CMPT 102 Introduction to Scientific Computer Programming

Transcription

1 CMPT 102 Introduction to Scientific Computer Programming Control Structures for Loops Janice Regan, CMPT 102, Sept 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) Janice Regan, CMPT 102, Sept

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 Janice Regan, CMPT 102, Sept 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 Janice Regan, CMPT 102, Sept

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

4 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); Janice Regan, CMPT 102, Sept 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); Janice Regan, CMPT 102, Sept

5 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 Janice Regan, CMPT 102, Sept 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 Janice Regan, CMPT 102, Sept

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

7 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); Janice Regan, CMPT 102, Sept Endfile controlled Loop Initial statement int nscan: nscan = scanf( &lf, v1); Loop condition nscan == EOF T Statement 1; Statement m; F nscan = scanf( &lf, v1); Update statement Janice Regan, CMPT 102, Sept

8 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); Janice Regan, CMPT 102, Sept Flag controlled Loop Initial statement int nscan: nscan = scanf( &lf, &lf, &lf, v1, v2, v3); Loop condition nscan <= 3 T Statement 1; Statement m; F nscan = scanf( &lf, &lf, &lf, v1, v2, v3); Update statement Janice Regan, CMPT 102, Sept

9 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); Janice Regan, CMPT 102, Sept 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); Janice Regan, CMPT 102, Sept

10 Input validation Loop Initial statement int nscan: nscan = scanf( &lf, v1); Loop condition If v1 is valid F T printf ( invalid input, please reenter v1 ); nscan = scanf( &lf, v1); Update statement Janice Regan, CMPT 102, Sept 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 Janice Regan, CMPT 102, Sept