C glossary I - a - c

auto :  
auto in c is a storage class specifier. Auto indicates the variable is a automatic. Which means it is created automatically and destroyed automatically. This variable has a block scope.  
This keyword is redundant.
All non-static local variables are auto by default.

#include <stdio.h> int main()                  { auto int a; int b; ---- }

Here both a and b are auto variables.

break:
This is used to break from a loop or a switch case. If used outside of loops and switches, you get a syntax error. 

  Note: 

1. If used in a nested loop, break breaks from inner most loop
2. break can not be used to break out of a function. For that one needs
   to use return statement instead.
3. If break is not used in switch statement at the end of each case, execution will continue to next case block also.

 

1  a=1; 2  while(a<10) 3  { 4     printf("%d ",a); 5      if(a==3) 6        break; 7      a++; 8  }

1 scanf("%d",&a); 2 switch(a%2) 3{ 4   case 1: printf("%d is even ",a); 5   case 0: printf("%d is odd ",a); 6 }

  In the first example break causes the loop to print 1 to 3 instead of 1 to 9. When a is 1, break stops the loop.
In the second example, as break is not used, if the number is even, the code segment prints both even and odd .e.g. 8 is even 8 is odd. To correct the error, one should use break; at the end of line number 4.

boolean  
Unfortunately C does not have boolean data type. Instead it treats all non-zero values as true and zero as false.       

int a=1; if(a)       printf("a is true");  a=0;   if(!a)    printf("a is false");

 Here if (a) checks the condition if a is zero or non-zero. Since a is non-zero, it is true and printf is executed and print a is true. 


Be careful about the conditions such as  
      if(a=2) 
This condition is always true for any value of a. Because expression inside brackets is assignment and its value is non-zero.

case :  
case is used for individual values inside a switch statement. If the switch expression matches any of the case values, the subsequent statements till a break are executed. If none of the case values are matching, then default statements are executed.   

   Note:

• case values must be constants or constant expressions
• case values must be integer
• Since character constants are treated as int, case values can also be
  character literals

------ switch(m+n) { case 3: printf("three");//valid           break;                               case 3+2: printf("five");//valid. constant expression                   break; case m: printf("n is zero");//invalid.                                //non-constant expression not allowed              break; case <0:printf("Negative");//invalid - relational op.s not allowed } --------

char: 
Is a datatype in C. char is stored using a single byte and stored internally as integer ASCII (or other code which machine uses to represent characters) code. For all practical purposes C treats char as a byte sized integer. In fact you can use char in place of int to save memory.    char can also be signed char or unsigned char. Default depends on the compiler

#include<stdio.h> int main() {   char c = 'A';   printf("%c %d \n",c,c);   int m=68;   c = m;   printf("%c",c);   c = '0';   m=c;   printf("c=%c m=%d\n",c,m);   return 0; }

#include<stdio.h> int main() {   unsigned char c;   for (c = 0;c<128;c++)   {     printf("c=%c ASCII code = %d\n",c,c);   }   return 0; }

This program prints A 65 D c=0 m=48 In line 7 c is assigned to an integer 68. Hence c will be 'D'. In line 10 m is assigned to c and m will be 48 which is ASCII code for 0. Second program prints the ASCII codes and corresponding characters for 0 to 127. continue: continue when used inside a loop, skips the current iteration of the loop and goes to next iteration. continue can only be used inside for, while or do-while statements.
 

#include<stdio.h> int main() { int i; for(i=1;i<100;i++) {  if(i%3==0)          continue;  printf("%d\t",i); }   return 0;  }

Program given above prints all integers from 1 to 100 excluding numbers divisible by 3. When i%3 is 0, next statements are skipped and loop will continue for next value of i