1_Introduction to C programming - Copy.ppt

Instructor: NICHOLAS KATENDE email: katsnic@live.com Tel:+250-783-933978 INTRODUCTION TO COMPUTER PROGRAMMING

Overview 2  Introduction to C programming Language.  Structure of a C program.  Operators, Expressions, and Statements.  Control Structures (Branching).  Iterative Structures (Loops).  Functions.  Arrays.  Structures and pointers  Implementation Using Borland Compiler .

Programming Languages  A programming language is a language that a computer can understand and provides a programmer an environment to write and execute programs in it. A number of programming languages exist but the choice mainly depends on the nature of the problem at hand and the programmer’s ability to use the language.  There has been a great revolution in programming languages right from the time when programming started. The first generation of programming languages involved the use of machine language, the second involved the use of assembly language, the third generation involved highlevel programming languages like C, C++, Java e.t.c, the forth generation involved languages mainly used for database manipulation like Structured Query Language (SQL) and the fifth generation involved languages mainly used for artificial intelligence and neural networks like ProLog and LISP 3

Introduction to C Programming 4 What are Programming Languages ? Programming Languages are used to specify, design, and build software systems. Programming languages evolve with the systems they are used to construct. C is a good example of how this system takes place. C as a programming language C has been standardized (ANSI C) and spawned new languages (C++,Stroustrup,1986) that improve C. The basic characteristics of C are:  Small in size  Structured (Extensive use of functions)  Less error prone

Cont’d … 5  Designed for systems programming (low level programming of the type that is used in the implementation of an operating system).  C is high level than assembler but still close to hardware and allows direct manipulation of many systems accepts: pointers, memory allocation and bitwise manipulation.  C programming language provides high level construct (functions and data structures) that facilitate programming without loosing too much performance. Being a low level language, C gives a lot of freedom to the programmer:

Cont’d… 6 It has the advantage that good programmers can implement very efficient programs in a compact manner and it has the disadvantage that most of us are not good programmers and the freedom C grants is usually translated in error prone, messy code.

Why learn C ? 7  Compact, Fast and Powerful.  Standard for program development (widely accepted).  Its everywhere (Portable).  Supports modular programming.  Useful for all applications  C is a native language for UNIX.  Easy to interface with system devices/assembler routines.

Syntax, Semantics and Program Errors  Syntax  Syntax of a programming language means the rules that dictate how exactly the vocabulary elements of the language can be combined to form statements. Each programming language has its own unique syntax. For example, in English we know that a sentence must end with a question mark, a full stop or an exclamation mark. That is the rule that must be obeyed  During compilation, all syntax rules are checked. If a program is not syntactically correct, the compiler will issue error messages. Such errors could be like attempt to add two integers but the addition operator in not put between them or forgetting to close a procedure/function. 9

CONT’  Semantics  The semantics of a statement in a programming language define what will happen when that statement is executed. In other words, semantics refer to the meaning of the statement.  Programming languages are generally unambiguous, which means the semantics are well defined. This means there’s only one and only one interpretation of each statement. Natural languages e.g. French and English are full of ambiguities. 10

CONT’  Program Errors  You will encounter three types of errors as you develop programs  Compile time errors. These are errors identified by the compiler when it is compiling your program. They may include syntax errors and wrong data types. A program will not execute until when a compile time error is corrected.  Runtime errors. These occur during the execution of a program and cause a program to abort abnormally e.g. division by zero, infinite looping.  Logical errors. The program compiles and runs normally but it produces wrong results. A logical error may occur when:  A value is calculated incorrectly e.g instead of addition you subtract.  When a graphical button does not appear in the correct place.  Logical errors are the most difficult to debug because they manifest themselves in many ways when their root cause is different.  Debugging is the process of finding and correcting errors/bugs in a program. The figure below shows the basic program development process. 11

Program Development and the Top-Down Design Approach During the program development process, the following steps must be followed; Step1. Understand the problem Step2. Analyzing the problem we're given Step3. Dissect the problem into manageable pieces Step4. Developing a solution technique (algorithm) Step5. Documenting the program/technique Step6. Translating (implementing) the technique into code Step7. Compiling and running the program Step8. Testing the results with appropriate data The above steps must be followed carefully. After step 8, if there are logical errors (i.e. program running properly but results are not correct), you must go back to step 6.

