The concept of C language array

stay 《C Large summary of language data output and lightweight advanced 》 In this section we give an example , It's the output of a 4×4 The integer matrix of , The code is as follows ：

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int a1=20, a2=345, a3=700, a4=22;
    int b1=56720, b2=9999, b3=20098, b4=2;
    int c1=233, c2=205, c3=1, c4=6666;
    int d1=34, d2=0, d3=23, d4=23006783;
    printf("%-9d %-9d %-9d %-9d\n", a1, a2, a3, a4);
    printf("%-9d %-9d %-9d %-9d\n", b1, b2, b3, b4);
    printf("%-9d %-9d %-9d %-9d\n", c1, c2, c3, c4);
    printf("%-9d %-9d %-9d %-9d\n", d1, d2, d3, d4);
    system("pause");
    return 0;
}

Running results ：

20	345	700	22
56720	999	20098	2
233	205	1	6666
34	0	23	23006783

Matrices share 16 It's an integer , We define a variable for each integer , That is to say 16 A variable . that , To reduce the number of variables , Make development more efficient , Can you define a variable for multiple data ？ such as , Put the integer of each line in a variable , Or the 16 All integers are put in one variable . The answer, of course, is yes , The way is to use arrays （Array）.

The concept and definition of array

We know , To put data into memory , You must allocate memory space first . Put in 4 It's an integer , You have to allocate 4 individual int Type of memory space ：

int a[4];

such , Just allocate it in memory 4 individual int Type of memory space , common 4×4=16 Bytes , And gave them a name , It's called a.
We call such a set of data an array （Array）, Every piece of data it contains is called an array element （Element）, The number of data contained is called array length （Length）, for example int a[4]; It defines a length of 4 Integer array , The name is a.
Each element in the array has a sequence number , This number comes from 0 Start , Not from the familiar 1 Start , It's called subscript （Index）. When using array elements , Indicate the subscript , In the form of ：

arrayName[index]

arrayName Is the array name ,index Subscript . for example ,a[0] It means the first one 0 Elements ,a[3] It means the first one 3 Elements .
Next, let's take the first line 4 Put an integer into the array ：

a[0]=20;
a[1]=345;
a[2]=700;
a[3]=22;

there 0、1、2、3 Is array subscript ,a[0]、a[1]、a[2]、a[3] It's an array element .
In the learning process , We often use circular structures to put data into arrays （ That is, assign values to array elements one by one ）, Then use the loop structure to output （ That is, read the values of array elements in turn ）, Now let's demonstrate how to 1~10 Put these ten numbers into the array ：

#include <stdio.h>
int main(){
    int nums[10];
    int i;
   
    // take 1~10 Put it in an array 
    for(i=0; i<10; i++){
        nums[i] = (i+1);
    }
   
    // Output the array elements in turn 
    for(i=0; i<10; i++){
        printf("%d ", nums[i]);
    }
   
    return 0;
}

Running results ：

1 2 3 4 5 6 7 8 9 10 

Variable i Is an array subscript , It's also a cyclic condition ; Use array subscripts as loop conditions , When the last element is reached, the loop ends . Array nums The largest subscript of is 9, That is, no more than 10, So we specify that the condition of the cycle is i<10, once i achieve 10 You have to end the cycle .
Change the code above , Let the user type 10 A number and put it into the array ：

#include <stdio.h>
int main(){
    int nums[10];
    int i;
   
    // Read user input from the console 
    for(i=0; i<10; i++){
        scanf("%d", &nums[i]);  // Pay attention to the address character  &, Don't forget 
    }
   
    // Output the array elements in turn 
    for(i=0; i<10; i++){
        printf("%d ", nums[i]);
    }
   
    return 0;
}

Running results ：

22 18 928 5 4 82 30 10 666 888
22 18 928 5 4 82 30 10 666 888 

The first 8 Line code ,scanf() An address is required to read data （ Addresses are used to indicate where data is stored ）, and nums[i] Represents a specific array element , So we need to add... To the front & To get the address .
Finally, let's summarize how arrays are defined ：

dataType  arrayName[length];

dataType Is the data type ,arrayName Is the array name ,length Is array length . for example ：

float m[12];  // Define a length of  12  Floating point array 
char ch[9];  // Define a length of  9  Character array of 

It should be noted that ：
1) The data type of each element in the array must be the same , about int a[4];, Each element must be int.
2) The length of the array length  Preferably an integer or constant expression , for example 10、20*4 etc. , In this way, it can run under all compilers ; If length Contains variables , for example n、4*m etc. , In some compilers, errors will be reported ,
3) When accessing array elements , The value range of subscript is 0 ≤ index < length, Too big or too small will cross the border , Cause array overflow , Something unpredictable happens , Please pay attention to the big housework .

Array memory is continuous

An array is a whole , Its memory is continuous ; in other words , Array elements are next to each other , There is no gap between each other . The following figure illustrates int a[4]; Storage in memory ：

「 Array memory is continuous 」 This is important , So I used a headline to emphasize . Continuous memory for pointer operations （ Access array elements through pointers ） And memory processing （ Copy of whole memory 、 Write, etc ） Provides convenience , This allows the array to act as a cache （ A block of memory that temporarily stores data ） Use . You may not understand what this sentence means for the moment , After learning the pointer and memory, you will naturally understand .

Initialization of an array

The above code defines the array first and then assigns a value to the array , We can also assign values while defining an array , for example ：

int a[4] = {20, 345, 700, 22};

The value of the array element is determined by { } Surround , The values are separated by , Separate .

For the initialization of array, we need to pay attention to the following points ：
1) You can assign values to just a few elements . When { } When the number of median is less than the number of elements , Assign values to only the preceding elements . for example ：

int a[10]={12, 19, 22 , 993, 344};

To give only to a[0]~a[4] 5 Element assignment , And the back 5 Elements are automatically initialized to 0.

When the assigned elements are less than the total elements of the array , The remaining elements are automatically initialized to 0：

• about short、int、long, It's integer. 0;
• about char, It's the characters '\0';
• about float、double, It's a decimal 0.0.

We can initialize all the elements of the array to 0：

int nums[10] = {0};
char str[10] = {0};
float scores[10] = {0.0};

Because the remaining elements are automatically initialized to 0, So just give it to the third party 0 The first element is assigned to 0 that will do .

2) You can only assign values to elements one by one , You can't assign values to an array as a whole . For example, to 10 All elements are assigned to 1, I can only write ：

int a[10] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};

Instead of writing ：

int a[10] = 1;

3) Such as assigning values to all elements , So when defining an array, you can not give the length of the array . for example ：

int a[] = {1, 2, 3, 4, 5};

Equivalent to

int a[5] = {1, 2, 3, 4, 5};

Last , We use arrays to output a 4×4 Matrix ：

#include <stdio.h>
int main()
{
    int a[4] = {20, 345, 700, 22};
    int b[4] = {56720, 9999, 20098, 2};
    int c[4] = {233, 205, 1, 6666};
    int d[4] = {34, 0, 23, 23006783};
    printf("%-9d %-9d %-9d %-9d\n", a[0], a[1], a[2], a[3]);
    printf("%-9d %-9d %-9d %-9d\n", b[0], b[1], b[2], b[3]);
    printf("%-9d %-9d %-9d %-9d\n", c[0], c[1], c[2], c[3]);
    printf("%-9d %-9d %-9d %-9d\n", d[0], d[1], d[2], d[3]);
    return 0;
}

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