How Can I Check for Prefixes and Extract Numeric Substrings in C ?

Checking Prefix and Extracting Numeric Substring in C

In Python, checking if a string starts with a specific prefix and converting a substring to an integer is a straightforward task. However, in C , it may not be immediately apparent how to achieve similar functionality.

To determine if a string starts with a certain substring, we can utilize the rfind function with the pos parameter set to zero. This ensures the search is confined to the beginning of the string. For instance:

<code class="cpp">std::string str = "tititoto";
if (str.rfind("titi", 0) == 0) {
    // The string starts with "titi"
}</code>

In the above example, pos is set to zero, which limits the search to the prefix. Hence, rfind returns 0 if the string commences with the specified substring. Otherwise, it returns std::string::npos, indicating failure.

With C 20 and later, the process becomes simpler due to the introduction of starts_with in std::string and std::string_view.

<code class="cpp">std::string str = "tititoto";
if (str.starts_with("titi")) {
    // The string starts with "titi"
}</code>

To extract a numeric substring from a string, we can use std::stoi. For example, if we have a string such as "--foo=98", we can extract the numeric value as follows:

<code class="cpp">std::string arg = "--foo=98";
std::size_t pos = arg.find("--foo=");
if (pos != std::string::npos) {
    std::string foo = arg.substr(pos + sizeof("--foo=") - 1);
    int foo_value = std::stoi(foo);
}</code>

In this case, we use find to locate the position of the "--foo=" prefix. If found, we extract the substring using substr and convert it to an integer using std::stoi.

These techniques provide efficient and concise solutions for working with strings in C .

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