Review and Read: Textbook Chapters 1-6, Lectures 11-24, General Programming Tips

1. Review • Read textbook chapters 1 - 6 • Read Lectures 11 - 24 • General Programming Tips: • use Top-Down Design • use Incremental Programming • use Functions to simplify code and create modules

2. Using Variables • You may declare variables in C using the data type you need - e.g.: • Integers (int, long int, short int) • Floating point (float, double) • Characters (char) • Examples of variable declarations: int meatballs ; • float area ;

3. Declaring Variables • When we declare a variable: • space in memory is set aside to hold that data type • That space is associated with the variable name • Visualization of the declaration • int meatballs ; meatballs FE07

4. Keywords in C • int long • register return • short signed • sizeof static • struct switch • typedef union • unsigned void • volatile while • auto break • case char • const continue • default do • double else • enum extern • float for • goto if

5. Arithmetic Operators • Name Operator Example • Addition + num1 + num2 • Subtraction - initial - spent • Multiplication * fathoms * 6 • Division / sum / count • Modulus % m % n

6. Modulus % • The expression m % n yields the remainder after m is divided by n. • Modulus is an integer operation. - Both operands MUST be integers. • Examples : 17 % 5 = 2 • 6 % 3 = 0 • 9 % 2 = 1 • 5 % 8 = 5

7. Integer Division • If both operands of a division expression are integers, you get an integer answer. The fractional portion is thrown away. • Examples : 17 / 5 = 3 • 4 / 3 = 1 • 35 / 9 = 3 • Division where one operand is a floating point number will produce a floating point answer. Automatic promotion.

8. Arithmetic Operators Rules of Operator Precedence • Operator(s) Precedence & Associativity • ( ) Evaluated first. If nested - innermost first. If on same level - left to right. • * / % Evaluated second. If many, they are evaluated left to right • + - Evaluated third. If there are several, evaluated left to right. • = Evaluated last, right to left.

9. Relational Operators • < less than • > greater than • < = less than or equal to • > = grater than or equal to • = = is equal to • != is not equal to • Relational expressions evaluate to the int • value 1 (True) or the int value 0 (False) • All of these operators are called binary operators because they take two expressions as operands

10. True or False • Arithmetic expressions also evaluate to true of false. • Any expression that has a zero value is false. ( 0 ) is False • Any expression that has a non-zero value is true. ( 1 ) is True

11. Structured Programming • All programs can be written in terms of only 3 control structures • The sequence structure • Unless otherwise directed, the statements are executed in the order in which they are written. • The selection structure • Used to choose among alternative courses of action • The repetitive structure • Allows that an action is to be repeated while some condition remains true.

12. A Selection Structurethe if statement • if ( condition is “true” ) • { • statement(s) • } • if ( value = = 0 ) • { • printf (“The value you entered was zero\n”); • }

13. Gotcha int a = 2; if (a = 1) { printf (“ a is one \n”); } else if (a == 2) { printf (“ a is two \n ”); } else { printf (“ The vaue of a is %d \n”, a); }

14. Example while loop • children = 10 ; • cookies = 1024 ; • while ( children > 0 ) • { • children = children - 1; • cookies = cookies / 2 ; • }

15. Using a Sentinel Value • We could let the user keep entering the grades and when he’s done enter some special value that signals us that he’s done. • This special signal value is called a sentinel value. • We have to make sure that the value we choose as the sentinel isn’t a legal grade. (ie. can’t use 0 as the sentinel )

16. The Priming Read • When we use a sentinel value to contol a while loop, we have to get the first value from the user before we encounter the loop so that it will be tested and the loop can be entered. • This is known as a priming read. • We have to give significant thought to the initialization of variables, the sentinel value and getting into the loop.

17. The cast operator ( ) • We can use a cast operator to create a temporary value of the desired type, to be used in a calculation. • Does NOT change the variable’s type or how it is stored. • Is only good for the statement it’s in. • Often used to avoid integer division. • Used anytime we want to temporarily change a type for a calculation.