Top-Down Design Approach  Many problems are too large to immediately grasp and solve all the details; top-down design approach tries to address this issue.  Take a problem, divide it into smaller logical sub-problems, and remember how they fit together  For each sub-problem, divide it into still smaller sub- problems  Continue sub-dividing until each little problem is small enough to be easily solved  Solving a collection of small problems thus allows us to solve one much larger problem 13

Structure of a C Program 14 // my first program #include <stdio.h> main () { printf (“Hello Worldn”); // prints hello world return 0; } C is case sensitive. All commands must be in lower case.

Muhirwe Jackson, Mak, FCIT Entering and Compiling welcome.c 1. Open Borland C++ compiler. 1. Then open a new blank page 2. Use the keyboard to type the welcome.c source code exactly as shown above Press Enter at the end of each line. 3. Save the source code. You should name the file welcome.c

Muhirwe Jackson, Mak, FCIT Entering and Compiling welcome.c 4. Verify that welcome.c is on disk by listing the files in the directory or folder. You should see welcome.c within this listing. 5. Compile and link welcome.c 6. Check the compiler messages. If you receive no errors or warnings, everything should be okay.

Muhirwe Jackson, Mak, FCIT Entering and Compiling welcome.c 7. If you made an error typing the program, the compiler will catch it and display an error message. For example, if you misspelled the word printf as prntf, you would see a message similar to the following: 1. Error: undefined symbols: _prntf in welcome.c (welcome.obj)

Muhirwe Jackson, Mak, FCIT Entering and Compiling welcome.c 8. Go back to step 2 if this or any other error message is displayed. Open the welcome.c file in your editor. Check your codes, make any necessary corrections, and continue with step 3. 9. Your first C program should now be compiled and ready to run. If you display a directory listing of all files named welcome (with any extension), you should see the following: 10. welcome.c, the source code file you created with your editor welcmoe.obj or welcome.o, which contains the object code for welcome.c

Muhirwe Jackson, Mak, FCIT Entering and Compiling welcome.c 11. WELCOME.EXE, the executable program created when you compiled and linked WELCOME.C 12. 9. To execute, or run, WELCOME.EXE, simply enter WELCOME. The message This is my first C progam is displayed on-screen.

Cont’d… 21 The above program is the typical program that programmer apprentices write for the first time, and its result is the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in C, but it already contains the fundamental components that every C program has. We are going to look line by line at the code we have just written: // my first program in C This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is.

Cont’d… 22 #include <stdio.h> Lines beginning with a hash sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include<stdio.h> tells the preprocessor to include the stdio.h standard file. This specific file (stdio.h) includes the declarations of the basic standard input-output library in C, and it is included because its functionality is going to be used later in the program. .h The .h portion is a language extension which denotes and includes header files and the < > characters around the name stdio.h header file tells C to check in the system area for the file called stdio.h

Cont’d… 23 { This is a brace. As the name implies, braces come in packs of two, i.e. for every open brace there must be a matching close. Braces allow to lump pieces of a program together. Such a lump of program is often called a block. A block can contain the declaration of variables used within it, followed by a sequence of program statements which are executed in order. In this case the braces enclose the working parts of the function main (). When the compiler sees the matching close brace at the end it knows that it has reached the end of the function and can look for another (if any). The effects of an un-paired brace are invariably fatal and can lead to error in the program. main() The line corresponds to the beginning of definition of the main function. The main function is the point by where all C programs start their execution independent of its location within the source code. Hence its essential for all C programs to have the main() function.

Cont’d… 24 printf (“Hello Worldn”); Instructs the computer to perform an action  Specifically prints the string of characters within the (“ ”),  Entire line is called a statement and all statements end with a semicolon (;)  Escape character ()  (n) is the new line character. Printf represents the standard output stream in C, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen). Notice that the each statement in C ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).

Escape Sequences  n new line  t tab  r carriage return  a alert  backslash  ” double quote

Cont’d… 26 return 0; The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C console program. comments Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code. C supports two ways to insert comments: // Line comment /* Block comment */

Variables, Data types 27 In order to go a little further on and to become able to write programs that perform useful tasks that really save us work we need to introduce the concept of variable. Let us think that I ask you to retain the number 5 in your mental memory, and then I ask you to memorize also the number 2 at the same time. You have just stored two different values in your memory. Now, if I ask you to add 1 to the first number I said, you should be retaining the numbers 6 (that is 5+1) and 2 in your memory. Values that we could now for example subtract and obtain 4 as result. The whole process that you have just done with your mental memory is a simile of what a computer can do with two variables. The same process can be expressed in C++ with the following instruction set: a=5; b=2; a=a+1; result= a-b;

Cont’d… 28 Identifiers A valid identifier is a sequence of one or more letters, digits or underscore characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. Only letters, digits and single underscore characters are valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline character (_ ), but in some cases these may be reserved for compiler specific keywords or external identifiers, as well as identifiers containing two successive underscore characters anywhere. In no case they can begin with a digit. Examples of identifiers; interest_rate, tax_rate, new_vat etc.

