Скачать или смотреть How to Check if std::vector const char * Contains a Given Character Sequence

Discover the best methods to check for a character sequence in `std::vector const char * `. We break down the solution in simple steps to help you improve performance in C+ + .
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This video is based on the question https://stackoverflow.com/q/68491675/ asked by the user 'Mateusz Bahyrycz' ( https://stackoverflow.com/u/13469160/ ) and on the answer https://stackoverflow.com/a/68491726/ provided by the user 'Jonathan S.' ( https://stackoverflow.com/u/16264289/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When working with C+ + , handling string comparisons can often lead to confusion, especially when dealing with different data types. A common issue arises when trying to check if a std::vector<const char *> contains a particular character sequence. If you’ve found yourself in a situation where your code isn’t performing as expected, you’re not alone!

In this guide, we will explain the problem, explore why std::find isn't suitable for const char *, and provide a high-performance solution using std::find_if and strcmp.

The Problem

If you declare a std::vector<std::string> like this:

[[See Video to Reveal this Text or Code Snippet]]

You can easily check if it contains a specific string, say "abc", using the following code snippet:

[[See Video to Reveal this Text or Code Snippet]]

However, the situation changes when you use std::vector<const char *>:

[[See Video to Reveal this Text or Code Snippet]]

If you try to apply the same logic:

[[See Video to Reveal this Text or Code Snippet]]

This code does not work as you might expect. Why is that?

Understanding the Issue

The crux of the problem lies in how std::find operates. This function calls operator==, which, for const char *, compares the pointer values rather than the contents of the strings they point to. This means that two different string literals that contain the same characters could still have different pointer values.

Why It Doesn’t Work

std::find is comparing memory addresses (pointer values) rather than the actual string contents.

As a result, unless the exact pointer is found in the vector, it will return false — which is misleading since the contents can be identical.

The Solution

To solve this issue, we need to compare the actual string contents. One effective way to do this is by using std::find_if in conjunction with the strcmp function.

High-Performance Approach

Here’s an efficient function to check if a particular character sequence exists within a std::vector<const char *>:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Function:

std::find_if: This function allows us to specify a custom condition to determine if an element exists within the range.

Lambda function: The lambda takes each const char* from the vector and compares it with the target string pStr using strcmp.

Performance: Since you specified an interest in performance, this method is efficient as it only traverses the vector once and compares the actual string content instead of pointer values.

Conclusion

When working with std::vector<const char *>, it's crucial to remember how comparisons work and choose the right tools for the job. Using std::find_if in combination with strcmp ensures that you correctly determine if a character sequence is present. This strategy not only resolves the issue of pointer value comparison but also meets performance needs effectively.

By employing these techniques in your C+ + applications, you can confidently handle string comparisons within vector structures, ultimately leading to cleaner, more reliable code.