Lab 4 reading material formatted io and arithmetic expressions
- 1. CS F111 Lab 4: Input/Output Arithmetic Expressions
- 2. 2 Advanced Output Formatting • A few of many other things you can do: – Control number of decimals • 3.1 vs 3.100000 – Exponential (scientific) or decimal notation • 3.1 vs 3.1E0 – Control total width (including spaces) • _______3.1 vs __3.1
- 3. 3 Output Format Examples • %10.2f _ _ _ _ 1 2 3 . 5 5 float • %10.4f _ _ 1 2 3 . 5 5 0 0 • %.2f 1 2 3 . 5 5 • %10d _ _ _ _ _ _ _ 4 7 5 int • %-10d 4 7 5 _ _ _ _ _ _ _ • %10c _ _ _ _ _ _ _ _ _ a char
- 4. 4 Output Expressions •Basic rule: –% placeholders in the format must match variables in the input list – MUST! match on for-one in number, order, and type. int studentID ; double grade ; scanf (“%d %lf”, &studentID , &grade ) ;
- 5. 5 Multiple Inputs What happens if types don’t match? printf -- garbled output scanf -- unpredictable errors and don’t forget the &
- 6. Exercise: print the values of a and b. scanf(“%d %d”,a,b); scanf(“ %d %d”,&a,&b); scanf(“%d %d”,&a,&b); scanf(“%d %d ”,&a,&b); scanf(“%d %d ”,&a,&b); scanf(“%d %d”,&a,&b,&c); scanf(“%d %d”,&b); scanf(“%d %d n”,&a,&b); scanf(“hello %d %d”,&a,&b); scanf(“%d%d”,&a,&b); scanf(“%f %f”,&a,&b);
- 7. Exercise: to study formatted input int a,b,d; char c; scanf(“%2d %5d”,&a &b); printf(“%d %dn”,a,b); i/p 50 31426 o/p 50 31426 i/p 31426 50 o/p 31 426 scanf(“%d ”,&d); /*What if there is this line: second input*/ scanf(“%d”,&c); /* c is char variable */ printf(“%dn”,c); i/p 25 o/p 50
- 8. Exercise: to study formatted input scanf(“%d %*d %d”,&a, &b); printf(“%d %d”,a,b); i/p 123 456 789 o/p 123 789 printf(“%11.4e”,-a); /* “%w.p e” what should be the minimum difference w-p ? */
- 9. 9 • space (‘ ’), tab (‘t’), newline (‘n’) are “whitespace” • Whitespace is skipped by scanf for int (“%d”), and double (“%lf”) – This means the user can type spaces before a number and they are ignored • Not skipped for char input “%c” – each character typed, including spaces, is used Whitespace
- 10. Reading single characters • Another important pair of functions for keyboard input/output is getchar/putchar • getchar reads a single character • putchar writes a single character #include <ctype.h> char c; c = getchar(); /* blocks until data is entered if more than one char is entered only first is read */ putchar(c); /* prints value of c to screen */
- 11. 11 • Arithmetic expressions • Integer and floating-point (double) types • Unary and binary operators • Precedence • Associativity • Conversions and casts • Symbolic constants
- 12. We need precise rules that define exactly what an expression means: What is the value of 4 - 4 * 4 + 4? Arithmetic on a computer may differ from everyday arithmetic or math: (1.0 / 9.0) * 9.0 could be 0.99999998213 2 / 3 is zero in C, not .667 (!) Why Study Expressions?
- 13. double area, radius; area = 3.14 * radius * radius; assignment statement expression Execution of an assignment statement: Evaluate the expression on the right hand side Store the value of the expression into the variable named on the left hand side Assignment Statement: Review
- 14. 14 Expressions are things that have values A variable by itself is an expression: radius A constant by itself is an expression: 3.14 Often expressions are combinations of variables, constants, and operators. area = 3.14 * radius * radius; Expressions
- 15. 15 Expression Evaluation Some terminology: – Data or operand means the integer or floating-point constants and/or variables in the expression. – Operators are things like addition, multiplication, etc. • The value of an expression will depend on the data types and values and on the operators used – Additionally, the final result of an assignment statement will depend on the type of the assignment variable.