Fundamental data types 29 We store variables in our computer’s memory, but the computer has to know what kind of data we want to store in them since its going to occupy the same amount of memory to store a simple number than a simple letter or a large number, and they are not going to be interpreted the same way. The memory in our computer is organized in bytes. A byte is the minimum amount of memory we can manage in C. a byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition the computer can manipulate more complex data type that come from grouping several bytes, such as long number and non-integer numbers.

Description of each data type 31

Declaration of Variables 32 In order to use a variable in C, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid variable identifier. For example: int a; float my_number; or float mynumber; if you are going to declare more than one variable of the same data type, we declare all of them in a single statement by separating their identifiers with commas for example: int a, b, c;

Cont’d… 33 This has exactly the same meaning as: int a; int b; int c; The integer data types char, short, long and int can be either signed or unsigned depending on the range of number needed to be represented. Signed types can represent both negative and positive values, whereas unsigned types can only represent positive values (and zero). This can be specified by using either the specifier signed or the specifier unsigned before the type name. For example: unsigned short int NumberOfSisters; signed int MyAccountBalance

Cont’d… 34 By default, if we do not specify either signed or unsigned most compiler settings will assume the type to be signed hence instead of the second declaration above we could have written int MyAccountBalance; with exactly the same meaning (with or without the keyword signed). An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed or unsigned if you intend to store numerical values in a char-sized variable.

Cont’d… 35 short and long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types: short is equivalent to short int and long is equivalent to long int. The following two variable declarations are equivalent: short year; short int year; Finally, signed and unsigned may also be used as standalone type specifiers, meaning the same as signed int and unsigned int respectively. The following two declarations are equivalent: unsigned NextYear; unsigned int NextYear; To see what variable declarations look like in action within a program, we are going to see the C code of the example about your mental memory proposed at the beginning of this section:

Cont’d… 36 // operating with variables #include <stdio.h> main () { // declaring variables: int a, b; int result; // process: a = 5; b = 2; a = a + 1; result = a - b; // print out the result: printf ( “Result = %d”, result); // terminate the program: return 0; }

Cont’d… 38 A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block. Note: Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration. The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa.

Initialization of Variables 39 When declaring a regular local variable, its value is by default undetermined. But you may want a variable to store a concrete value at the same moment that it is declared. In order to do that, you can initialize the variable. There are two ways to do this in C: 1) type identifier = initial_value ; e.g. int a = 0; 2) type identifier (initial_value) ; e.g. int a(0); Both ways are valid and equivalent in C programming.

Cont’d… 40 // initialization of variables #include <stdio.h> main () { int a=5; // initial value = 5 int b(2); // initial value = 2 int result; // initial value undetermined a = a + 3; result = a - b; printf (“Result = %d”, result) return 0; }

Scanf ();  Now that we have mastered the intricacies of printf you should find scanf very easy. The scanf function works in much the same way as the printf. That is it has the general form:  scanf (control string,variable,variable,...)  In this case the control string specifies how strings of characters, usually typed on the keyboard, should be converted into values and stored in the listed variables. However there are a number of important differences as well as similarities between scanf and printf.  The most obvious is that scanf has to change the values stored in the parts of computers memory that is associated with parameters (variables). 41

scanf("%d,&a);  #include <stdio.h> main() { int a,b,c; printf("nThe first number is "); scanf("%d",&a); printf("The second number is "); scanf("%d",&b); c=a+b; printf("The answer is %d n",c); } 42

Cont.  The first instruction declares three integer variables: a, b and c. The first two printf statements simply display message on the screen asking the user for the values. The scanf functions then read in the values from the keyboard into a and b. These are added together and the result in c is displayed on the screen with a suitable message. Notice the way that you can include a message in the printf statement along with the value. 43

OPERATORS & EXPRESSIONS 1) Assignment Operators 2) Arithmetic operators. 3) Compound Assignment. 4) Increase and Decrease. 5) Relational and equality operators. 6) Logical Operators. 7) Conditional. 8) Comma. 9) SizeOf () *. 10) Precedence of Operators. 11) Basic Input-Output 44

