Fundamentals of C Structure of a C Program 1 Our First Simple - PDF document

Fundamentals of C Structure of a C Program 1

Our First Simple Program Comments - Different Modes 2

Comments - Rules Preprocessor Directives Preprocessor directives start with # e.g. #include copies a file into the source code before compilation 2 forms…..we will usually see the first #include <systemFilename> #include “undefinedFilename” Within “ ” : personal file in your own directory 3

We will almost always use #include <stdio.h> • stdio.h is the standard input/output header • Contains code required for I/O (Input/Output) • .h files are header files Includes header ; The .obj part on Linking NO code is generally included except on linking • Header files contain definitions The function : main() ALL Programs must have a main() function. Later we will have add other functions. Different Formats: main( ) int main (void ) ***The book likes this void main(void) int main( ) 4

main( ) , void main(void) End with return; ************************** int main (void ) , int main( ) End with return 0; Back To Our First Program 5

Variables A Variable is a block of memory that stores data. A variable has a particular “Type”, i.e. it stores a particular kind of data A variable is named with an appropriate Identifier (or Name) Suppose we have a variable declared as follows: int number_of_days This reserves a block of memory which holds an integer 6

Schematic view: number_of_days 365 In the course of the program we might want to change the value in the variable. So that later we might want: 7361 number_of_days • The “actual” view of memory: 4 bytes 01100000 number_of_days 11001010 10010100 00100111 So a variable takes a fixed number of bytes in memory 7

Rules for Naming Variables 1. First character: alphabetic or underscore 2. Consist only of alphanumeric or underscores 3. Only first 31 characters count 4. Cannot duplicate a reserved word Legal/Illegal Variable Names Use meaningful names Legal Illegal a $sum student_name 2names salary stdnt number time_of_day int 8

Standard C Data Types Primitive C Data Types Data Type C-Implementation void void char (1 byte) character unsigned short int (1 byte) unsigned int (2 or 4 bytes) unsigned long int (4 or 8 bytes) integer short int (1 or 2 byte) int (2 or 4 bytes) long int (4 or 8 bytes) float (4 bytes) double (8 bytes) floating point long double (10 bytes) 9

Variables : Declaration Variables : Initialization How we put a value in a variable • At compile time in the declaration: e.g. int x=5; • At run time by an assignment: e.g. x =5; • At run time by an input statement: e.g.scanf(“%d”,&x); 10

No variable is initialized until you do so! You can initialize a variable in the declaration char code = ‘B’; char letter = ‘B’; int i = 0; int age = 65; float pie = 3.1415; float salary 27.57; double variable2 = 3.1415926535; Special Characters ASCII Character Symbolic Name Null character ’\0’ Alert (bell) ’\a’ Backspace ’\b’ ’\t’ Horizontal tab ’\n’ Newline Vertical Tab ’\v’ 11

Special Characters ASCII Character Symbolic Name Form Feed ’\f’ Carriage return ’\r’ Single quote ’\’’ ’\”’ Double Quote ’\\’ Backslash Strings : Constants 12

Preprocessing : #define Defines constants #define name token Replaces the name with the token Example: #define PI 3.1415926535 #define SIZE 1000 Syntax for Define : #define • No equals sign • No semicolon at the end 13

Constants To define a constant define a “variable” (usually, U.C.) with keyword: const This is how it works: const float PI = 3.1415926; This does NOT work: const float PI; PI = 3.1415926; /* not allowed to change it */ May be collected into a header file for general use Standard Input and Output 14

To OUTPUT you require #include <stdio.h> printf(format string, data list); Field specifiers are inside the format string i.e. Controls how the data looks when printed Standard printf statements #include <stdio.h> void main( ) /* This codes prints the values of two variables */ { int a = 57; int b = 145; printf( "%d\n%d\n", a,b); return; } 15

What happened? The statements int a = 57; a 57 int b = 145; b 145 The statement printf( "%d\n%d\n", a , b); prints: 57 145 Field Specifiers %<flag><minimum width><precision><size>code Size Code Type Example Codes: none c char %c h d short int %hd none d int %d l or L d long int %Ld none f float %f none f double %f L f long double %Lf 16

