delataX;, // WRONG:, for( x = xMin; x <= xMax; x += deltaX ) { . . ., // RIGHT:, for( i = 0; i < nLoops; i++ ) {, x = xMin + i * deltaX;, ..., 25, , Review, What value will be stored by the following C++, code:, double answer = 3 + 5 / 2 * 4 / 5.0;, A. 0.2, B. 3.2, C. 4.0, D. 4.6, E., 5.0, 26, , 13

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7/17/2018, , Review, Given the following code, what will be stored in k?, int i = 5, j = 4, k = 3;, k /= i++ % --j;, A., B., C., D., E., , 0, 1, 1.5, 3, infinity ( divide by zero error. ), 27, , Infinite Loop Knock Knock Joke, Knock knock, , Who’s there?, , Knock, , Knock who?, , 28, , 14