Objective:

• Compile and run an existing C program
• Examine the basic structure of a C program
• Look at some data types
• Use some input/output (I/O) functions
• Look at the if/else statement
• Look at the while statement
• Look at program flow by using the for statement.
• break and continue
• Logical operators.

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    Getting Started

• mkdir ~/programs
• Download the examples to your computer c1.tar
• tar -xvf c1.tar
• vi ex1.c

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Compiling and Running

• Compiling: Try the command gcc ex1.c -o ex1 -lm the compliler would have made an executable called ex1 Let's look at the results.
• Runing: Try ./ex1, you will get the output of your code. You can use Ctrl-C to force the program to exit.

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The Program.

ex1.c
/* ..... */(comment everything between /* and */ ) or // ..... (comment the line beginning with //)

These are the comments. Anything between /* .... */ or after // will be ignored by the compiler.
    Comments in a program are extremely important. One reason is that they allow other people to be able to follow the "flow" of your program.

#include <stdio.h>
#include <math.h>

    These are preprocessor directives. Preprocessing occurs before a program is compiled. This line tells the preprocessor to include the contents of the header files stdio.h and math.h. These header files contain information about the functions used in the program like printf() and exp(). The #include preprocessor directive is normally used to include standard library header files such as stdio.h and stdlib.h .

int main()
        { .... }

    All C programs need a  main function where excution begins. The parentheses after main indicate that it is a function. It defines a function main that takes no arguments and returns an integer value. The left brace { must begin the body of every function and the right brace } must end each function. The program between the braces is called a block.

C programs stop when

• The end of main is reached
• An exit() call is reached
• The program is interrupted in some way
• The program crashes

Basic Data Types and Declarations

float a,b;
int i,n;

    These are variable declarations. These two lines define two different types of variables: reals (float) and integers (int).  Some simple data types are listed below.



Basic I/O

printf(" ..... ");
scanf("%d",&n);

    These are two basic IO functions. (Remeber that they are functions becuase they have ()).  The function definitions for printf and scanf are kept in the header file stdio.h. The printf function prints what is ever between the qoutation marks to the screen. The scanf function takes input from the keyboard. Note that scanf takes two arguments: "%d" and &n . The first argument is called a format control string, which contains the conversion specifer %d, and it tells scanf what to expect to receive, which in this case is an integer. The second argument tells scanf to store the value received into the memory location where the variable n is located.

Arithmetic Operators

ex2.c
The binary arithmetic operators are   +, -, *, /, and the modulus operator %, see wikibooks for more details.

Increment and Decrement Operators

C provides two unusual operators for incrementing and decrementing variables. The increment operator ++ adds 1 to its operand; the decrement operator -- subtracts 1. As will be seen in for loop statement.

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if/else

ex3.c
The if selection structure performs an indicated action only when the condition is true.

Example:
               if (grade >= 60)
            printf("Passed\n");

The if/else allows an action to be performed when the condition is true and an alternate statement to be performed when the condition is false.

Example:

            if (grade >= 60)
          printf("Passed\n");
      else
          printf("Failed\n");

To test for multiple cases you can use nested if/else structures. (Also look at the switch/case structure)

 Example:

           if (grade >= 90)
         printf("A\n");
     else if (grade >= 80)
         printf("B\n");
     else if (grade >= 70)
         printf("C\n");
           else if (grade >= 60)
         printf("D\n");
     else
         printf("F\n");

You can also include multiple statements within the if/else structure with the left and right brace {}.

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For loops

ex4.c
for(i=0;i<=n;i++)
{
  ...statements..
}
    The for statement. This is used to execute code iteratively.
    i=0 is the initialization.
    i<=n is the condition which if it is true will execute the statements in the body of the for loop.
    i++ is executed after the statements are done. i++ increments the value of i by 1.

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While loops.

The while is a repetition structure (much like the for). The while will keep executing its statements until the condition becomes false.

Example: Consider a program segment designed to find the first power of 2 larger than 1000:

            product = 2;

     while (product <= 1000)
         product = 2 * product;

The condition in this example is product <= 1000 and the statement is product = 2 * product;. The program will not even consider the statements in a while if the condition if initially false.

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Conditions and Logical Operators

So far we have seen some equality and relational operators ( >, <, <=, >=, == and !=) to create simple conditions in if/else and while structures. In C we can combine multiple simple conditions into a more complex condition by using logical operators. The logical operators are && (logical AND), || (logical OR),  and ! (logical NOT or negation).

Examples:

1)
    if (gender == 0 && age >= 65)
      seniorMales++;

2)
     if (semesterAverage >= 90 || finalExam >= 90)
      printf("Student grade is A\n");

3)
     if (!(grade == sentinelValue))
      printf("The next grade if %f\n");

(Note: Don't confuse the assignment operator = with the equality operator ==.)

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break and continue.

ex5.c
The break and continue statements are used to alter the flow of control. The break causes immediate exit from a for, while, do/while, or switch statement. The continue statement when executed in a while, for, or do/while causes all other statements in that control statement to be skipped and the next iteration to be performed.

Examples:

1)
    for (x = 1; x<=10; x++) {
      if (x == 5)
          break;

      printf("%d ",x);
  }

2)
    for (x = 1; x <= 10; x++) {
      if (x == 5)
          continue;

      printf("%d ",x);
  }

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A Sample Program

ex6.c


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Note: These web pages were written with the reference text The C Programming Language by Brian W.Kernighan & Dennis M. Ritchie