C language portal - Array

October Today morning to see the parade passion, but in the afternoon or to continue to write C language, in front of this very simple

int number[100];

scanf("%d" , &number[i]);

First, the definition of an array

1. <Type> variable name [number of elements]
2. The number of elements must be an integer

Array Features

1. All elements have the same data type
2. Once created, the size can not be changed
3. (Memory elements in the array are ordered in this continuous)

Second, the array example

Write a program, an unspecified number of input [0,9] in the range of integer count the number of times each number appears, enter -1 for the end

Arrays are usually used the following steps:

#include <stdio.h>

int main(void)
{

    // 数组的大小
    const int number = 10;
    int x;
    // 定义数组
    int count[number];
    int i;

    // 初始化数组
    for (i = 0; i < number; i++)
    {
        count[i] = 0;
    }

    scanf("%d" , &x);
    while( x != -1){
        if(x >= 0 && x <= 9){
            // 数组参与运算
            count[x] ++;
        }
        scanf("%d" , &x);
    }

    // 遍历数组输出
    for (i = 0; i < number; i++)
    {
        printf("%d:%d\n", i , count[i]);
    }

    return 0;
}

Third, the array operation

Integrated array initialization

int a[] = {2,4,6,7,1,3,5,9,11,13,23,14,32};

Integrated positioning initialization

    int a[10] = {[0] = 2 , [2] = 3,6};

    int i;
    for (i = 0; i < 10; ++i)
    {
        printf("%d\t", a[i]);
    }
    // 2       0       3       6       0       0       0       0       0       0

1. With [n] is given in the initialized data positioned
2. No data is positioned behind the contact position in front of
3. Value of zero padding other locations
4. May not be given array size, let the compiler count
5. Particularly suitable for the initial data sparse array

Array size

1. sizeof gives the entire contents of the array occupied by the size in bytes

sizeof(a)/sizeof(a[0]);

1. sizeof (a [0]) gives the size of a single element of the array, then divided by the number of cells is obtained array
2. Such code, once the initial array data to modify, without modifying the code traversal

Array-valued

1. Array variable itself can not be assigned
2. Put all the elements of an array to another array, you must traverse

Fourth, look for the array elements

When the array as a function parameter, then another parameter often must be passed in the array size

When the array as a parameter:

1. Size of the array can not be given in []
2. The number of elements can not be calculated using the array sizeof

#include <stdio.h>

int main(void)
{
    int a[] = {2,4,6,7,1,3,5,9,11,13,23,14,32,};
    int x;
    int loc;
    printf("请输入一个数字：\n");
    scanf("%d" , &x);


    loc = search(x, a, sizeof(a)/sizeof(a[0]));
    if (loc != -1)
    {
        printf("%d在第%d个位置上\n", x , loc);
    }else{
        printf("%d不存在\n", x);
    }
    return 0;
}

int search(int key , int a[] , int length)
{
    int ret = -1;
    int i;
    for (i = 0; i < length; i++)
    {
        if (a[i] == key)
        {
            ret = i;
            break;
        }
    }
    return ret;
}

Five, prime

Can be determined whether the number is divisible known and prime <x of

#include <stdio.h>

int main(void)
{
    const int number = 10;
    int prime[10] = {2};
    int count = 1;
    int i = 3;
    while(count < number){
        if (isPrime(i,prime,count))
        {
            prime[count++] = i;
        }

        // 进行调试
        {
            printf("i=%d \tcnt=%d\t", i , count );
            int i;
            for (i = 0; i < number; i++)
            {
                printf("%d\t", prime[i]);
            }
            printf("\n");
        }
        i++;
    }

    for ( i = 0; i < number; i++)
    {
        printf("%d", prime[i]);
        if ( (i+1)%5)
        {
            printf("\t");
        }else{
            printf("\n");
        }
    }
    return 0;
}


int isPrime(int x, int knownPrimes[], int numberofKnowPrimes)
{
    int ret = 1;
    int i;
    for (i = 0; i <numberofKnowPrimes ; i++)
    {
        if ( x % knownPrimes[i] == 0)
        {
            ret = 0;
            break;
        }
    }
    return ret;
}

Six, two-dimensional array

int a[3][5]
// 通常可以理解为a是一个3行5列的矩阵

Traversing the two-dimensional array

for(i = 0; i<3; i++){
    for(j = 0; j<5; j++){
        a[i][j] = i * j;
    }
}

// a[i][j]是一个int，表示第i行第j列上的单元

Two-dimensional array initialization

int a[][5] = {
    {0,1,2,3,4,},
    {2,3,4,5,6,},
};

1. The number of columns is to be given, the number of rows may have to set compiler
2. {} Per line, separated by commas
3. The last comma can exist
4. If omitted, showing zero padding