Specification of Width Value %d %4d 12 12 12 123 123 123 1234 1234 1234 12345 12345 12345 Precision specification for floats: %7.2f /* this prints the float in a field of seven characters with two characters after decimal: nnnn.dd */ 17

Another Example of printf #include <stdio.h> int main() /* This codes prints the values of two variables */ { int months = 9; float salary = 145.35; printf( "%d\n%7.2f\n", months , salary); return 0; } Another Example of printf #include <stdio.h> int main( ) /* This codes prints the values of two variables */ { int months = 9; float salary = 145.35; printf( "the number of months is%4d\nthe salary is%7.2f\n", months,salary); return 0; } 18

The output the number of months is 9 the salary is 145.35 Input: Getting information into memory: (reading) Requires: #include <stdio.h> scanf(format string, address list); Again : Field specifiers inside the format string 19

Standard scanf Statements scanf(format string, address list); scanf(“%d%f”, &age, &weight); At keyboard type 23 60.75 60.75 Result: weight 23 age Field Specifiers %<flag><maximum width><size>code Remember : There is no precision issue here !!! Codes: Size Code Type Example none c char %c h d short int %hd none d int %d l or L d long int %Ld none f float %f none f double %f L f long double %Lf 20

Rules for scanf formats Addresses of a variable are specified with: &variableName A variety of rules apply to conversion Rules for scanf formats 1. Initial whitespace is ignored (except %c) 2. The conversion operation process until: i. End of file is reached ii. Maximum characters are processed iii. A whitespace character is found after a digit iv. An error is detected 21

Rules for scanf format strings 1. A field specifier for each variable 2. Other characters must be exactly matched 3. Cannot end format string with whitespace Examples of scanf scanf(“%d%d%d%c”,&a,&b,&c,&d); scanf(“%d%d%d %c”,&a,&b,&c,&d); scanf(“%-8d%-8d%d”,&a,&b,&c); %-8d /* left justify flag */ 22

Working with the variables: We want to operate on variables: e.g add two variables or subtract them. An Expression: rateOfPay*hours – tax Operators e.g., + plus - minus * multiply / divide % mod You create expressions out of operators. 23

Some Expressions 35 2 * 3 + 4 23 + b * 6 -salary Use parenthesis to clarify complicated expressions: (food + drinks)*(1+gst+pst) Assignment Assignment expressions evaluate to the expression on the right of the assignment operator. 24

Simple Assignment Contents of Contents of Expression Value of Result of Variable x Variable y Expression Expression 10 5 x=y+2 7 x = 7 10 5 x=x/y 2 x=2 10 5 x=y%4 1 x=1 Operator Precedence Operators have a built in order of precedence which is over ridden by using parenthesis 1+2*3+4 answer 11 1*2 + 3*5 answer 17 -5*6+2 answer -28 25

Operators precedence (on the same level they are equal) 1. ( ) takes precedence 2. unary minus -7 unary plus +3 3. * Multiply / Divide 4 % Modulus 5. + add - subtract Examples Operators • Examples of C statements using these : • The following is a program fragment only • int i, j ,k, l, m; • i = j + k; • i = j + k * l / m; • /* Here i gets the value j + (k*l/m) */ • i = j * k + l * m • /* Here i gets the value (j*k) + (l*m) */ 26

Examples Operators • Examples of C statements using these : • There is one division operator for integer and float but it means different things. → • 2 / 5 0 • 2.0 / 5.0 → 0.4 • % is the modulus (remainder) operation → • 5 % 2 1 → 2 • 2 % 5 Unary operator + and - +a – evaluates to the contents of a -a – evaluates to the negative contents of a 27

Unary Expressions Contents of Expression Value of Contents of x Before x After Expression 10 ++x 11 11 10 --x 9 9 Unary operator ++ and -- Post-increment and decrement i++ i-- /*First use the value ; then do operation*/ Pre-increment and decrement ++i --i /*First do operation; then use the value */ 28