learn/doc/c_array_pointer at main

C: array, pointer

Reference

array

Let's say you have thousands of values to store in your program. One way, you could:

int num1, num2, num3, num4, num5;
int num6, num7, num8, num9, num10;
...
int num1000, num1001, num1002, num1003, num1004;
...

This is too verbose and error-prone. You should use array:

#include <stdio.h>

int main(void)
{
	int arr0[2];
	arr0[0] = 10;
	arr0[1] = 20;
	int sum0=0, i;
	for (i=0; i<2; i++)
		sum0 += arr0[i];
	printf("sum0: %d\n", sum0);
	// sum0: 30

	int len=50;
	double arr1[len];
	int j;
	for (j=0; j<len; j++)
		arr1[j] = (double)j;
	double sum1=0;
	int k;
	for (k=0; k<len; k++)
		sum1 += arr1[k];
	printf("sum1: %f\n", sum1);
	// sum1: 1225.000000

	int arr2[3]={0, 1, 2};
	int i2;
	for (i2=0; i2<3; i2++)
		printf("%d ", arr2[i2]);
	printf("\n");
	// 0 1 2
	
	int arr3[]={0, 1, 2}; // automatically sized to 3
	int i3;
	for (i3=0; i3<3; i3++)
		printf("%d ", arr3[i3]);
	printf("\n");
	// 0 1 2

	int arr4[5]={1}; // automatically fill up with 0
	int i4;
	for (i4=0; i4<5; i4++)
		printf("%d ", arr4[i4]);
	printf("\n");
	// 1 0 0 0 0 

	printf("arr4 sizeof: %ld\n", sizeof(arr4));             // arr4 sizeof: 20
	printf("int sizeof: %ld\n", sizeof(int));               // int sizeof: 4
	printf("arr4 length: %ld\n", sizeof(arr4)/sizeof(int)); // arr4 length: 5

	printf("\ntype 3 integers: ");
	int arr[3];
	scanf("%d", &arr[0]);
	scanf("%d", &arr[1]);
	scanf("%d", &arr[2]);
	int ia;
	for (ia=0; ia<3; ia++)
		printf("%d ", arr[ia]);
	printf("\n");
	// type 3 integers: 100 200 300
	// 100 200 300

    return 0;
}

`char`

C represents string literals with double quotes:

And you can save this string in character array type. In C, string is an array of characters, and the null character \0 is automatically inserted at the end of all character arrays. C needs \0 to differentiate between two types of array of characters, one that cannot be printed with %s, the other that can be printed with %s:

#include <stdio.h>

int main(void)
{
	char array[7] = "Hello";
	int i;
	for (i=0; i<7; i++)
		printf("%d: %c\n", i, array[i]);
	printf("\n");
	for (i=0; i<7; ++i)
		printf("%d: %c\n", i, array[i]);
	printf("\n");
	/*
		0: H
		1: e
		2: l
		3: l
		4: o
		5: 
		6: 

		0: H
		1: e
		2: l
		3: l
		4: o
		5: 
		6: 
	*/

	char str[]="Hello World!";
	printf("str length: %ld\n", sizeof(str)/sizeof(char));
	// str length: 13
	
	// we need null character to differentiate between these two:
	char charArray[]={'H', 'e', 'l', 'l', 'o'};
	printf("charArray: %s\n", charArray); // charArray: Hello�

	char charArrayString[]={'H', 'e', 'l', 'l', 'o', '\0'};
	printf("charArrayString: %s\n", charArrayString); // charArrayString: Hello
	int idx=0;
	while (charArrayString[idx] != 0)
	{
		printf("%c", charArrayString[idx]);
		idx++;
	}
	printf("\n");
	// Hello

	charArrayString[3] = '\0';
	printf("charArrayString: %s\n", charArrayString); // charArrayString: Hel

	charArrayString[1] = 0;
	printf("charArrayString: %s\n", charArrayString); // charArrayString: H

    return 0;
}

pointer

Pointer is a variable to store the address value of data(variable). &num returns the address of num. int * num defines a pointer type variable num. * is also used to dereference or access the memory that the pointer points to.

#include <stdio.h>

int main(void)
{
	int num = 1;
	int * pnum;   // declare pointer-type pnum
	pnum = &num;  // store the address of num to pnum
	printf("%p\n", pnum); // 0x7ffdfbe9a224

	double f = 1.5;
	double * fp = &f;
	*fp = 100.10;
	printf("%f\n", f); // 100.100000

	int * pt = NULL;    // assign NULL pointer if not sure what to store
	printf("%p\n", pt); // (nil)

    return 0;
}

array, pointer

First take a look at this code:

#include <stdio.h>

int main()
{
	int arr[3] = {0, 1, 2};

	printf("arr: %p\n", arr);         // 0x7ffcb4d51010
	printf("&arr[0]: %p\n", &arr[0]); // 0x7ffcb4d51010
	printf("&arr[1]: %p\n", &arr[1]); // 0x7ffcb4d51014
	printf("&arr[2]: %p\n", &arr[2]); // 0x7ffcb4d51018

	// (X)
	// arr = &arr[1];

	return 0;
}

The name of an array represents the address of an beginning element (first element in the array). That is, array name is a pointer to the array.
Each int takes 4 bytes of memory, so int array element differs 4 bytes in its memory addresses.
The memory address of an array cannot be changed, unlike other pointer variables.

