CMPT 102 Introduction to Scientific Computer Programming

Transcription

1 CMP 102 Introduction to Scientific Computer Programming Control Structures while Loops continue; and break; statements Janice Regan, CMP 102, Sept Control Structures hree 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, CMP 102, Sept

2 Basic Loops When one action is to be repeated a number of times a loop is used. Loops are repetition structures here 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, CMP 102, Sept A while Loop in C while ( condition ) /* Series of actions to be taken */ /* each time the loop is executed */ /* loop is executed when condition is rue */ action 1; action 2; /* When condition is false execute following actions */ actions; Janice Regan, CMP 102, Sept

3 Structure of while loop condition Statement 1; Statement n; Janice Regan, CMP 102, Sept Counting while statement Initial statements start = n end = m loopindex = start; Repeats Statements m-n times Loop condition loopindex < end loopindex += step; Update statement irst Statement: Last Statement; Janice Regan, CMP 102, Sept

4 Example while Loop in C /* Sum the Integers from 1 to 25 inclusive */ X = 1; Sum = 0; while ( X <= 25 ) Sum += X; X++; printf( %d\n, Sum); Janice Regan, CMP 102, Sept do while Loop in C do /*Series of actions to be taken */ /*each time the loop is executed */ /*Always executed on first pass */ /* Subsequently executed if condition is rue */ action 1; action 2; while ( condition ); /* When condition is false execute following actions */ actions; Janice Regan, CMP 102, Sept

5 Structure of do while Loop Statement 1; Statement n; condition Loop condition Janice Regan, CMP 102, Sept Counting do while statement Initial statement start = n end = m loopindex = start; Repeats Statements m-n times irst Statement: Last Statement; Update statement loopindex += step; loopindex < end Loop condition Janice Regan, CMP 102, Sept

6 Example do while Loop /* Sum the Integers from 1 to 25 inclusive */ X = 1; sum = 0; do sum += X; X++; while ( X <= 25 ); printf( %d, sum); Janice Regan, CMP 102, Sept Differences do vs do while /* Sum Integers from n to m */ n = 4: m = 3: sum = 0; do sum += n; n++; while ( n <= m ); printf( %d, sum); After code sum = 4 Body of loop executes once /* Sum Integers from n to m */ n = 4: m = 3: sum = 0; while ( n <= m ) sum += n; n++; printf( %d, sum); After code sum = 0 Body of loop does not execute Janice Regan, CMP 102, Sept

7 Infinite Loops When using while or do..while loops it is possible that the loop condition is always true. If the variable or expression tested in the loop condition is not modified in the action statements within the loop, then if the condition is true for the first loop test it remains true for ever Even if the variable or expression changes it may satisfy the test condition for ever An infinite loop will cause your program to execute forever If your program is caught in an infinite loop you can terminate it by typing <CNRL>C into your linux or cygwin command window Janice Regan, CMP 102, Sept Uses of an infinite loop o make an infinite loop useful, you must have a way to exit from it It is possible to exit from a loop by using a break; statement (discussed below) If you use an infinite loop intentionally it is usually a loop you intend to execute many times until Some condition is met OR he loop has executed the maximum number of times If you use an infinite loop intentionally you should always exit after a maximum number of times through the loop, just in case the condition you expect to take you out of the loop never occurs. Janice Regan, CMP 102, Sept

8 he Break Statement: Loops Sometimes you wish to exit from a loop even if the loop condition is still true or example if your loop is reading and processing data and an error occurs that can be recognized but cannot be corrected, you may wish to print an error message and exit the loop immediately break; A C statement that causes you to exit the loop you are in Program execution will continue at the statement following the loop Janice Regan, CMP 102, Sept break statement Loop condition Condition Statement 1; condition2 Statement n; break; Janice Regan, CMP 102, Sept

9 he Continue Statement: Sometimes you wish to execute only some of the statements in the body of the loop for a particular value of the loop index or example you may wish to skip the last half of the body of the loop if (loopindex/14)= = 1 continue; A C statement that causes you to go from anywhere in the loop to the update statement and loop condition Program execution will continue at the update statement of the loop Janice Regan, CMP 102, Sept continue statement Condition Statement 1; condition2 Statement n; continue; Janice Regan, CMP 102, Sept

10 continue in Counting while Update statement Start = n Initial statement End = m loopindex = start; Loop condition loopindex += step; loopindex < end irst Statement: condition2 Last Statement; Repeats Statements m-n times continue; Janice Regan, CMP 102, Sept Example infinite Loop in C /* he user inputs a positive integer */ /* the routine prints the square of the number, */ /* to terminate the user enters -999 */ while ( 1 ) printf("enter a positive integer " ); scanf("%d", &positiveintnum); if(positiveintnum < 0) if( positiveintnum == -999) break; printf("error*** negative input ***\n"); continue; printf("%d\n", positiveintnum*positiveintnum); Janice Regan, CMP 102, Sept