18. Using a Sentinel (continued) • printf (“Enter grade, -1 to end : “); • scanf (“%d”, &grade); • while (grade != -1) • { • total = total + grade ; • counter = counter + 1 ; • printf (“Enter grade, -1 to end : “); • scanf (“%d”, &grade); • } • average = ( float ) total / counter ; • printf (“The average was %.2f\n”, average) ;

19. Increment and Decrement Operators • The Increment Operator ++ • The Decrement Operator -- • Precedence - lower than (), but higher than * / and % • Associativity - right to left • Increment and decrement operators can only be applied to variables, NOT to constants or expressions

20. The for loop Repetitive Structure • The for loop handles details of the counter-controlled loop automatically • The initialization of the the loop control variable, termination conditional test and modification are handled in for loop structure for ( i = 1; i < 11; i++) { initialization modification } test

21. A for loop that countsfrom 0 to 10 for (i = 0; i < 11 ; i++) { printf (“%d”, i); } printf (“\n”);

22. do-while loop example do { printf (“Enter a positive number: “); scanf (“%d”, &num); if (num < = 0) { printf (“ \n That is not positive, try again \n ”); } } while (num <= 0);

23. for vs while • use a for loop when your program “knows” exactly how many times to loop • use a while loop when there is a condition that will terminate your loop

24. Nested for loops for (i = 1; i < 5; i++) { for (j = 1; j < 3; j+) { if (j % 2 = = 0) { printf (“O”); } else { printf (“X”); } } printf (“\n”); } How many times is the ‘if’ statement executed? What is the output ??

25. The char data type • The char data type holds a single character char ch; • The char is held as a one-byte integer in memory. The ASCII code is what is actually stored, so we can use them as characters or integers, depending on our purpose • Use scanf (“%c”, &ch); to input 1 char

26. Character Example #include <stdio.h> main ( ) { char ch; printf (“Enter a character: “); scanf (“%c”, &ch); printf (“The value of %c is %d.\n”, ch, ch); } If the user entered an A the output would be The value of A is 65.

27. The getchar ( ) function • We can also use the getchar() function that is found in the stdio library • The getchar ( ) function reads one charcter from stdin and returns that character (value) • The value can then be stored in either a char variable or an integer variable

28. getchar () example #include <stdio.h> main ( ) { char grade; printf (“Enter a letter grade: “); grade = getchar ( ); printf (“\nThe grade you entered was %c.\n”, grade); }

29. switch example switch (day) { case 0: printf (“Sunday\n”); break; case 1: printf (“Monday\n”); break; case 2: printf (“Tuesday\n”); break; case 3: printf (“Wednesday\n”); break; case 4: printf (“Thursday\n”); break; case 5: printf (“Friday\n”); break; case 6: printf (“Saturday\n”); break; default: printf (“Error -- unexpected value for day\”); break; }

30. break • The last statement of each ‘case’ in the switch should be (99 % of the time) break; • The break causes program control to jump to the right brace of the switch • Without the break, the code flows into the next case. This is almost never what you want.

31. Logical Operators • Logical operators are used for combining condition • && is AND if ( (x > 5) && (y < 6) ) • || is OR if ( (z == 0) || (x > 10) ) • ! is NOT if (! (bob >42) )

32. Operator Precedence & Associativity ( ) left to right/inside-out ++ -- ! + (unary) - (unary) (type) right to left * / % left to right + (addition) - (subtraction) left to right < <= > >= left ot right == != left to right && left to right || left to right = += -= *= /= %= right to left , (comma) right to left

33. Examining PrintMessage function #include <stdio.h> void PrintMessage (void); function Prototype main ( ) { PrintMessage ( ); function call } void PrintMessage (void) function header { printf (“A message for you:\n\n”); function printf (“Have a Nice Day!\n”); body }

34. Another version ofPrintMessage void PrintMessage (int counter); main ( ) { int num; printf (“Enter an integer: “); scanf (“%d”, &num); PrintMessage (num); one argumentmatches the one } of type intformal parameter of type int void PrintMessage (int counter) { int i; for (i = 0; i < counter; i++) { printf (“Have a nice day\n\n”); } }

35. Local Variables • Functions only “see” their own local variable. This includes main ( ) • The variables that are passed to the function are matched with the formal parameters in the order they are passed • The parameters are declarations of local variables. The values passed are assigned to those variables • Other local variables can be declared within the function

36. Data Types andConversion Specifiers Data Type printf scanf conversion conversion float %f %f double %f %lf long double %Lf %Lf int %d %d long int %ld %ld unsigned int %u %u unsigned long int %lu %lu short int %hd %hd char %c %c

