CS 201 Repetition loops Types Of Loops n

CS 201 - Repetition loops

Types Of Loops n n n Counting Loop (while, for) Sentinel-Controlled Loop (while, for) Endfile-Controlled Loop (while, for) Input Validation Loop (do-while) General Conditional Loop (while, for)

While Loop (same as Java) n n n while (condition) { Do Something } When condition is true, Do Something and return to while. When condition is false, skip and exit loop. i = 0; while ( i<10 ) { printf(“%d “, i); i=i+1; }

0 1 2 3 4 5 6 7 8 9

For Loop (almost like Java) n n n No internal type statement allowed! for (initialization; test; update) { Do Something } for(i=0; i<10; i=i+1) { printf(“%dn”, i); } (same output) for (int j=0; j<10; j=j+1) { … } NOT ALLOWED IN C !

Interpretation n n First the initialization expression(s) is(are) executed. Then the test expression(s) is(are) evaluated. n n n If true, statements are executed, update expression(s) is(are) executed, and test is evaluated again. If false, loop exits. Last expression controls multiple expressions.

What Prints? int i=0, j=0; for (i=0, j=-2; i<8, j<2; i=i+1, j=j+2) printf(“%d %dn”, i, j);

What Prints? int i=0, j=0; for (i=0, j=-2; i<8, j<2; i=i+1, j=j+2) printf(“%d %dn”, i, j); 0 -2 10

Do While Loop (just like Java) n n n Do {something} while ( expression); Always does “something” at least once! Test expression. n n If true, return and to something again. If false, exit loop at that point.

What prints? int i=0; do { i=i+3; printf(“%dn”, i); } while (i<12);

What prints? int i=1; 4 do 7 { 10 i=i+3; printf(“%dn”, i); } while (i<12); 13

Compound Assignment n n n n Like Java. x += 3; is the same as x = x + 3; In general: var op = expression; Is the same as var = var op expression; += -= *= /= %=

Increment and Decrement Operators n n n Like Java Prefix and postfix versions ++i i++; is the same as i = i + 1; j--; is the same as j = j – 1; However k=++i; is different than k=i++;

Prefix increment n What prints? int i=0, j=0; printf(“%dn”, i++); printf(%dn”, ++i); j = i++; printf(“%dn, j);

Prefix increment n What prints? int i=0, j=0; printf(“%dn”, i++); printf(%dn”, ++i); j = i++; printf(“%dn, j); 0 2 2 What’s the value of i here?

Prefix increment n What prints? int i=0, j=0; printf(“%dn”, i++); printf(%dn”, ++i); j = i++; printf(“%dn, j); 0 2 2 What’s the value of i here? 3

Sentinel-Controlled Loops that run as long as needed. Signal the end of the loop with a special value (negative for ages, etc. ) n n Get a line of data While the sentinel value has not been encountered n n Process the data line. Get another line of data.

What prints? int i, sum=0; scanf(“%d”, &i); while(i!=-9) { sum+=i; scanf(“%d”, &i); } printf(“%dn”, sum); /* input = 1 3 5 7 -9 */

What prints? int i, sum=0; scanf(“%d”, &i); while(i!=-9) { sum+=i; scanf(“%d”, &i); } printf(“%dn”, sum); /* input = 1 3 5 7 -9 */ 16

What prints? int i, sum=0; scanf(“%d”, &i); while(i!=-9) { sum+=i; scanf(“%d”, &i); } printf(“%dn”, sum); /* input = -9 */

What prints? int i, sum=0; scanf(“%d”, &i); while(i!=-9) { sum+=i; scanf(“%d”, &i); } printf(“%dn”, sum); /* input = -9 */ 0

EOF n n Many library functions return helpful data as the value of the function? For example, scanf returns the number of successful conversions. File functions can return a value equal to EOF (a special defined variable). You can send this to scanf with ctrl-D (linux)

int i, sum=0, status=0; What prints? status = scanf(“%d”, &i); while(status!=EOF) { sum+=i; status = scanf(“%d”, &i); } printf(“%dn”, sum); /* input = */ 2 4 6 ctl-d

int i, sum=0, status=0; What prints? status = scanf(“%d”, &i); while(status!=EOF) { sum+=i; status = scanf(“%d”, &i); } printf(“%dn”, sum); /* input = */ 2 4 6 ctl-d 12

Nested Loops n n Usually used to work with two dimensional arrays (later). Simply put the definition for one loop in the body of another loop.

What Prints? int a=0, b=0, sum=0; for(a=0; a<6; a+=2) for(b=0; b>4; b--) sum=sum+1; printf(“%d”, sum);

What Prints? int a=0, b=0, sum=0; for(a=0; a<6; a+=2) for(b=0; b>-4; b--) sum=sum+1; printf(“%d”, sum);

Debugging n n Use a source level debugger as part of your IDE. Use a command line debugger like gdb. Add printf statements to trace execution. (Be careful when removing the printf statements that you don’t introduce errors. )

∞ Loops n It’s hard to distinguish between a complex calculation loop and an infinite loop without debugging statements.