C言語のきほん｜文字列内を探す（strchr、strstr）

Hello world!

#include <string.h>

char *strchr(const char *s, int c);

char str[] = "Hello world!";
char *pos = strchr(str, 'o');

printf("位置:%d\n", (int)(pos - str));

pos - str

char *pos = strchr(str, 'x');

if (pos != NULL) {
    printf("見つかりました。\n");
} else {
    printf("見つかりませんでした。\n");
}

#include <string.h>

char *strstr(const char *s1, const char *s2);

char str[] = "Hello world!";
char *pos = strstr(str, "world");

printf("位置:%d\n", (int)(pos - str));

C language is fun

lang

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

int main(void)
{
    char text[] = "Learning C language is fun.";
    char *pos1;
    char *pos2;

    /* 文字 'a' を探す */
    pos1 = strchr(text, 'a');
    if (pos1 != NULL) {
        printf("文字 a は %d 文字目に見つかりました。\n", (int)(pos1 - text));
    } else {
        puts("文字 a は見つかりませんでした。");
    }

    /* 部分文字列 \"lang\" を探す */
    pos2 = strstr(text, "lang");
    if (pos2 != NULL) {
        printf("文字列 lang は %d 文字目に見つかりました。\n", (int)(pos2 - text));
    } else {
        puts("文字列 lang は見つかりませんでした。");
    }

    return 0;
}

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

int main(void)
{
    char months[][20] = {
        "January", "February", "March", "April",
        "May", "June", "July", "August",
        "September", "October", "November", "December"
    };

    int count = sizeof(months) / sizeof(months[0]);

    puts("月の名前と文字 r の有無");

    for (int i = 0; i < count; i++) {
        if (strchr(months[i], 'r') != NULL) {
            printf("%2d月: %-10s  r を含む\n", i + 1, months[i]);
        } else {
            printf("%2d月: %-10s  r を含まない\n", i + 1, months[i]);
        }
    }

    return 0;
}

"Tomato", "Carrot", "Cabbage", "Pumpkin", "Onion",
"Radish", "Spinach", "Potato", "Lettuce", "Peas"

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

int main(void)
{
    char vegetables[][20] = {
        "Tomato", "Carrot", "Cabbage", "Pumpkin", "Onion",
        "Radish", "Spinach", "Potato", "Lettuce", "Peas"
    };

    int count = sizeof(vegetables) / sizeof(vegetables[0]);
    int max_index = 0;
    int best_index = -1;
    int best_pos = 1000;

    for (int i = 1; i < count; i++) {
        if (strcmp(vegetables[i], vegetables[max_index]) > 0) {
            max_index = i;
        }
    }

    for (int i = 0; i < count; i++) {
        char *pos = strchr(vegetables[i], 'a');
        if (pos != NULL) {
            int index = (int)(pos - vegetables[i]);
            if (index < best_pos) {
                best_pos = index;
                best_index = i;
            }
        }
    }

    printf("辞書順で一番大きい名前の野菜: %s\n", vegetables[max_index]);

    if (best_index != -1) {
        printf("文字 a が最も前に現れる野菜: %s\n", vegetables[best_index]);
    } else {
        puts("文字 a を含む野菜はありません。");
    }

    return 0;
}

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

int main(void)
{
    char result[31] = "";
    char input[100];

    puts("文字列を繰り返し入力してください。（30文字で終了します）");
    puts("? を含む入力は連結しません。");

    while (strlen(result) < 30) {
        printf("追加する文字列> ");

        if (fgets(input, sizeof(input), stdin) == NULL) {
            break;
        }

        size_t len = strlen(input);
        if (len > 0 && input[len - 1] == '\n') {
            input[len - 1] = '\0';
        }

        if (strchr(input, '?') != NULL) {
            puts("? が含まれているので、この入力は無視します。");
            continue;
        }

        size_t remain = 30 - strlen(result);
        strncat(result, input, remain);
    }

    puts("完成した文字列:");
    printf("%s（%zu文字）\n", result, strlen(result));

    return 0;
}

A red book and a red pen were placed on the red desk.

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

int main(void)
{
    char src[] = "A red book and a red pen were placed on the red desk.";
    char dest[200] = "";
    char *current = src;
    char *found;

    while ((found = strstr(current, "red")) != NULL) {
        strncat(dest, current, (size_t)(found - current));
        strcat(dest, "blue");
        current = found + 3;
    }

    strcat(dest, current);

    puts("元の文字列:");
    puts(src);
    puts("");
    puts("置換後の文字列:");
    puts(dest);

    return 0;
}

char books[][40] = {
    "C Programming Basics",
    "Linux Start Guide",
    "Data Structure",
    "Network Basics",
    "Database Intro",
    "Web Design"
};

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

int main(void)
{
    char books[][40] = {
        "C Programming Basics",
        "Linux Start Guide",
        "Data Structure",
        "Network Basics",
        "Database Intro",
        "Web Design"
    };

    int count = sizeof(books) / sizeof(books[0]);

    puts("Data を含むタイトル");

    for (int i = 0; i < count; i++) {
        if (strstr(books[i], "Data") != NULL) {
            puts(books[i]);
        }
    }

    return 0;
}