C/C++实现：不使用字符串函数判断单词是否在句子中

Got it, let's break this down. We need to check if a target word exists as a standalone word in a sentence—meaning it can't just be a substring of another word (like "cat" in "category" doesn't count). And we can't use any built-in string functions like strstr(), strlen(), or strcmp(). Here's a straightforward approach:

Core Logic Steps #

• Manual Length Calculation: First, we'll write a helper function to get the length of any string by counting characters until we hit the null terminator ('\0').
• Character-by-Character Matching: Traverse the sentence, and for each position, check if the following characters match the target word exactly.
• Boundary Check: Once we find a full match of the target word, we need to verify it's a standalone word—this means the character before the match is either a space or the start of the sentence, and the character after is either a space or the end of the sentence.

Complete Code Implementation #

#include <iostream>
using namespace std;

// Helper function to get string length without using strlen()
int getStringLength(char str[]) {
    int length = 0;
    while (str[length] != '\0') {
        length++;
    }
    return length;
}

// Function to check if target word exists as standalone in sentence
bool isWordPresent(char sentence[], char target[]) {
    int sentenceLen = getStringLength(sentence);
    int targetLen = getStringLength(target);

    // If target is longer than sentence, it can't exist
    if (targetLen > sentenceLen) {
        return false;
    }

    // Traverse each possible starting position in the sentence
    for (int i = 0; i <= sentenceLen - targetLen; i++) {
        bool match = true;
        // Check if current segment matches target
        for (int j = 0; j < targetLen; j++) {
            if (sentence[i + j] != target[j]) {
                match = false;
                break;
            }
        }

        // If full match found, check word boundaries
        if (match) {
            // Check left boundary: start of sentence or space before
            bool leftValid = (i == 0) || (sentence[i - 1] == ' ');
            // Check right boundary: end of sentence or space after
            bool rightValid = (i + targetLen == sentenceLen) || (sentence[i + targetLen] == ' ');

            if (leftValid && rightValid) {
                return true;
            }
        }
    }
    return false;
}

int main() {
    char sentence[1000], target[100];

    cout << "Enter the sentence: ";
    // Use fgets to read entire line (including spaces)
    fgets(sentence, sizeof(sentence), stdin);
    // Replace newline character from fgets with null terminator
    int senLen = getStringLength(sentence);
    if (senLen > 0 && sentence[senLen - 1] == '\n') {
        sentence[senLen - 1] = '\0';
    }

    cout << "Enter the target word: ";
    cin >> target;

    if (isWordPresent(sentence, target)) {
        cout << "The word exists in the sentence." << endl;
    } else {
        cout << "The word does NOT exist in the sentence." << endl;
    }

    return 0;
}

How It Works #

1. getStringLength: Simple loop that counts characters until it hits the null terminator—no library functions needed.
2. isWordPresent:
   • First, we rule out impossible cases where the target is longer than the sentence.
   • For each starting index in the sentence, we check if the next targetLen characters match the target exactly.
   • When a full match is found, we check the boundaries to ensure it's a standalone word (not part of a longer word).
3. main: Handles input (using fgets to capture the entire sentence including spaces, then cleaning up the newline character) and calls our check function to output the result.

Test Cases #

• Case 1: Sentence = "I love coding in C++", Target = "coding" → Returns true (valid standalone word).
• Case 2: Sentence = "I love coding in C++", Target = "cod" → Returns false (only a substring of "coding").
• Case 3: Sentence = "Hello world", Target = "Hello" → Returns true (starts the sentence).
• Case 4: Sentence = "Hello world", Target = "world" → Returns true (ends the sentence).

内容的提问来源于stack exchange，提问作者Paras Jain