- 16. 16 Arithmetic Types: Review C provides two different kinds of numeric values • Integers (0, 12, -17, 142) Type int Values are exact Constants have no decimal point or exponent • Floating-point numbers (3.14, -6.023e23) Type float, double Values are approximate (12-14 digits precision typical) Constants must have decimal point and/or exponent
- 17. 17 Operator Jargon • Binary: operates on two operands • 3.0 * b • zebra + giraffe • Unary: operates on one operand • -23.4 • C operators are unary or binary • Puzzle: what about expressions like a+b+c? – Answer: this is two binary ops, in sequence
- 18. 18 •Constants of type double: –0.0, 3.14, -2.1, 5.0, 6.02e23, 1.0 3 –not 0 or 17 •Operators on doubles: –unary: - –binary: +, -, *, / –Note: no exponentiation operator in C Expressions with doubles
- 19. 19 •Declarations • double height, base, radius, x, c1, c2; •Sample expressions (not statements): • 0.5 * height * base • ( 4.0 / 3.0 ) * 3.14 * radius * radius * radius • - 3.0 + c1 * x - c2 * x * x Example Expressions with doubles
- 20. 20 •Constants of type int: –0, 1, -17, 42 –not 0.0 or 1e3 •Operators on ints: –unary: - –binary: +, -, *, /, % Expressions with ints
- 21. 21 •Integer operators include integer division and integer remainder: • symbols / and % •Caution: division looks like an old friend, but there is a new wrinkle! int Division and Remainder 2 rem 1 100 )299 200 99 2 rem 99 100 )299 200 99
- 22. 22 / is integer division: no remainder, no rounding 299 / 100 6 / 4 5 / 6 int Division and Remainder % is mod or remainder: 299 % 100 99 6 % 4 2 5 % 6 5 2 1 0
- 23. 23 •Given: total_minutes 359 •Find: hours 5 • minutes 59 •Solution in C: Expressions with ints: Time Example hours = total_minutes / 60 ; minutes = total_minutes % 60 ;
- 24. 24 •int radius; •double volume; •double pi = 3.141596; •volume = ( 4/3 ) * pi * radius *radius * radius; A Cautionary Example
- 25. 25 Sometimes only ints make sense the 15th spreadsheet cell, not the 14.997th cell Doubles may be inaccurate representing “ints” In mathematics 3 • 15 • (1/3) = 15 But, 3.0 * 15.0 * (1.0 / 3.0) might be 14.9999997 Last, and least operations with doubles is slower on some computers doubles often require more memory Why Use ints? Why Not doubles Always?
- 26. Resolving Complex Expressions • C has precedence, associativity rules it uses to determine the order operators are evaluated • Precedence: some operators are executed before others • Associativity: when more than one operator in an expression has the same precedence then associativity rules are used to order
- 27. 27 •Precedence determines the order of evaluation of operators. •Is a + b * a - b equal to ( a + b ) * ( a - b ) or a + ( b * a ) - b ?? And does it matter? •Try this: • 4 + 3 * 2 - 1 • Order of Evaluation 7 9 (4 + 3) * (2 - 1) = 4 + (3 * 2) - 1 =
- 28. 28 Precedence rules: –1. do ( )’s first, starting with innermost –2. then do unary minus (negation): - –3. then do “multiplicative” ops: *, /, % –4. lastly do “additive” ops: binary +, - Operator Precedence Rules
- 29. 29 •Precedence doesn’t help if all the operators have the same precedence •Is a / b * c equal to • a / ( b * c ) or ( a / b ) * c ?? •Associativity determines the order among consecutive operators of equal precedence •Does it matter? Try this: 15 / 4 * 2 Precedence Isn’t Enough
- 30. 30 •Associativity determines the order among consecutive operators of equal precedence •Does it matter? Try this: 15 / 4 * 2 • (15 / 4) * 2 = 3 * 2 = 6 • 15 / (4 * 2) = 15 / 8 = 1 Associativity Matters
- 31. 31 •Most C arithmetic operators are “left associative”, within the same precedence level • a / b * c equals (a / b) * c • a + b - c + d equals ( ( a + b ) - c ) + d •C also has a few operators that are right associative. •HW- Which are they? Associativity Rules
