1. © Janice Regan, CMPT 102, Sept. 2006 0 CMPT 102 Introduction to Scientific Computer Programming Control Structures for Loops

2. © Janice Regan, CMPT 102, Sept. 2006 1 Control Structures  Three methods of processing a program  In sequence  Branching  Looping  Branch: Altering the flow of program execution by making a selection or choice  Loop: Altering the flow of program execution by repetition of a particular block of statement(s)

3. © Janice Regan, CMPT 102, Sept. 2006 2 Basic Loops  When one action is to be repeated a number of times a loop is used. Loops are repetition structures  There are two common types of loops  while loop or do...while loop Used to continue repetition while a condition holds Can also be used (along with a counter variable) to repeat a particular number of times  for loop Specifically designed to repeat a particular number of times

4. © Janice Regan, CMPT 102, Sept. 2006 3 What kind of loop  When you know how many time you want to execute the block of code use a counting loop  A counting while loop, you set up increment and test your counter  A for loop, give the critical values the for loop sets up, increments and tests for you  When you have no idea how many times the loop will be executed  Loop will stop executing when some condition becomes true Use a condition based while loop

5. © Janice Regan, CMPT 102, Sept. 2006 4 for statement T loopIndex < end loopIndex += step; loopIndex = start; Initial statement Loop condition First Statement: ….. Last Statement; F Repeats Statements m-n times Update statement start = n end = m

6. © Janice Regan, CMPT 102, Sept. 2006 5 for Loop in C for (initial statement; loop condition; update statement) { /*Series of actions to be taken */ /*each time the loop is executed*/ action 1; action 2; }

7. © Janice Regan, CMPT 102, Sept. 2006 6 Example for Loop in C SumOfInt = 0; for (index=0 ; index <= 10 ; index++) { /*Loop reads 11 integers and */ /*determines the sum of those integers*/ printf("enter an integer "); scanf( "%d", &newnum); printf( "%d\n", newnum); SumOfInt += newnum; } /* print the value of the sum to the screen */ printf( "%d", SumOfInt);

8. © Janice Regan, CMPT 102, Sept. 2006 7 Example while Loop in C sumOfInt = 0; index = 0; while( index <= 10 ) { /*Loop reads 11 integers and */ /*determines the sum of those integers*/ printf("enter an integer "); scanf( "%d", &newnum); printf( "%d\n", newnum); sumOfInt += newnum; index++; } /* print the value of the sum to the screen */ printf( "%d", sumOfInt);

9. © Janice Regan, CMPT 102, Sept. 2006 8 Condition controlled loops  Types of conditions for controlling while loops  General condition controlled  Sentinel controlled  Flag controlled  Endfile controlled (special sentinel/flag value)  Input validation loop

10. © Janice Regan, CMPT 102, Sept. 2006 9 Sentinel Controlled loops  A sentinel value is a special value of a particular variable known as a sentinel variable.  The sentinel value of a sentinel variable must be a value of the variable that is not encountered in normal operation of the loop  When the sentinel variable has the sentinel value at the end of the statements in the loop, execution will pass to the next statement following the loop  Often used for reading unknown amounts of input data

11. © Janice Regan, CMPT 102, Sept. 2006 10 Sentinel Controlled loop while ( sentinel variable != sentinel value ) { /*Series of actions to be taken each time*/ /*the loop is executed */ /*one of these actions will change the */ /*value of the sentinel variable */ action 1; action m changes the value of sentinel variable; action n; }

12. © Janice Regan, CMPT 102, Sept. 2006 11 Sentinel controlled Loop T Sentinelvariable == Sentinelvalue Sentinelvariable =Start; Statement 1; ….. Statement n changes value of Sentinel value; …… Statement m; Initial statement Loop condition F

13. © Janice Regan, CMPT 102, Sept. 2006 12 Sentinel Controlled loop /* The user inputs a positive integer the routine prints the square of the number, */ /* to terminate the user enters -999 */ printf("enter a positive integer" ); scanf("%d", &positiveIntNum); while ( positiveIntNum != -999 ) { if(positiveIntNum < 0) { printf("error*** negative input ***\n"); } else { printf("%d\n", positiveIntNum*positiveIntNum); } printf("enter a positive integer" ); scanf("%d", &positiveIntNum); }

14. © Janice Regan, CMPT 102, Sept. 2006 13 Endfile controlled Loop T nscan == EOF int nscan: … nscan = scanf(“&lf”, v1); Initial statement Loop condition F Statement 1; … Statement m; Update statement

15. © Janice Regan, CMPT 102, Sept. 2006 14 Endfile controlled Loop /* Data is read from a file, one line (three values at a time */ /* Each time a line is read the cost is updated */ /* After all data is read the cost is printed to the screen */ FILE * infile; double v1,v2, v3, cost; int nscan; fopen(“filename”,”r”); /* should check if file has been opened */ /*after reading nscan holds the number of items read, or an error condition */ nscan = fscanf(infile, “&lf”, v1); while ( nscan != EOF ) { cost += v1; nscan = fscanf(“&lf”, v1); } fclose(infile); printf( “Total cost is %f”, cost);

16. © Janice Regan, CMPT 102, Sept. 2006 15 Flag controlled Loop T nscan <= 3 int nscan: … nscan = scanf(“&lf, &lf, &lf”, v1, v2, v3); Initial statement Loop condition F Statement 1; … Statement m; Update statement

17. © Janice Regan, CMPT 102, Sept. 2006 16 Flag controlled Loop /* Data is read from a file, one line (three values at a time */ /* Each time a line is read the cost is updated */ /* After all data is read the cost is printed to the screen */ FILE * infile; double v1,v2, v3, cost; int nscan; fopen(“filename”,”r”); /* should check if file has been opened */ /*after reading nscan holds the number of items read, or an error condition */ nscan = fscanf(infile, “&lf, &lf, &lf”, v1, v2, v3); while ( nscan == 3 ) { cost += (v2 + v1 / v3); nscan = fscanf(“&lf, &lf, &lf”, v1, v2, v3); } fclose(infile); printf( “Total cost is %f”, cost);

18. © Janice Regan, CMPT 102, Sept. 2006 17 Flag Controlled loop /* The user inputs a positive integer the routine prints the square of the number, */ /* to terminate the user enters -999 */ printf("enter a positive integer" ); scanf("%d", &positiveIntNum); while ( positiveIntNum != -999 ) { if(positiveIntNum < 0) { printf("error*** negative input ***\n"); } else { printf("%d\n", positiveIntNum*positiveIntNum); } printf("enter a positive integer" ); scanf("%d", &positiveIntNum); }

19. © Janice Regan, CMPT 102, Sept. 2006 18 Input validation Loop T If v1 is valid int nscan: … nscan = scanf(“&lf”, v1); Initial statement Loop condition F printf (“invalid input, please reenter v1”); Update statement

20. © Janice Regan, CMPT 102, Sept. 2006 19 Input validation loop  Use when requesting data from user  Do not use when reading from a file  When an error occurs reading a file a subsequent read will read the same incorrect data  You will create an infinite loop by using an input validation loop  When an error is detected reading from a file, reading must be terminated,  if more information from the file is required the program should be terminated with an error message