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C語言練習題：字串(C language exercise: String)

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練習一：字串輸入
撰寫一可以輸入字串並輸出所輸入字串的程式。

Exercise 1: Input a string
Design a program to input a string and displays it.

練習一參考解法：
Exercise 1 solution:
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/*
Input a string and displays it.
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE];

    printf("Input a string:");
    // Get any string with space method 1
    scanf("%[^\n]%*c", str);
    printf("Your string: %s\n", str);

    printf("Input a string:");
    // Get any string with space method 2
    gets(str);
    printf("Your string: %s\n", str);

    printf("Input a string:");
    // Get any string with space method 3
    fgets(str, SIZE, stdin);
    printf("Your string: %s\n", str);

    return 0;
}


練習二：字串長度
撰寫一可以算出字串長度的程式，請不要使用strlen()函數。

Exercise 2: String's Length
Write a program to find the length of a string without using strlen() function.

練習二參考解法：
Exercise 2 solution:
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/*
Find the length of a string without using strlen() function.
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], cnt = 0;

    printf("Input a string:");

    // Get any string with space method 1
    //scanf("%[^\n]%*c", str);

    // Get any string with space method 2
    //gets(str);

    // Get any string with space method 3
    fgets(str, SIZE, stdin);

    while(str[cnt] != '\0' && ++cnt < SIZE);

    printf("Length:%d\n", cnt);

    return 0;
}


練習三：字元
設計可以將一字串的每個字元以空白區隔做輸出。

Exercise 3: Characters
Design a program to split a string into characters.

練習三參考解法：
Exercise 3 solution:
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/*
Split a string into characters.
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], idx = 0;;

    printf("Input a string:");

    gets(str);

    while(str[idx] != '\0')
        printf("%c ", str[idx++]);

    return 0;
}


練習四：大小寫轉換
設計可以將字串的小寫字母轉換成大寫字母，大寫字母轉換成小寫字母。例如 HelLoWoRLD 經轉換後，變成hELlOwOrld。

Exercise 4: Invert Case
Design a program that inverts string case. For example:  HelLoWoRLD will be inverted to hELlOwOrld.

練習四參考解法：
Exercise 4 solution:
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/*
Invert Case
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], idx = 0;;

    printf("Input a string:");

    gets(str);

    while(str[idx] != '\0')
    {
        char c = str[idx];

        // Convert to upper case.
        if(c >= 'a' && c <= 'z')
            c = c - 32;
        // Convert to lower case.
        else if(c >= 'A' && c <= 'Z')
            c = c + 32;

        printf("%c", c);
        idx++;
    }

    return 0;
}


練習五：反轉字串
設計可以將字串反轉的程式，例如 Hello World 會變成 dlroW olleH。

Exercise 5: Reverse String
Design a program to reverse any string. For example: Hello World ==> dlroW olleH

練習五參考解法：
Exercise 5 solution:
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/*
Reversed string
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], idx = 0;;

    printf("Input a string:");

    gets(str);

    while(str[idx] != '\0')
        idx++;

    while(idx >= 0)
        printf("%c", str[--idx]);
    return 0;
}


練習六：母音或子音
設計一個可以算出字串中的母音字母的個數與子音字母的個數。

Exercise 6: Vowel or consonant
Design a program that finds the total of vowel or consonant in a string.

練習六參考解法：
Exercise 6 solution:
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/*
Vowel or consonant
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], idx = 0;
    int vowelCnt = 0, consonantCnt = 0;

    printf("Input a string:");

    gets(str);

    while(str[idx] != '\0')
    {
        char c = str[idx];
        switch(c)
        {
            case 'a':
            case 'A':
            case 'e':
            case 'E':
            case 'i':
            case 'I':
            case 'o':
            case 'O':
            case 'u':
            case 'U':
                vowelCnt++;
                break;

            default:
                if( (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') )
                    consonantCnt++;
                break;
        }
        idx++;
    }

    printf("Total vowel:%d\tTotal consonant:%d\n", vowelCnt, consonantCnt);
    return 0;
}


練習七：迴文
設計可以判斷迴文的程式。例如：madam 是迴文。

Exercise 7: Palindrome
Design a program that checks whether a string is palindrome or not.

練習七參考解法：
Exercise 7 solution:
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/*
Palindrome
Author: Holan
*/

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

int main()
{
    const int SIZE = 100;
    char str[SIZE], idx = 0;
    int isPalindrome = 1;

    printf("Input a string:");

    gets(str);

    while(str[idx] != '\0')
        idx++;

    int i = 0;
    while(i < idx)
    {
        if(str[i++] != str[--idx])
        {
            isPalindrome = 0;
            break;
        }
    }

    if(isPalindrome != 0)
        printf("Yes\n");
    else
        printf("No\n");

    return 0;
}


練習八：出現次數
設計一個可以統計每個字元在字串中出現的次數。

Exercise 8: Character Frequency
Design a program that calculates each character's frequency in a string.

練習八參考解法：
Exercise 8 solution:
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/*
Character Frequency
Author: Holan
*/

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

int main()
{
    const int CHR_NO = 255;
    const int SIZE = 1000;
    char str[SIZE];
    int frequency[CHR_NO];

    for(int i = 0; i < CHR_NO; i++)
        frequency[i] = 0;

    printf("Input a string:");

    gets(str);

    for(int i = 0; str[i] != '\0'; i++)
    {
        frequency[(int)(str[i])]++;
    }

    for(int i = 0; i < CHR_NO; i++)
    {
        printf("%c:%d\n", i, frequency[i]);
    }

    return 0;
}


練習九：出現次數最高的字元
設計一個可以統計每個字元在字串中出現的次數，並找次數最高的字元與次數。

Exercise 9: Highest frequency character 
Design a program to find the highest frequency character in a given string.

練習九參考解法：
Exercise 9 solution:
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/*
Highest Character Frequency
Author: Holan
*/

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

int main()
{
    const int CHR_NO = 255;
    const int SIZE = 1000;
    char str[SIZE];
    int frequency[CHR_NO];
    int maxFreq = 0;
    char mChr;

    for(int i = 0; i < CHR_NO; i++)
        frequency[i] = 0;

    printf("Input a string:");

    gets(str);

    for(int i = 0; str[i] != '\0'; i++)
    {
        frequency[(int)(str[i])]++;
    }

    for(int i = 0; i < CHR_NO; i++)
    {
        if(frequency[i] > maxFreq)
        {
            maxFreq = frequency[i];
            mChr = (char)(i);
        }
    }

    printf("%c %d\n", mChr, maxFreq);
    return 0;
}


練習十：取代空白字元
設計可以將字串中的空白字元以特定的字元取代。

Exercise 10: Spaces replacing
Design a program that replaces all spaces in a given with a specific character.

練習十參考解法：
Exercise 10 solution:
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/*
Spaces replacing
Author: Holan
*/

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

int main()
{
    const int SIZE = 1000;
    char str[SIZE];
    char aChr;


    printf("Input a string:");
    gets(str);

    printf("Enter a special character:");
    scanf("%c", &aChr);

    for(int i = 0; str[i] != '\0'; i++)
    {
       if(str[i] == ' ') str[i] = aChr;
    }


    printf("The input string becomes %s\n", str);
    return 0;
}


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