Structured Programming. Jon Macey

Transcription

1 Structured Programming Jon Macey

2 Structured Programming Structured programming is an attempt to formalise the process of program development. There are several basis for a theorem of structured programming, some of which go back the mid 1940 s we will concentrate on the following papers :- "Flow Diagrams, Turing Machines and Languages with Only Two Formation Rules" Bohm and Jacopini (1966) Flow chart techniques for structured programming Nassi and Shneiderman 1973

3 Structured Programming Theorem The structured programming theorem states that any computable function may be built from three building blocks :- 1. sequences of operations. 2. selections ( executing one of two operations based on a boolean value). 3. iteration (repetition) repeating an operation until some sentinel value is reached.

4 Sequence A sequence is a linear execution of program commands The sequence of the two commands A and B makes sure that after entering the sequence A is always NS Chart A B flowchart A followed by B and the exit can only be reached by doing this sequence B

5 Selection At it s simplest selection asks a question and will execute a sequence if the answer to the question is true This is know as a selection monadic(1) It is also possible to execute different sequences depending upon the answer (true / false) This is know as a dyadic (2) selection. 1) (Philosophy / Logic) (Mathematics) Logic Maths (of an operator, predicate, etc.) having only a single argument place 2) (Philosophy / Logic) Logic Maths (of a relation, predicate, etc.) relating two terms; binary Compare monadic, polyadic

6 Monadic Selection condition true true condition false false A A Here we check a condition and if true execute A else continue

7 dyadic Selection true condition false true condition false A B A B In this version we execute A if the condition is true else we execute B

8 multiple selection Depending upon the programming language there are different methods for doing multiple selection. Some of these methods are language constructs, whilst some are the nesting of if / else clauses? condition value 1 value 1 value 3 default value A B C D A B C D A

9 iteration (loop execution) We can break iteration into 3 different categories iterative (head controlled loop) repetitive (foot controlled loop) continuous (never ending loop) We can also use a technique called recursion (which we will look at later in the year)

10 Head controlled loop condition A A? true false This loop structure will check the condition first and then execute the function To enter this structure the exit condition must be true in the first place. Once this is complete the condition is checked again and repeated until the condition is false This is know as a do-while loop

11 foot controlled loop A A condition? false true In this loop construct the function A is executed at least once Then depending upon the condition may be executed again This is know as a do-until loop

12 continuous loop A A A continuous loop has an entrance but no exit condition or check. These kinds of loops can be useful for systems that need to run forever without interruption (embedded systems) However they may also be problematic and can be caused by bugs in code.

13 continuous with exit(s) It is possible to construct loops with multiple exit conditions Whilst these are common, there are sometimes issues with maintenance / debugging using page A exit? B exit? C these constructs.

14 Structured Programming We will return to these constructs soon, and see how we can build them in different programming languages. But first we are going to introduce notational technique to help describe the syntax of programming languages. This will aid us in explaining the programming constructs in a more formal way before jumping into specific programming language syntax

15 BNF (Backus Naur Form) In the 1950 s both John Backus and Noam Chomsky invented the same notation which has become widely used to describe programming language syntax. Chomsky is a linguist and never intended to apply this work to computers Backus however was involved in the specification for a programming language called ALGO and used this context free grammar to describe syntax which he called BNF. It was only later that the connection between the two systems were made

16 Meta-languages A meta-language is a language to describe another language. BNF uses abstractions for syntactic structures. A BNF is a set of rules written like this <symbol> ::= expression symbol is a syntactic variable (or non-terminal) meaning a variable that can be replaced The expression is what the symbol is representing and is usually a finite combination of other symbols

17 alternatives In BNF alternatives may be separated by the vertical bar <digit>:== "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" In this case we state that a digit can be any of the numbers 0 through 9

18 (E)xtended- BNF E-BNF has a different syntax from the previous BNF, notation but has a number of advantages of basic BNF. BNF < digit >:== "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" <> no longer needed E-BNF digit excluding zero = "1" "2" "3" "4" "5" "6" "7" "8" "9"; digit = "0" digit excluding zero; Note ; to end statement

19 E-BNF, rule A comma indicates a concatenation for example eleven = "1", "1"; one hundred and eleven = "1", eleven; or perhaps? one hundred and eleven = eleven, "1";

20 E-BNF {} Expressions that may be omitted or repeated can be represented through curly braces digit excluding zero = "1" "2" "3" "4" "5" "6" "7" "8" "9"; digit = "0" digit excluding zero; natural number = digit excluding zero, {digit};