37. Commonly Used Header Files header file Contains function prototypes for <stdio.h> the standard input/output library functions & information used by them <math.h> the math library functions <stdlib.h> the conversion of number to text, text to number, memory allocation, random numbers and other utility functions <time.h> maninpulating time and date <ctype.h> functions that test characters for certain properties and that can convert case

38. Manipulating what rand() returns • Since rand() returns unsigned integers in a large range, we often have to manipulate the return value to suit our purposes • Suppose we want only random numbers in the range from 0 to 5 • num = rand () % 6 • How about 1 to 6? • num = 1 + rand( ) % 6; • How about 5 to 20? • num = 5 + rand ( ) % 16;

39. srand ( ) and rand ( ) • The pseudo-random number generator needs an unsigned int as it’s seed • Although it produces what appear to be random numbers, if we use the same seed, we get the same sequence of random numbers • To get different random numbers each time we run our program, we have to give a different seed each time

40. Array Declarations • int array [5] ; • This declaration sets aside a chunk of memory that’s big enough to hold 5 integers. • It does not initialize those memory locations to 0 or any other value. • Initializing an array may be done with an array initializer, as in : • int array [5] = { 5, 2, 6, 9, 3 } ; array 5 2 6 9 3 0 1 2 3 4

41. Indexing Array Elements • Values of individual elements can be found by indexing into the array. In our example, array [0] is equal to 5 and array [3] is equal to 9. • The integer in square brackets is called the subscript. • The subscript could also be an expression that evaluates to an integer. • In our example, array is the name of the array.

42. Filling Arrays • Since many arrays are quite large, using an array initializer is impractical. • Large arrays are often filled using a for loop. • for ( i = 0; i < 100; i++) • { • rolls [ i ] = 0 ; • } • would set every element of the 100 element array, rolls, to 0.

43. Arrays and Pointers • The array name alone (without [ ] ) is just a variable that contains the starting address of the block of memory where the array is held. • A pointer is just a variable that holds an address. • So the array name alone is a pointer to the array. • Pointers have types. If an array is an array of ints, then the name of that array has the type pointer to int or intpointer.

44. Passing Arrays to Functions • The function prototype : • void FillArray ( int array[ ], int numElems); • The function definition header: • void FillArray ( int array[ ], int numElems) • The function call: • FillArray ( array, SIZE); • Notice that we are passing only the name of the array (an address) and that we aren’t returning anything (the function is void)

45. Passing Arrays to Functions • If an individual element of an array such as temper[3] is passed to a function, it is passed by value not address. So the array is not modified. • Not like when &temper[0] or temper is passed. • Example on page 230 of text.

46. Multiple Subscripted Arrays • A common use of multiple subscripted arrays is to represent tables of values consisting of information arranged in rows + columns. • For example: a [ 2 ] [ 3 ] • row column

47. Multiple Subscripted Arrays • void printArray ( int a [ ] [ 3 ] , int SIZE_R , int SIZE_C ) • { • ... • } • When we receive a single-subscripted array as an argument to a function, the array brackets are empty in the functions parameter list. • The first subscript of a multiple-subscripted array is not required either, but all subsequent subscripts are required

48. Final Exam for CS 104, Spring 2002 • To study for final exam do: • start studying for the final exam well before the exam date so you can ask questions • ask questions in class or come to the office hours or email us if you have questions • study textbook chapters 1-6 • study lecture slides 11 - 25 • use final exam master question list on class web site • no answers provided • not all questions have necessarily been covered in this course (what we didn't cover will also not show up in the final exam)

49. Final Exam for CS 104, Spring 2002 • Tips for final exam: • make sure you can follow what programs do - in the final you'll be required to read and understand what some small programs do and write down their output (I find this rather easy but you need to FOCUS + CONCENTRATE to make sure you don't mix things up) • you will also be required to write some small programs and functions and arrays so make sure you know how to do that • ALWAYS WRITE DOWN SOMETHING that shows me that you worked on the problem - you might get points for trying to solve the problems even if the solution is wrong. YOU WILL NEVER GET ANY POINTS IF YOU LEAVE QUESTIONS BLANK - that especially counts for multiple choice,... questions!

50. Final Exam for CS 104, Spring 2002 • Date, Time and Place of Final Exam: • Wed, May 15th, 2002 at 7:00 - 9:00 pm in AC IV 150 • usual time and place • different date/time than listed in UMBC schedule!