Again, array name is also a pointer:

#include <stdio.h>

int main()
{
	int arr[3] = {0, 1, 2};

	printf("*arr: %d\n", *arr);  // 0
	*arr += 100;
	printf("*arr: %d\n", *arr);  // 100
	arr[0] -= 50;
	printf("*arr: %d\n", *arr);  // 50

	int * ptr = &arr[0];
	// int * ptr = arr;
	
	printf("%d %d\n", *ptr, *arr);     // 50 50
	printf("%d %d\n", ptr[0], arr[0]); // 50 50
	printf("%d %d\n", ptr[1], arr[1]); // 1 1
	printf("%d %d\n", ptr[2], arr[2]); // 2 2

	return 0;
}

pointer arithmetic

#include <stdio.h>

int main()
{
	int * pt0 = 0x0010;  // increase by 4
	printf("%p %p\n", pt0+1, pt0+2); // 0x14 0x18

	double * pt1 = 0x0010;  // increase by 8
	printf("%p %p\n", pt1+1, pt1+2); // 0x18 0x20

	int arr[3] = {0, 1, 2};
	int * ptr = arr;
	printf("%d %d %d\n", *ptr, *(ptr+1), *(ptr+2));
	// 0 1 2
	
	printf("%d\n", arr[2] == *(arr+2)); // 1

	return 0;
}

`char` array

In C, there are two way to represent strings:

#include <stdio.h>

int main()
{
	char s0[] = "abc";
	// *s0 = "Hello";   // (X)
	printf("%s\n", s0); // abc
	s0[0] = 'X';
	printf("%s\n", s0); // Xbc

	char * s1 = "abc";
	s1 = "Hello";
	printf("%s\n", s1); // Hello
	s1[0] = 'X';        // (X)
	printf("%s\n", s1); // (X)

	return 0;
}

pointer array

#include <stdio.h>

int main()
{
	int n0=0, n1=1, n2=2;
	int * arr[3] = {&n0, &n1, &n2};
	printf("%d\n", *arr[0]); // 0
	printf("%d\n", *arr[1]); // 1
	printf("%d\n", *arr[2]); // 2

	char * sArr[3] = {"AA", "BB", "CC"};
	printf("%s\n", sArr[0]); // AA
	printf("%s\n", sArr[1]); // BB
	printf("%s\n", sArr[2]); // CC

	return 0;
}

pointer function

#include <stdio.h>

// argument is passed by value!
// make sure to pass pointer if you want to update!
void updateArray(int * arrPt, int len, int delta)
{
	int i;
	for (i=0; i<len; i++)
		arrPt[i] += delta;
}

void printArray(int * arrPt, int len)
{
	int i;
	for (i=0; i<len; i++)
		printf("%d ", arrPt[i]);
	printf("\n");
}

int main()
{
	int arr[3] = {1,2,3};
	updateArray(arr, sizeof(arr) / sizeof(int), 10);
	printArray(arr, sizeof(arr) / sizeof(int));
	// 11 12 13

	return 0;
}

swap

#include <stdio.h>

void swapValue(int v1, int v2)
{
	int temp = v1;
	v1 = v2;
	v2 = temp;
}

void swapPointer(int * pt1, int * pt2)
{
	int temp = *pt1; // dereference
	*pt1 = *pt2;
	*pt2 = temp;
}

int main()
{
	int num1 = 1;
	int num2 = 2;
	swapValue(num1, num2);
	printf("num1 num2: %d %d\n", num1, num2);
	// 1 2

	swapPointer(&num1, &num2);
	printf("num1 num2: %d %d\n", num1, num2);
	// 2 1

	return 0;
}

function pointer

#include <stdio.h>

void printAdd(int v1, int v2)
{
	printf("%d + %d = %d\n", v1, v2, v1 + v2);
}

void printString(char * str)
{
	printf("%s\n", str);
}

int main()
{
	int num1 = 1, num2 = 2;
	void (*fpAdd)(int, int) = printAdd;
	fpAdd(num1, num2);
	// 1 + 2 = 3

	char * str = "Hello World!";
	void (*fpStr)(char *) = printString;
	fpStr(str);
	// Hello World!

	return 0;
}

double pointer

#include <stdio.h>

/*
void myFunc(TYPE * arr) {}
void myFunc(TYPE arr[]) {}

void myFunc(TYPE ** arr) {}
void myFunc(TYPE * arr[]) {}
*/

void printStrings(int argc, char * argv[])
{
	int i;
	for (i=0; i<argc; i++)
		printf("%s ", argv[i]);
	printf("\n");
}

int main(void)
{
	char * str[3] = {
		"A", "B", "C"
	};

	printStrings(3, str);
	// A B C

	return 0;
}