- 32. 32 The Full Story... • C has about 50 operators & 18 precedence levels… • A "Precedence Table" shows all the operators, their precedence and associativity. – Look in any C reference manual • When in doubt: check the table • When faced with an unknown operator: check the table
- 33. Precedence Table 33 Highest on top ++ -- (Postfix) ++ -- (Prefix) * / % + - << >> < > & | && ||
- 34. 34 •Mathematical formula: ________ - b + b2 - 4 a c ---------------------- 2 a #include <math.h> (- b + sqrt ( b * b - 4.0 * a * c) ) / ( 2.0 * a ) Compiling requires gcc squareroot.c -lm Precedence and Associativity: Example
- 35. 35 b * b - 4.0 * a * c Depicting Expressions - 67.05 -1.0 15.2 * -4.0 * -60.8 2.5 2.5 * 6.25
- 36. 36 •What is 2 * 3.14 ? •Compiler will implicitly (automatically) convert int to double when they occur together: •int + double double + double (likewise -, *, /) •2*3 * 3.14 (2*3) * 3.14 6 * 3.14 6.0 * 3.14 18.84 •2/3 * 3.14 (2/3) * 3.14 0 * 3.14 0.0 * 3.14 0.0 • Strongly recommend you avoid mixed types: e.g., use 2.0 / 3.0 * 3.14 instead. Mixed Type Expressions
- 37. Promotion Hierarchy short int long int float double char • Operands of the lower type can be implicitly converted to the higher type • Occurs when an operator expects the higher type (adding together two floats when one is an int)
- 38. 38 •int total, count, value; •double avg; •total = 97 ; count = 10; •avg = total / count ; /*avg is 9.0!*/ •value = total*2.2; /*bad news*/ Conversions in Assignments implicit conversion to int – drops fraction with no warning implicit conversion to double
- 39. 39 Use a cast to explicitly convert the result of an expression to a different type Format: (type) expression Examples (double) myage (int) (balance + deposit) This does not change the rules for evaluating the expression itself (types, etc.) Good style, because it shows the reader that the conversion was intentional, not an accident Explicit Conversions: cast
- 40. 40 •int total, count ; •double avg; •total = 97 ; count = 10 ; • • /* explicit conversion to double (right way)*/ •avg = (double) total / (double) count); /*avg is 9.7 */ • Using Casts /* explicit conversion to double (wrong way)*/ avg = (double) (total / count) ; /*avg is 9.0*/
- 41. 41 •Named constants: •#define PI 3.14159265 •#define HEIGHT 50 •#define WIDTH 80 •#define AREA (HEIGHT * WIDTH) –... •circle_area = PI * radius * radius ; •volume = length * AREA; #define - Symbolic Constants Note: = and ; are not used for #define () can be used in #define
- 42. 42 Centralize changes No "magic numbers" (unexplained constants) use good names instead Avoid typing errors Avoid accidental assignments to constants double pi ; vs. pi = 3.14 ; #define PI 3.14 ... ... pi = 17.2 ; PI = 17.2 ; syntax error Why #define?
- 43. 43 •Every variable, value, and expression in C has a type •Types matter - they control how things behave (results of expressions, etc.) •Lots of cases where types have to match up •Start now: be constantly aware of the type of everything in your programs! Types are Important
- 44. 44 • Write in the clearest way possible to help the reader • Keep it simple; break very complex expressions into multiple assignment statements • Use parentheses to indicate your desired precedence for operators when it is not clear • Use explicit casts to avoid (hidden) implicit conversions in mixed mode expressions and assignments • Be aware of types Advice on Writing Expressions
- 45. Home Work 45 •Meaning of – Syntax – Semantics of a programming language •Find the Output: – value=value++ + value++; – value=++value + ++value; – value=value++ + ++value;
- 46. Does Terminology Matter? Lots of new terminology today! "variable", "reserved word", "initialization", "declaration", “statement”, "assignment", etc., etc. You can write a complicated program without using these words But you can't talk about your programs without them! Learn the exact terminology as you go, and get in the habit of using it. C-46