C language tutorial (2) - printf and data types that come with c

Table of Contents

printf

Data Type

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printf

printf outputs the content to stdout (standard output, the final destination is the display), which can be used like this:

printf("hello");printf("hello %s", "xxx");printf("the var va is %d\n", va); //\n is a newline // is a comment

More help information can be entered

man 3 printf

If you want to use printf, you need to write it at the top of the code

#include

#include is a preprocessing command. During preprocessing, copy and paste the content of the file in <> to this location

Where can I find the files in

<>?

In linux, the files in <> are in

/usr/local/include/usr/lib/gcc/i386-redhat-linux/4.1.2/include/usr/include...

Look inside.

While #include "" is preprocessing, copy and paste the contents of the file in "" to this location, first search in the current directory, if not found, then go to the place where <> was found.

Data Type

The data type is the access rule to the address space

How computers store data

Computers can only store binary data, namely 0s and 1s.

For signed numbers, the most significant bit is the sign bit, 0 is positive and 1 is negative.In computers, storing positive numbers is directly converted to binary.To store a negative number, you need to first convert the absolute value of the number to binary, then invert the bits, and finally add 1.

For unsigned numbers, there is no sign bit, and it can be directly converted to binary.

Therefore, if an unsigned number 6 is subtracted from a signed number -20, it is a binary number

0000 0110

With binary numbers

1110 1100

Subtract, the result is

0001 1010

If this number is viewed according to the rules of signed numbers, it is 26 in decimal.

This stuff is often tested in interview questions.

sizeof

The sizeof operator can know the number of bytes accessed by a data type or the number of bytes of the address space occupied by a variable. For example, if you want to know the number of bytes accessed by an int type, you can write the following code

sizeof(int)

int type

The int type accesses 4 bytes

short type

short type accesses 2 bytes

char type

Char type access 1 byte

float type

float type accesses 4 bytes, which is the type of decimal

double type

The double type accesses 8 bytes and is a decimal type

Variable Definition

Data type + variable name; a variable is defined

Also available

Data type + variable name = value; initialize variable

For example, define the int variable va, which is initialized to 1.

int va=1;

Variable usage

You can use the variable name directly.For example, output the value of va

printf("va=%d\n", va);

Pointer

A pointer is actually a data type

Let's look at this code

int *p;

* indicates that p is a pointer type.A pointer type variable stores an address.int means that when accessing the address in the variable of the pointer type, the access rules of int are followed.

So, how to get the address of the specified variable?

&+variable can get the address of the specified variable.

For example, to create a pointer type variable, the address of the variable va is stored in the pointer type variable, and the access rules of int are followed when accessing the address in the pointer type variable.The code can be written as

int *p=&va;

Read the contents of the address stored by p, use *p;

printf("va=%d\n", *p);

C language has an interview question called distinguishing whether a computer is big endian or little endian.

The big endian mode means that the high byte of the data is stored in the low address of the address space, and the low byte of the data is stored in the high address of the address space

Little-endian mode means that the high byte of the data is stored in the high address of the address space, and the low byte of the data is stored in the low address of the address space

How to tell?code is

#includeint main(void){short a=0x01;//0x.. is a hexadecimal number, 0x01 is written as a short type is 00000000 00000001char *c=&a;//When accessing the address in the variable, follow the access rules of char, if it is big endian, then *c=0if((*c)==0){//if usage: if(expression){code}.If the expression is not 0, the code in {} is executed.==, if the left and right sides are not equal to 0printf("big\n");}else{//else follows the if, using the method: else{code}.If the expression of if is 0, execute the code in {}printf("small\n");}return 0;}

At this point, we have learned almost the data types that come with the C language.Next we should learn about user-defined data types