Overview of C

Problem Solving and Program Design in C

Overview of C

2.1 C Language Elements

              One advantage of C is that it lets you write programs that resemble everyday

English. Even though you do not yet know how to write your own programs, you

can probably read and understand the program.

Function main

         The two-line heading = int and main(void)

marks the beginning of the main function where program execution begins. Every

C program has a main function. The remaining lines of the program form the body

of the function which is enclosed in braces {, } .

SYNTAX: int

main(void)

{

function body

}

2.2 Variable Declarations and Data Types

Variable Declarations

            The memory cells used for storing a program’s input data and its computational

results are called variables because the values stored in variables can change

(and usually do) as the program executes. The variable declarations in a C

program communicate to the C compiler the names of all variables used in a program.

Syntax Display for Declarations

SYNTAX: int variable_list ;

double variable_list ;

char variable_list ;

EXAMPLES: int count,

large;

double x, y, z;

char first_initial;

char ans;

Data Types

            A data type is a set of values and a set of operations on those values. Knowledge

of the data type of an item (a variable or value) enables the C compiler to correctly

specify operations on that item

            Data Type int In mathematics, integers are whole numbers. The int data type

is used to represent integers in C.

            Data Type double A real number has an integral part and a fractional part that

are separated by a decimal point. In C, the data type double is used to represent

real numbers.

2.3 Executable Statements

 The executable statements follow the declarations in a function. They are the C

statements used to write or code the algorithm and its refinements. The C compiler

translates the executable statements into machine language; the computer executes

the machine language version of these statements when we run the program.

Assignment Statements

An assignment statement stores a value or a computational result in a variable,

and is used to perform most arithmetic operations in a program.

2.4 General Form of a C Program

Now that we have discussed the individual statements that can appear in C programs,

we review the rules for combining them into programs. We also discuss the

use of punctuation, spacing, and comments in a program.

Program Style Spaces in Programs

The consistent and careful use of blank spaces can improve the style of a program.

A blank space is required between consecutive words in a program line.

The compiler ignores extra blanks between words and symbols, but you may

insert space to improve the readability and style of a program

Program Style Using Comments

Each program should begin with a header section that consists of a series of comments

specifying

■ the programmer’s name

■ the date of the current version

■ a brief description of what the program does

2.5 Arithmetic Expressions

To solve most programming problems, you will need to write arithmetic expressions

that manipulate type int and double data. This section describes the operators

used in arithmetic expressions, and rules for writing and evaluating the expressions.

Operators / and %

When applied to two positive integers, the division operator ( / ) computes the integral

part of the result of dividing its first operand by its second.

Characters as Integers

Since characters are represented by integer codes, C permits conversion of type

char to type int and vice versa. For example, you could use the following to find

out the code your implementation uses for a question mark:

qmark_code = (int)'?';

printf("Code for ? = %d\n", qmark_code);

2.6 Formatting Numbers in Program Output

C displays all numbers in its default notation unless you instruct it to do otherwise.

This section explains how to specify the format or appearance of your output.

Formatting Values of Type int  Specifying the format of an integer value displayed by a C program is fairly easy.

Formatting Values of Type double To describe the format specification for a type double value, we must indicate both

the total field width needed and the number of decimal places desired.

2.7 Interactive Mode, Batch Mode, and Data Files

        There are two basic modes of computer operation: batch mode and interactive

mode. The programs that we have written so far run in interactive mode. In interactive

mode , the program user interacts with the program and types in data while

it is running.

Input Redirection

The miles-to-kilometers conversion program rewritten as a batch

program. We assume here that the standard input device is associated with a batch

data file instead of with the keyboard. In most systems, this association can be

accomplished relatively easily through input/output redirection using operating system

commands.

Output Redirection

You can also redirect program output to a disk file instead of to the screen. Then

you can send the output file to the printer (using an operating system command) to

obtain a hard copy of the program output.

2.8 Common Programming Errors

As you begin to program, soon you will discover that a program rarely runs correctly

the first time it executes. Murphy’s Law, “If something can go wrong, it will,”

seems to have been written with the computer program in mind. In fact, errors

are so common that they have their own special name— bugs —and the process of

correcting them is called debugging a program.

Syntax Errors

A syntax error occurs when your code violates one or more grammar rules of

C and is detected by the compiler as it attempts to translate your program. If a

statement has a syntax error, it cannot be translated and your program will not be

executed.

Run-Time Errors

Run-time errors are detected and displayed by the computer during the execution

of a program. A run-time error occurs when the program directs the computer to

perform an illegal operation, such as dividing a number by zero. When a run-time

error occurs, the computer will stop executing your program and will display a diagnostic

message that indicates the line where the error was detected.

Undetected Errors

Many execution errors may not prevent a C program from running to completion,

but they may simply lead to incorrect results. Therefore, it is essential that you

predict the results your program should produce and verify that the actual output

is correct.

Logic Errors

Logic errors occur when a program follows a faulty algorithm. Because logic

errors usually do not cause run-time errors and do not display error messages,

they are very difficult to detect. The only sign of a logic error may be incorrect

program output. You can detect logic errors by testing the program thoroughly,

comparing its output to calculated results. You can prevent logic errors by carefully

desk checking the algorithm and the program before you type it in.

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