DAY9 C Language Notes: 2D Arrays, Strings, and Functions

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1. Two-Dimensional Integer Arrays

A two-dimensional array is a matrix-like data structure composed of multiple one-dimensional arrays.
It’s mainly used to store tabular or planar data.

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1.1 Definition

// Define a 2D array with 2 rows and 3 columns
int a[2][3];

// Fully initialized
int a[2][3] = {{1,2,3},{4,5,6}};
int b[2][3] = {1,2,3,4,5,6};

// Partially initialized
int a1[2][3] = {1,2,3,4};      // Uninitialized elements are set to 0
int b2[2][3] = {{1,2},{3}};
int c1[2][3] = {0};            // All elements are initialized to 0
int c2[2][3] = {{0}};

Notes:

• Both row and column numbers must be integer constants.
• The data type cannot be void.
• You can omit the row number, letting the compiler infer it.

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1.2 Accessing Array Elements

int a[2][3];  // a has 2 elements, each being a 1D array of length 3

// Memory layout:
a[0][0] a[0][1] a[0][2]
a[1][0] a[1][1] a[1][2]

// Type relations:
a      -> int [][3]
a[0]   -> int []
a[0][0]-> int

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1.3 Characteristics

Property	Description
Uniformity	All elements share the same data type
Continuity	Stored in contiguous memory blocks
Orderliness	Elements are indexed in order

A two-dimensional array is essentially a collection of multiple one-dimensional arrays.

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1.4 Size and Dimension Calculation

int array_test[2][3] = {0};
int row = sizeof(array_test) / sizeof(array_test[0]);        // number of rows
int col = sizeof(array_test[0]) / sizeof(array_test[0][0]);  // number of columns
int size = sizeof(array_test);                               // total bytes

printf("size:%d row:%d col:%d\n", size, row, col);

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1.5 Traversing a 2D Array

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main() {
    int arr[2][3] = {0};
    int row = sizeof(arr)/sizeof(arr[0]);
    int col = sizeof(arr[0])/sizeof(arr[0][0]);
    srand(time(NULL));

    for(int i=0;i<row;i++)
        for(int j=0;j<col;j++)
            arr[i][j] = rand() % 50;

    for(int i=0;i<row;i++)
        for(int j=0;j<col;j++)
            printf("row:%d, col:%d, value:%d\n", i, j, arr[i][j]);

    return 0;
}

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2. Two-Dimensional Character Arrays

Two-dimensional character arrays are often used to store multiple strings.

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2.1 Definition and Initialization

char str[2][100] = {"hello", "world"};

char str[5][32] = {{"hello"}, {"world"}, {"1234"}, {"how"}, {"you ###"}};
char str2[5][32] = {"hello","world","1234","how","you ###"};
char str3[5][32] = {0};
char str4[5][32] = {{0}};
char str5[][32] = {"hello","world","how","are","you"};

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2.2 Access and Memory Layout

str -> char [][100]
str[0] -> char [100]
str[0][0] -> 'h'
str[1][0] -> 'w'
sizeof(str) = 2 * 100 * sizeof(char) = 200 bytes

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2.3 Common String Library Functions

Function	Description
strcpy(dest, src)	Copy string
strcmp(s1, s2)	Compare strings (returns 0 if equal)
strcat(dest, src)	Concatenate strings
strlen(str)	Get string length

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2.4 Manual String Comparison Logic

int i = 0;
int ret = 0;

while(1) {
    if(str[i] == str2[i] && str[i] != 0) {
        i++;
    } else {
        ret = str[i] - str2[i];
        break;
    }
}
printf("ret %d\n", ret);

Equivalent built-in function:

int ret = strcmp(str, str2);

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2.5 Input and Output Example

#include<stdio.h>
#include<string.h>

int main() {
    char str_test[3][100] = {0};
    for(int i=0;i<3;i++) {
        printf("input str %d:\n", i+1);
        gets(str_test[i]);
    }
    for(int i=0;i<3;i++) {
        printf("str %d, content: %s\n", i+1, str_test[i]);
    }
    return 0;
}

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3. Functions in C

Functions are the basic building blocks of modular programming, allowing large problems to be decomposed into smaller, reusable components.

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3.1 Definition and Usage

Syntax:

type function_name(type param1, type param2, ...) {
    // function body
    return expression;
}

Example:

int add(int a, int b) {
    int c = a + b;
    return c;
}

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3.2 Function Declaration and Invocation

int add(int, int); // Declaration

int main() {
    int a = 10, b = 20;
    int c = add(a, b);
    printf("%d\n", c);
    return 0;
}

Key Points:

• A declaration is required if the definition comes after its use.
• Return type cannot be an array.
• If not explicitly defined, the return type defaults to int.
• Any code after return will not execute.

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3.3 Example: Basic Function

#include<stdio.h>
int add(int a,int b){
    int c = a + b;
    return c;
}

int main(){
    int a=10, b=20;
    int c=add(a,b);
    printf("%d\n", c);
    return 0;
}

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3.4 Example: Finding the Maximum Value

#include<stdio.h>
int getmax(int a,int b) {
    return a >= b ? a : b;
}
int main() {
    int a,b;
    scanf("%d %d",&a,&b);
    printf("%d\n", getmax(a,b));
    return 0;
}

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3.5 Example: Printing Multiples of 3 (1–100)

#include<stdio.h>

int isThreeTimes(int a) {
    if (a % 3 == 0) return 1;
    return 0;
}

int main() {
    for(int i=1;i<=100;i++)
        if(isThreeTimes(i))
            printf("%d ", i);
    return 0;
}