Cont’d… Operators Once we know of the existence of variables and constants, we can begin to operate with them. For that purpose, C integrates operators. Unlike other languages whose operators are mainly keywords, operators in C are mostly made of signs that are not part of the alphabet but are available in all keyboards. This makes C code shorter and more international, since it relies less on English words, but requires a little of learning effort in the beginning. 45

The Fundamental C-Operators 1) Assignment Operator (=) The assignment operator assign a value to a variable. a=5; This statement assigns a value 5 to the variable a the part at the left of the assignment (=) operator is called the left value and the right one is called the right value. Other valid assignment expressions include; a) a=b; b) b=5; a=2+b; c) a=b=c=5; d) a=2+(b=5); 46 Same as

Cont’d… 2) Arithmetic Operators (+, -, /, %, *). a=11%3; 3) Compound Assignment (+=, -=, %=, /=, *=, &=, ^=, | =) Syntax:- Value += increase [value= value + increase] a+=b; is the same as: a=a+b; a-=5; …… a=a-5; a/=b; …… a=a/b; price*= unit +1;…. price= price*(unit+1); 47

Example of using compound assignment operators // compound assignment operators #include <stdio.h> #include <conio.h> main () { int a, b=3; a = b; a+=2; // equivalent to a=a+2 printf (“a= %d”, a); return 0; } 48

Cont’d… 4) increase and decrease (++, --) Examples: a++; a=a+1; b--; b=b-1; Given the following expressions determine the final values of a & b: b=3; b=3; a=++b; a=b++; a=4,b=4 a=3, b=4 Find out how this come about???? 49

Cont’d… 5) Relational and equality operators (==, !=, >, <, >=, <=) Some examples (7 == 5) // evaluates to false. (5 > 4) // evaluates to true. (3 != 2) // evaluates to true. (6 >= 6) // evaluates to true. (5 < 5) // evaluates to false 50

Cont’d… 6) Logical Operators C has the following logical operators, they compare or evaluate logical and relational expressions. Operator Meaning && Logical AND || Logical OR ! Logical NOT 51

Cont’d… Examples Logical AND (&&) a > b && x = = 10 The expression to the left is a > b and that on the right is x == 10 the whole expression is true only if both expressions are true i.e., if a is greater than b and x is equal to 10. Logical OR (||) The logical OR is used to combine 2 expressions or the condition evaluates to true if any one of the 2 expressions is true. Example a < m || a < n The expression evaluates to true if any one of them is true or if both of them are true. It evaluates to true if a is less than either m or n and when a is less than both m and n. 52

Cont’d… Logical NOT (!) The logical not operator takes single expression and evaluates to true if the expression is false and evaluates to false if the expression is true. In other words it just reverses the value of the expression. For example ! (x >= y) the NOT expression evaluates to true only if the value of x is neither greater than or equal to y. 53

Cont’d… 7) Conditional operators (?) The conditional operator evaluates an expression returning a value if that expression is true and a different one if an expression is evaluated as false. Its format is: condition? result1 : result2 If condition is true the expression will return result1, if it is not it will return result2. Examples,  7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5.  7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2.  5>3 ? a : b // returns the value of a, since 5 is greater than 3.  a>b ? a : b // returns whichever is greater, a or b. 54

Example // conditional operator #include <stdio.h> #include <conio.h> main () { int a,b,c; a=2; b=7; c = (a>b) ? a : b; printf (“c =%d”, c); return 0; } 55

Cont’d… In this example a was 2 and b was 7, so the expression being evaluated (a>b) was not true, thus the first value specified after the question mark was discarded in favor of the second value (the one after the colon) which was b, with a value of 7. 8) Comma operator ( , ) The comma operator (,) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered. For example, the following code: a = (b=3, b+2); 56

Cont’d… Grouping defines the precedence order in which operators are evaluated in the case that there are several operators of the same level in an expression. All these precedence levels for operators can be manipulated or become more legible by removing possible ambiguities using parentheses signs ( and ), as in this example: a=5 + 7 % 2; may be written either as: a= 5 + (7%2); OR a= (5+7)%2; depending on the operation we want to perform. 58

Basic Input / Output Using the standard input and output library, we will be able to interact with the user by printing messages on the screen and getting the user's input from the keyboard. C uses a convenient abstraction called stream to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract characters to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentially. 59

