Lesson 4
C Statements
1. Structure of a C program
2. Errors in a C program
3. Simple and Compound statements
4. Input /Output statements
5. Formatted I/O
6. Non-Formatted I/O
7. Storage Classes
structure of a c program
Before we study the basic building blocks of the C programming language, let us look at a bare minimum C program structure so that we can take it as a reference in the upcoming chapters.
Hello World Example
A C program basically consists of the following parts −
• Preprocessor Commands
• Functions
• Variables
• Statements & Expressions
• Comments
Let us look at a simple code that would print the words "Hello World" −
#include <stdio.h>
int main() {
/* my first program in C */
printf("Hello, World! \n");
return 0;
}
Let us take a look at the various parts of the above program −
· The first line of the program #include <stdio.h> is a preprocessor command, which tells a C compiler to include stdio.h file before going to actual compilation.
· The next line int main() is the main function where the program execution begins.
· The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program.
· The next line printf(...) is another function available in C which causes the message "Hello, World!" to be displayed on the screen.
· The next line return 0; terminates the main() function and returns the value 0.
Compile and Execute C Program
Let us see how to save the source code in a file, and how to compile and run it. Following are the simple steps −
· Open a text editor and add the above-mentioned code.
· Save the file as hello.c
· Open a command prompt and go to the directory where you have saved the file.
· Type gcc hello.c and press enter to compile your code.
· If there are no errors in your code, the command prompt will take you to the next line and would generate a.out executable file.
· Now, type a.out to execute your program.
· You will see the output "Hello World" printed on the screen.
$ gcc hello.c
$ ./a.out
Hello, World!
Make sure the gcc compiler is in your path and that you are running it in the directory containing the source file hello.c.
2. Errors in a C program
These errors occur because of wrongly typed statements, which are not according to the syntax or grammatical rules of the language. For example, in C, if you don’t place a semi-colon after the statement it results in a syntax error.
/* Syntatically wrong *
/ printf("Hello, World! \n")
/* Syntatically Correct */
printf("Hello, World! \n");
These errors are caught by compiler during compilation and can be fixed by correcting the syntax of the statement
3. Logical Error
These errors occur because of logically incorrect instructions in the program. Let us assume that in a program which was supposed to do addition of 2 numbers, it was wrongly written to perform subtraction. This logically incorrect instruction will produce wrong results. Detecting such errors is difficult as they only surface during run-time that too occasionally.
4.Simple and Compound Statements
A simple statement is one that does not contain another statement as a component. These statements are represented by capital letters A-Z. A compound statement contains at least one simple statement as a component, along with a logical operator, or connectives.
5.Input / Output Statements
C language has standard libraries that allow input and output in a program. The stdio.h or standard input output library in C that has methods for input and output.
scanf()
The scanf() method, in C, reads the value from the console as per the type specified.
Syntax:
scanf(“%X”, &variableOfXType);
printf()
The printf() method, in C, prints the value passed as the parameter to it, on the console screen.
Syntax:
printf(“%X”, variableOfXType);
6.Formatted I/O
Formatted console input/output functions are used to take one or more inputs from the user at console and it also allows us to display one or multiple values in the output to the user at the console. This function is used to read one or multiple inputs from the user at the console
7.Non-Formatted I/O.
Unformatted console input/output functions are used to read a single input from the user at console and it also allows us to display the value in the output to the user at the console. Reads a single character from the user at the console, without echoing it.
8. Storage Classes
Storage Classes are used to describe the features of a variable/function. These features basically include the scope, visibility and life-time which help us to trace the existence of a particular variable during the runtime of a program.C language uses 4 storage classes, namely.