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Program Control

Program Control. Selection Repetition Altering the flow of control Unconditional branch More operators RSI – Repetitive Stress Injury. Selection – 3 Ways. If structure (single selection) If/else structure (double, multiple selection) if(condition1) statement1;

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Program Control

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  1. Program Control • Selection • Repetition • Altering the flow of control • Unconditional branch • More operators • RSI – Repetitive Stress Injury

  2. Selection – 3 Ways • If structure (single selection) • If/else structure (double, multiple selection) if(condition1) statement1; else if(condition2) statement2; . . . . . .; else statementN; • Switch structure (multiple selection)

  3. If Structure • If no parenthesis, then only the 1st statement following the if statement is associated with it • if(a>b) c=3; d=4; • So d=4; will always be evaluated

  4. If/else Ambiguity • If one if statement doesn’t have an ending else & another one does, may be ambiguous • Resolved by associating the else with the closest if if(a>b) if(c>d) e=3; else d=4; • Must use braces to associate with the outside if if(a>b){ if(c>d) e=3; }else d=4;

  5. Switch Structure • Handles a series of decisions • Must include a break statement • If not, each case statement will be executed until encounter another break statement or reach end • Several cases can execute the same statements • By listing case statements together • Can only test constant integral expressions

  6. /* Counting A,B,C or a,b,c (counting.txt)*/ #include <stdio.h> int main(){ int a=0,b=0,c=0,e=0,ch=0; while((ch=getchar()) != EOF){ switch(ch){ case 'A': case 'a': a++; break; case 'B': case 'b': b++; break; case 'C': case 'c': c++; break; default: e++; break;/*defensive programming*/ } } printf("\na=%d\nb=%d\nc=%d\nextra=%d",a,b,c,e); return 0; }

  7. Character I/O • Text input or output = stream of characters • Standard library functions for char I/O char c; c = getchar(); /*input*/ putchar(c); /*output*/ • Will get or put one character at a time from the input or output text stream

  8. End-of-File Marker • EOF defined in ANSI standard as a negative integer • EOF key combinations on different systems: • UNIX systems: ctrl-d • IBM PC and compatibles: ctrl-z • Macintosh: ctrl-d

  9. Repetition – 3 Ways • while (condition) {statements;} • while(1+2*5){ printf(“infinite loop ”); } • do{statements} while(condition) • do{printf(“one”); } while(1 < 0); • for(initialize, condition, increment) • for(i=0; i<5; i=i+1) {printf(“five”);} • Note:For conditions in C, • false = 0 & true = nonzero (anything else)

  10. for = = while • for(expr1;expr2;expr3) statement; • Is equivalent to: • expr1; while(expr2){ statement; expr3; } • This will create an infinite loop: • for(;;){ . . .}

  11. Comma in For Loops • Can put commas in for loops • Evaluated left to right #include <stdio.h> int main() { int a=0,b=0; for(a=0, b=10; a<b; a++, b--) printf("a=%d b=%d\n",a,b); return 0; }/*see comma.txt*/

  12. Alter Flow of Control • break; • Immediately exit from while, for, do/while, or switch statements • continue; • Skips the remaining statements • Performs the next iteration of while, for, or do/while loop

  13. Unconditional Branch #include <stdio.h> int main(){ start: printf("hello"); gotostart; return 0; } /*see goto.txt*/

  14. Goto • As a general rule, not a good idea to use goto • Might use when need to escape from a deeply nested structure (since break will only exit from innermost loop) for(. . .) { for(. . .){ if(need_to_stop) goto stop_loop; }} stop_loop: Continue with the program. . .

  15. ASCII Character Codes • '0' 48 dec 0x30 0011 0000 • '9' 57 dec 0x39 0011 1001 • 'A' 65 dec 0x41 0100 0001 • 'Z' 90 dec 0x5A 0101 1010 • 'a' 97 dec 0x61 0110 0001 • 'z' 122 dec 0x7A 0111 1010

  16. Formatted Output • Conversion character tables • Chapter 9, Fig. 9.1, 9.3, 9.6 • %c (char), %d (dec), %x (hex), %f (float) char c = 'A'; int i = 66; printf("c=%c i=%c\n", c, i); //c=A i=B printf("c=%d i=%d\n", c, i); //c=65 i=66 printf("c=%x i=%x\n", c, i); //c=41 i=42 • What’s the output if we set i = 66 + 1024? • See format.c

  17. Formatted Output i=50 i= 50 i=0.000000 f= 5.368900 f=5.369 f= 5.369 int i = 50; float f = 5.3689; printf("i=%d\n", i); printf("i=%4d\n", i); printf("i=%f\n", i); printf("f=%10f\n", f); printf("f=%.3f\n", f); printf("f=%10.3f\n", f);

  18. Class Exercise 1 • See exercise1.txt

  19. Relational & Logical Operators • Relational: >, >=, <, <=, ==, != • Logical: &&, ||, ! • Evaluated left to right • Evaluation stops as soon as true or false is determined • Ex: if(a && b && c) • Will stop if “b && c” is false • Often see if(!x), which means if(x==0)

  20. Type Conversions • Implicit conversion – if an operation has operands of different types, the “narrower” one will be converted to the “wider” one • 2.0 / 5 evaluates to 0.4 (a float) • Explicit conversion – can force a conversion with a cast • (type-name) expression • ((int)2.0) / 5 evaluates to 0 (an integer) • a = sqrt((double) x); (a = double, x = integer)

  21. Increment & Decrement Oper. • Preincrement • Assign old value, then add 1 • Postincrement • Add 1, then assign new value • For example, if a = 3 b = ++a; /*(b=4, a=4)*/ b = a++; /*(b=4, a=4)*/ /*(b=4, a=5)*/

  22. Assignment Operators x = x + 5; • Can also be written as x += 5; a *= b + 5; • Evaluates to a = a *(b + 5); • And not a = a * b + 5;

  23. Conditional Expressions if (x>y) a=0; else a =1; • Can also be written as x>y ? a = 0 : a = 1;

  24. Precedence • Precedence & of operators • See table in Dietel & Dietel, Appendix C

  25. Class Exercise 2 • See exercise2.txt

  26. Class Exercise 3 • See exercise3.txt

  27. Class Exercise 4 • Do exercise 4.26 from Deitel & Deitel textbook

  28. Repetitive Strain Injury (RSI) • Risk factors • Over 2 hours of keyboard use a day • Marathon keyboard use (deadlines, video games) • High stress work • Sitting for a long time • Poor posture • Symptoms (in neck, shoulders, back, arms, or hands) • Tightness • General soreness • Dull ache • Numbness • Loss of strength

  29. Repetitive Strain Injury (RSI) • How to avoid • Avoid long sessions & take frequent breaks • Take “micro-break” (shift position, stretch) • Proper position (back straight, head over shoulders, elbows at right angle, wrists straight, fingers curved) • Type lightly • Get plenty of rest & sleep • Web site:http://www.rsihelp.com/

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