#02 (06/04/2025)

Principles of C language

  1. A C program consists of a set of functions.
  2. A function in C is a code with the syntax: 
    type name(type var)
{
code.....
return value;
}
  3. A function must be defined before it is used.
  4. A function must return a value whose type must be declared (one of int, float, double, char). The last line of a function must be a return xx; statement where xx is a value to be returned upon exit.
  5. A function must take arguments and must have a placeholder () even if there is no argument.
  6. The content of a function must be enclosed by "{" and "}".
  7. A special function, int main(), is the one which is executed first (it is recommended that this function returns an integer value of 0).
  8. All the variables used within a function must be declared.
  9. It is important to distinguish different types of numbers (examples shown).

Skeleton C programs

int main()
{
return 0;
}

#include <stdio.h>
int main()
{
printf("Hello world\n");
return 0;
}

Another example

#include <stdio.h>
#include <math.h>
int main()
{
float x, y;

x = 6.28;

y=sin(x);

printf("Sine of %f is %f.\n", x, y);

return 0;
}

$ gcc -lm MyProgram.c; a.out

C variables

Type of variables

You must use the proper format in the printf and scanf functions.
Type Content Format Range Example
int integers %d −2147483647  ∼ +2147483647 10
float floating numbers %f ±2.9387e−39  ∼ ±1.7014e+38 3.14
double double precision floating %lf 2−63  ∼ 2+63 3.14159265358979
char characters %c ASCII code 'a'
/*
Print a character
*/
#include <stdio.h>
int main()
{
char a='h';
printf("%c\n",a);
return 0;
}

/*
Print an integer
*/
#include <stdio.h>
int main()
{
int a=10;
printf("%d\n",a);
return 0;
}

/* Print a floating number */
#include <stdio.h>
int main()
{
float a=10.5;
printf("%f\n",a);
return 0;
}

/* Print floating numbers */
#include <stdio.h>
int main()
{
float a, b=9.0, c;
a=10.0; c=-2.3;
printf("a = %f\n",a);
printf("c = %f\n",c);
return 0;
}

Input/Output

printf("format",argument);
Examples: 
printf("Hello world !\n");
printf("Two integers are %d and %d.\n",a,b);
printf("Two floating numbers are %f and %f.\n",a,b);
printf("Three floating numbers are %f, %f and %f.\n",a,b,c);
Compare the following two programs: 
#include <stdio.h>
int main()
{
int a, b;
printf("Enter two integers separated by a comma = ");
scanf("%d, %d",&a, &b);
printf("a=%d b=%d\n", a, b);
return 0;
}
#include <stdio.h>
int main()
{
int a; float b;
printf("Enter an integer and a real number separated by a space = ");
scanf("%d  %f",&a, &b);
printf("a=%d b=%f\n", a, b);
return 0;
}

Operators for variables

Symbol Meaning
< a < b ; a is less than b.
< = a < = b ; a is less than or equal to b.
> a > b ; a is greater than b.
> = a > = b; a is greater than or equal to b.
== a = = b ; a is equal to b.
!= a ! = b ; a is not equal to b.
if (a==b) printf("a and b are equal.\n"); else printf("a and b are not equal.\n");

Logical operators:

Symbol Meaning
&& And
|| Or
! Not 
if (a>0 && a<100) printf("a is between 0 and 100.\n");

if (a>0 || a<-5) printf("a is positive or less than -5.\n");

int a=20;
if (!(a==10)) printf("a is not equal to 10.");

if (a==10)
 { printf("You entered 10.\n");
   return 0;
 } 
else ....

Increment/decrement operators
Symbol Meaning
++ b=++a a is incremented by 1 and assigned to b. Same as a=a+1;b=a;
b=a++ a is assigned to b first and incremented by 1. Same as b=a;a=a+1;
− − b=− −a a is decremented by 1 and assigned to b. Same as a=a−1; b=a;
b=a− − a is assigned to b first and decremented by 1. Same as b=a;a=a−1;
i=100;
i++;
is the same as 
i=100;
i=i+1;
Substitution operators
Symbol Meaning
= a = b b is assigned to a.
+= a + = b a + b is assigned to a. Same as a=a+b;
−= a − = b a − b is assigned to a. Same as a=a−b;
*= a * = b a * b is assigned to a. Same as a=a*b;
/= a / = b a / b is assigned to a. Same as a=a/b;
%= a % = b Remainder of a / b is assigned to a. Same as a=a%b; 
i=i+1
i+=1
i++
++i
The difference between ++i and i++ is that the former pre-increments i before any operation while the latter post-increments i after the operation is done.

Control statements

The following are five statements that can control the flow of the program:

if Statement

#include <stdio.h>
int main()
{
int i;
printf("Enter an integer ");
scanf("%d",&i);

if (i>1 && i<100)
 printf("The number is between 1 and 100.\n");
else
 printf("The number is not in that range.\n");
return 0;
}


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On 03 Jun 2025, 13:27.