One-dimensional array
1. Array action: Arrays are often used to store data needed by programs.
2. Composition: Arrays consist of a series of elements of the same data type.
3. Array use: When an array is required, tell the compiler how many elements are contained in the array and what type of these elements are by declaring the array. The compiler correctly creates arrays based on this information. Types that can be used for ordinary variables, and array elements can be used.
/* Array declaration*/
int main(void)
{
float candy[365]; /* Array containing 365 float type elements */
char code[12]; /*Array containing 12 char type elements*/
int states[50]; /*Array containing 50 elements of type int */
...
}
//Square brackets ([]) indicate that candy, code, and states are arrays, and the numbers in square brackets indicate the number of elements in the array.4. How to access arrays: To access elements in an array, use the array subscripts (also known as indexes) to represent the elements in the array. The number of the array element starts at 0, so candy[0] represents the first element of the candy array, and candy[364] represents the 365 element, which is the last element.
5. Initialize Array:
int main(void)
{
int powers[8] = {1,2,4,6,8,16,32,64}; /* This initialization is supported starting with ANSI C */
...
}/* day_mon1.c -- Print days per month */
#include <stdio.h>
#define MONTHS 12
int main(void)
{
int days[MONTHS] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
int index;
for (index = 0; index < MONTHS; index++) {
printf("Month %2d has %2d days.\n", index + 1, days[index]); }
return 0;
}
/*The output of the program is as follows:
Month 1 has 31 days.
Month 2 has 28 days.
Month 3 has 31 days.
Month 4 has 30 days.
Month 5 has 31 days.
Month 6 has 30 days.
Month 7 has 31 days.
Month 8 has 31 days.
Month 9 has 30 days.
Month 10 has 31 days.
Month 11 has 30 days.
Month 12 has 31 days.*/#define MONTHS 12 Called a macro definition
Keep in mind that the number of an array element starts at 0 and exceeds the bounds called array subscript!!!
eg: demo program
/* day_mon2.c -- Let the compiler calculate the number of elements */
#include <stdio.h>
int main(void)
{
const int days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31 };
int index;
for (index = 0; index < sizeof days / sizeof days[0]; index++) //index is the meaning of array subscript
printf("Month %2d has %d days.\n", index + 1, days[index]);
return 0;
}Program explanation: sizeof is not a variable but an operator that gives the size of its object in bytes. So sizeof days is the size of the entire array in bytes, and sizeof day[0] is the size of the first element in the array in bytes. The size of the entire array divided by the size of a single element is the number of elements in the array.
8. How to assign an array element: After declaring an array, you can assign an array element with the aid of an array subscript (or index). For example, the following program segment assigns values to all elements of an array:
/* Assigning values to elements of an array */
#include <stdio.h>
#define SIZE 50
int main(void)
{
int counter, evens[SIZE];
for (counter = 0; counter < SIZE; counter++)
evens[counter] = 2 * counter;
...
}/* Some invalid array assignments */
#include <stdio.h>
#define SIZE 5
int main(void)
{
int oxen[SIZE] = {5,3,2,8}; /* Initialization OK */
int yaks[SIZE]; yaks = oxen; /* Not allow */
yaks[SIZE] = oxen[SIZE]; /* subscript out of bounds */
yaks[SIZE] = {5,3,2,8}; /* Not working */
···
}Notice that the array subscript is out of bounds: the last element of the oxen array is oxen[SIZE-1], so both oxen[SIZE] and yaks[SIZE] exceed the end of both arrays.
9. Specify the size of the array:
When referencing array elements, the following issues should be noted:int n = 5; int m = 8; float a1[5]; // Yes? float a2[5*2 + 1]; //Yes? float a3[sizeof(int) + 1]; //Yes? float a4[-4]; // No, the size of the array must be greater than 0 float a5[0]; // No, the size of the array must be greater than 0 float a6[2.5]; // No, the array size must be an integer float a7[(int)2.5]; // Yes, it has been cast to an integer constant float a8[n]; // Not allowed until C99 float a9[m]; // Not allowed until C99
(1) The value of a subscript expression must be an integer, which can be an integer constant, an integer variable, or an expression whose result is an integer.
(2) Only elements in the array can be used individually, not the entire array (except character arrays).
(3) Array subscripts in C start with 0, so the range of subscripts for arrays is 0 to L-1, where L is the length of the array. Arrays are not cross-checked in C. Array a contains six elements, if there is a statement a[10]=2;, Errors are compiled, but can cause errors in other variables or programs.
2-D Array
1. General format for type descriptions of two-dimensional arrays: type specifier Array name [Constant expression 1] [Constant expression 2]; Constant expression 1 defines the number of rows in the array and constant expression 2 defines the number of columns in the two-dimensional array x.