Cont’d… #include <stdio.h> void main() { int min, x = 0; while(x <= 0){ printf("Input number : "); //input scanf("%d", &x); min = x; // we assume first given number is the smallest } while (x != 0) { printf("Input number : "); // input scanf("%d", &x); if(x > 0 && x < min ) min = x; } printf("Smallest given number is: %dn", min); // output } . 60

Cont’d… 61 #include <stdio.h> void main() { int min=0, x=1; // in first iteration x=1 while (x != 0) { printf("Input number : "); scanf("%d", &x); //after first iteration x becomes != 1 if (x > 0) { if ((min==0) || (x < min)) min = x; } } printf("Smallest given number is: %dn", min); }

Program to add two numbers: 62 #include<stdio.h> int main() { int a,b,c; printf (“Enter first value:”); scanf(“%d”,&a); printf (“n Enter second value:”); scanf(“%d”,&b); c=a + b; printf(“%d + %d = %d” ,a ,b, c); return 0; }

Program for arithmetic operators: #include<stdio.h> #include<conio.h> void main() { float a, b; printf(“Enter value for a :n”); scanf(“%f”, &a); printf(“Enter value for b :n”); scanf(“%f”, &b); printf(“Addition of a and b =%fn”, a+b); printf(“Subtraction of a and b =%fn”, a-b); printf(“Multiplication of a and b =%fn”, a*b); printf(“Division of a and b =%fn”, a/b); getch(); } 63

Assignment 64 Write a program to solve a quadratic equation say, X2 -2x+3=0 , your program should include the following conditions: 1) if (b2 -4ac) =0, Real and Equal Roots 2) if (b2 -4ac) <0, Complex Roots. 3) if (b2 -4ac) > 0 Real Roots

Constants 65 C allows you to declare constants. When you declare a constant it is a bit like a variable declaration except the value cannot be changed. The const keyword is to declare a constant, as shown below: int const a = 1; const int a =2; Note : 1) You can declare a value before or after the type. Choose one and stick to it. 2) It is usual to initialize a const with a value as it cannot get a value any other way.

Example 1 66 #include <stdio.h> #define TAXRATE 0.10 main() { float balance, tax; balance = 72.10; tax = balance * TAXRATE; printf(“The tax on %.2 is %.2n”, balance,tax); } Output:- The tax on 72.10 is 7.21

Example 2 69 #include <stdio.h> #define MAGIC 10 // define MAGIC = 10 int main(void) // void specifies return type of main i.e. nothing should be returned to the operating system. { int i, fact, quotient; while (i++ < 3) { printf(”Guess a factor of MAGIC larger than 1: "); scanf("%d”, &fact); quotient = MAGIC % fact; if (0 == quotient) printf(”You got it!n”); else printf(”Sorry, You missed it!n”); } return 0; }

C Language Keywords 70 The following names are reserved by the C language. Their meaning is already defined, and they cannot be re-defined to mean anything else. Other than these names, you can choose any names of reasonable length for variables, functions etc. The names must begin with a letter or underscore (letters are better), and then can contain letters, numbers and underscores.

Introduction to Strings 71 A string is a sequence of characters. Any sequence or set of characters defined within double quotation symbols is a constant string. In C it is required to do some meaningful operations on strings they are:  Reading string, displaying strings  Combining or concatenating strings  Copying one string to another.  Comparing string & checking whether they are equal.  Extraction of a portion of a string . Strings are stored in memory as ASCII codes of characters that make up the string appended with ‘0’(ASCII value of null). Normally each character is stored in one byte, successive characters are stored in successive bytes.

Initialization of Strings. The initialization of a string must the following form which is simpler to one dimension array. char month1[ ]={‘j’,’a’,’n’,’u’,’a’,’r’,’y’}; Then the string month is initializing to January. This is perfectly valid but C offers a special way to initialize strings. The above string can be initialized char month1[]=”January”; The characters of the string are enclosed within a part of double quotes. The compiler takes care of string enclosed within a pair of a double quotes. The compiler takes care of storing the ASCII codes of characters of the string in the memory and also stores the null terminator in the end.

Example 73 #include < stdio.h > #include<string.h> main() { char month[15]; printf (“Enter the string”); gets (month); printf (“The string entered is %s”, month); } Note: More about Strings and Functions used will be covered in the course.

Cont’d… 74 Readings: Read about how flow chart diagrams are used to model computer programs and all the various symbols used in modeling. References 1) C by example By K. Noel.

1_Introduction to C programming - Copy.ppt

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