21 Usage Notation definition = concatenation, termination ; alternation option [...] repetition {...} grouping () terminal string... comment (*...*) exception - integer = "0" [" "], natural number;

22 Integers In computing the integer data type is used to represent the whole numbers (including zero) We can further refine the definition by stating if they are signed or unsigned digit excluding zero = "1" "2" "3" "4" "5" "6" "7" "8" "9"; digit = "0" digit excluding zero; unsigned integer = digit, {digit}; signed integer = unsigned integer + unsigned integer - unsigned integer;

23 variable declarations in C In C we can declare variables using the syntax <variable type> <variable identifier>; <variable type> <var1>,<var2>... <var n>; Where variable type indicates one of the C data types identifier is a valid name for a variable

24 valid variable names The following rules must be applied to C variable names must not begin with a number spaces are not allowed in names Only letters digits and _ are valid characters C keywords are not allowed digit = "0" "1" "2" "3" "4" "5" "6" "7" "8" "9"; letter = "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z" "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S" "T" "U" "V" "W" "X" "Y" "Z"; start char = "letter" "_"; variable name = start char, {digit} {letter};

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

26 integer data type In C / C++ we can specify an integer using the int keyword. The range of an integer is dependant upon the machine architecture but is usually a whole 16, 32 or 64-bit (2, 4 or 8 bytes, respectively) addressable word. By default the int data type is signed (can be positive or negative) Typical range is to

27 example #include <stdio.h> #include <stdlib.h> int main() { int a=10; int b=20; printf("a+b = %d \n",a+b); int abignumber = ; int one= 1; printf("abignumber+one = %d \n",abignumber+one); } return EXIT_SUCCESS;

28 ordinal data types The ordinal data types in C can be either signed or unsigned. C gives the programmer the following ordinal data types char, short int, long int each can be pre-fixed with the keyword unsigned

29 ordinal data types Data type Description char short int int long int Small data type only needs 1 byte / 8 bits of memory to store. Integer data type half the size of an integer Integer data type, size dependent upon platform using it Integer data type twice the size of the int data type

30 sizeof() In C/C++ sizeof is a unary operator that must be implemented by the developer of the compiler it appears as a C/C++ function when we use it but will return the size in bytes of the data type passed to it. The following program demonstrates sizeof

31 #include <stdio.h> #include <stdlib.h> int main() { printf("sizeof(char)= %ld \n",sizeof(char)); printf("sizeof(short int)= %ld \n",sizeof(short int)); printf("sizeof(int)= %ld \n",sizeof(int)); printf("sizeof(long int)= %ld \n",sizeof(long int)); printf("unsigned versions\n"); printf("sizeof(unsigned char)= %ld \n",sizeof(unsigned char)); printf("sizeof(unsigned short int)= %ld \n",sizeof(unsigned short int)); printf("sizeof(unsigned int)= %ld \n",sizeof(unsigned int)); printf("sizeof(unsigned long int)= %ld \n",sizeof(unsigned long int)); } return EXIT_SUCCESS; sizeof(char)= 1 sizeof(short int)= 2 sizeof(int)= 4 sizeof(long int)= 8 unsigned versions sizeof(unsigned char)= 1 sizeof(unsigned short int)= 2 sizeof(unsigned int)= 4 sizeof(unsigned long int)= 8

32 char The char data type is useful for representing ASCII characters It usually takes up 1 byte and can represent either 0 to +255 (unsigned) or -128 to +128 (signed) Whilst this is used to store numeric values we can use the convenience single quote method to assign a char from a character as shown in the next program

33 #include <stdio.h> #include <stdlib.h> int main() { char a='z'; printf("%c \n",a); a=42; printf("%c \n",a); } return EXIT_SUCCESS;

34 real numbers In computing we use floating point data types to represent real numbers (numbers with a fractional part) These numbers are always approximations as we have to move the decimal. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. Significant digits base exponent

35 real numbers in C C has two real data types float and double the long prefix may be used with double to increase the precision Type Specifiers Precision (decimal digits) Exponent range Minimum IEEE 754 Minimum IEEE 754 float 6 7.2(24 bits) ±37 ±38 (8 bits) double (53 bits) ±37 ±307(11 bits) long double (113 bits) ±37 ±4931 (15 bits)

36 #include <stdio.h> #include <stdlib.h> int main() { } float a=2.5; double b= ; long double c= ; printf("%f \n",a); printf("%lf \n",b); printf("%lf \n",c); return EXIT_SUCCESS; Note truncated printf output need to use %n.nf and specify decimal places to print e.g. %.8lf