As defined, the two-dimensional array x consists of three rows, three columns and nine elements. Element row and column subscripts start at 0. The nine elements are
x[0] [0] x[0] [1] x[0] [2]
x[1] [0] ×[1] [1] x[1] [2] x[2] [0] x[2] [1] x[2] [2]
2. Initialization of a two-dimensional array: < 1. Assign initial values to all elements
(1) Assign initial values by row. For example:int a[2] [3]={(1,2,3), (4,5,6)};
(2) Assign values by storage structure, all data in one curly bracket. For example:int a[2][3]={1, 2, 3, 4, 5, 6}; / _Equivalent to line assignment/
< 2. Initial values are given to some elements, and values of unassigned elements are 0.
(1) Assign initial values by row. For example:int a[2][3]={(1, 2, 3), (4)}; Then a[1][1], The value of a[1][2] is 0. (2) Assign values by storage structure, all data in one curly bracket. For example:int al2][3]=(1,2,3,4); Again a[1][1], The value of a[1][2] is 0.
< 3. Omit the number of rows in a two-dimensional array; do not omit the number of columns in a two-dimensional array. (1) Assign initial values by row. For example:int A[][3]={(1, 2, 3), (4)}; The system automatically defines the number of rows of array a to be 2. (2) According to the storage structure eye value, the number of rows of the array is the number of elements in curly brackets divided by the number of columns in the teaching group. For example:int A[][3]={1,2,3,4}; Curly brackets contain four elements, rounded up by four-thirds, the system automatically defines the rows of array a The number is 2.
3. Reference to a two-dimensional array: Once a two-dimensional array has been defined, each element of the two-dimensional array can be referenced in the following format. Array name [subscript] [subscript] For example:
int c[3] [2];
c[1] [1]=2;
c[1] [2]=C[1] [1]*2;
Demonstrate a more complex code
/* rain.c -- Calculate annual total precipitation, annual average precipitation, and monthly average precipitation over 5 years */
#include <stdio.h>
#define MONTHS 12 //Year Months
#define YEARS 5 //year
int main(void)
{// Initialize the array with precipitation data for 2010-2014
const float rain[YEARS][MONTHS] = {
{ 4.3, 4.3, 4.3, 3.0, 2.0, 1.2, 0.2, 0.2, 0.4, 2.4, 3.5, 6.6 },
{ 8.5, 8.2, 1.2, 1.6, 2.4, 0.0, 5.2, 0.9, 0.3, 0.9, 1.4, 7.3 },
{ 9.1, 8.5, 6.7, 4.3, 2.1, 0.8, 0.2, 0.2, 1.1, 2.3, 6.1, 8.4 },
{ 7.2, 9.9, 8.4, 3.3, 1.2, 0.8, 0.4, 0.0, 0.6, 1.7, 4.3, 6.2 },
{ 7.6, 5.6, 3.8, 2.8, 3.8, 0.2, 0.0, 0.0, 0.0, 1.3, 2.6, 5.2 }};
int year, month;
float subtot, total;
printf(" YEAR RAINFALL (inches)\n");
for (year = 0, total = 0; year < YEARS; year++)
{ // Total precipitation for each year and month
for (month = 0, subtot = 0; month < MONTHS; month++) subtot += rain[year][month];
printf("%5d %15.1f\n", 2010 + year, subtot);
total += subtot; // Total precipitation over 5 years
}
printf("\nThe yearly average is %.1f inches.\n\n", total / YEARS);
printf("MONTHLY AVERAGES:\n\n");
printf(" Jan Feb Mar Apr May Jun Jul Aug Sep Oct ");
printf(" Nov Dec\n");
for (month = 0; month < MONTHS; month++)
{ // Total precipitation per month, 5 years
for (year = 0, subtot = 0; year < YEARS; year++) subtot += rain[year][month];
printf("%4.1f ", subtot / YEARS); }printf("\n"); 662
return 0;
}
/*The following is the output of the program:
YEAR RAINFALL (inches)
2010 32.4
2011 37.9
2012 49.8
2013 44.0
2014 32.9
The yearly average is 39.4 inches. MONTHLY AVERAGES:
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
7.3 7.3 4.9 3.0 2.3 0.6 1.2 0.3 0.5 1.7 3.6 6.7 */for (year = 0, total = 0; year < YEARS; year++)
{ // Processing data for each year
for (month = 0, subtot = 0; month < MONTHS; month++)
...// Processing monthly data
...//Processing data for each year
}The second nested for loop has the same structure as the first, but the inner loop traverses year and the outer loop traverses month. Remember that every time an outer loop is executed, the inner loop is fully traversed. Therefore, before changing the month, traverse through the year to get the average precipitation over a period of five years in a month, and so on:
for (month = 0; month < MONTHS; month++)
{ // Processing monthly data
for (year = 0, subtot =0; year < YEARS; year++)
...// Processing yearly data
...// Processing monthly data
}Multidimensional Array