Operator-relevant

sizeof

From the C99 Standard

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

Example:

The result is 10.

    int i=10;
    long long t = sizeof(i++);
    printf("%d", i);

By the way, sizeof(x) is used to calculate how much memory x takes up. If x is an integer array whose elements are all int, sizeof(x)/sizeof(x[0]) returns the length of the array. However, if array is passed in as a function parameter, you can do it in this function body to it.

comma ,

The comma operator in c comes with the lowest precedence, and has left-to-right associativity.

Example:

The result is 9 1 20.

    int i,x,y;	
    i=x=y=0;
    do {
        ++i;
        if ( i%2 ) 
            x+=i, 
        i++;
        y +=i++;
    } while ( i<=7 );
    printf("%d %d %d", i, x, y);

&&

AND operator is left-to-right, if left expression is false, the right ones will not be executed.

Example:

The result is 0.

    int x=0, y=0, z=0;
    z = (x==1) && (y=2);
    printf("%d ", y);

Data type

char

Remember, char is signed!

char is used for variables that will store only one character. It always takes up 1 byte(8 bits) of memory, which means the range of values they can store is limited to 8 bits of information(-128 - 127).

Example:

The result is -1.

    char ch = -1;
	printf("%d\n", ch);

Control flow

if else

else should follow after if. In the code below, there are two expressions (the second one is empty) not included in curly braces. So the nearest expression before else is an empty expression instead of if expression, which will raise compiling error.

    if ( i<= 6 ) 
		printf("hello\n");;
	else
		printf("bye-bye\n");;

Array

Initialization

int my_array[10] = {2}; will initialize the elements at index 1-9 to be 0 by default.

More fancy way in C99, you can do int my_array[10] = {[1]=2, [5]=3} to assign 2 to second element, assign 3 to the sixth element, and the rest will be 0.

Number system

Octal number system

When you see an integer variable starting with 0, it’s an octal number, which has its base as ‘eight’.

Example:

The result is 10.

    int x=1, y=012;
    printf(“%d”,y*x++);

compilation

#include/#define is instruction of compiler, not a part of C language.

References

1.-.