37 Arithmetic expressions Most programs are algorithmic in nature which means we have to do some maths The table below shows the available arithmetic operators Operator Meaning Examples + addition - subtraction * multiplication / division is is is is 3.0 5*2 is *2.0=10.0 5/2 is 2 5.0/2.0 is 2.5 % remainder (modulus) 5%2 is 1

38 The / Operator When applied to two positive integers the division operator computes the integral part of the result dividing its first operand by its second For example 7.0 / 2.0 is / 2 is / is 2.99 (double value) 299 / 100 is 2 (integer value) If the / Operator is used with a negative and positive integer, the results vary from one C implementation to another For this reason you should avoid division by -ve integers

39 More on / It is also important not to do division by 0 as the program may crash, some modern compilers will try to warn of #include <stdio.h> #include <stdlib.h> int main() { printf("3/15 %d\n",3/15); printf("15/3 %d\n",15/3); printf("16/3 %d\n",16/3); printf("17/3 %d\n",17/3); printf("18/3 %d\n",18/3); printf("16/-3 %d\n",16/-3); printf("0/4 %d\n",0/4); printf("4/0 %d\n",4/0); } return EXIT_SUCCESS; this as seen with the program opposite

40 The % (modulus) Operator The remainder operator (%) returns the integer remainder of the result of dividing the first operand with the second For example the value of 7 % 2 is 1 The magnitude of m % n must always be lest than the division n 7/2 299/100 # # 7 2 = = 2 3 2= = %2= = 299 % 100 = 99

41 #include <stdio.h> #include <stdlib.h> int main() { printf("3 %% 5=%d\n",3%5); printf("5 %% 3=%d\n",5%3); printf("4 %% 5=%d\n",4%5); printf("5 %% 4=%d\n",5%4); printf("5 %% 5=%d\n",5%5); printf("15 %% 5=%d\n",15%5); printf("6 %% 5=%d\n",6%5); printf("15 %% 6=%d\n",15%6); printf("7 %% 5=%d\n",7%5); printf("15 %% -7=%d\n",15%-7); printf("8 %% 5=%d\n",8%5); printf("15 %% 0=%d\n",15%0); } return EXIT_SUCCESS;

42 Data type of an expression There are certain rules to define the results of mixing data types For example int a=10; int b=23; int c; c=a+b; // will result in a integer value However if we mix the types we will get different results depending upon the receiving variables data type double x; int n; x = 9 * 0.5; // will result in x = 4.5 n = 9 * 0.5; // will result in n = 4

43 Expressions with Multiple Operators There are rules as to how expressions are evaluated Parentheses Rule : All expressions in parentheses must be evaluated separately. Nested parenthesised expressions must be evaluated from the inside out, with the innermost expression evaluated first. Operator precedence rule : Operators in the same expression are evaluated in the following order. unary +, - first *, /, % next binary +,- last

44 Expressions with Multiple Operators Associativity Rule : Unary operators in the same subexpression and at the same precedence levels (such as + and -) are evaluated right to left. Binary operators in the same sub-expression and the same precedence level (such as + and -) are evaluated left to right. To help avoid problems with the order of evaluation it is best to use parenthesis x * y * z + a / b -c * d; can be written (x * y * z) + (a / b) - (c * d);

45 Mathematical Formulas as C/C++ expressions Mathematical Formula C Expression b 2 4ac b*b-4*a*c a + b c a+b-c a+b c+d (a + b) / (c + d) 1 1+x 2 1 / (1 + x * x ) a (b + c) a * -(b + c) Notice that C has no equivalent to x 2 so we have to evaluate it as x * x If any other power is required the pow(double x, double y) function must be used to evaluate x y

46 References Cornelia M. Yoder and Marilyn L. Schrag Nassi-Shneiderman charts an alternative to flowcharts for design. SIGSOFT Softw. Eng. Notes 3, 5 (January 1978), I. Nassi and B. Shneiderman Flowchart techniques for structured programming. SIGPLAN Not. 8, 8 (August 1973), Anderson Eike F Computer Programming in ANCI C Lecture notes NCCA Sebesta R. W. 2002, Concepts of Programming Languages, 5th edition, Addison Wesley, International Ed Daniel D. McCracken and Edwin D. Reilly Backus-Naur form (BNF). In Encyclopedia of Computer Science (4th ed.), Anthony Ralston, Edwin D. Reilly, and David Hemmendinger (Eds.). John Wiley and Sons Ltd., Chichester, UK Hanly. J. R Koffman E. B. 1999, Problem Solving & Program Design in C, 3rd Edition, Addison Wesley, International Ed

47 Further reading