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Problem with scanf() when there is fgets()/gets()/scanf() After it in C++

It seems like you might be trying to mix C and C++ input functions, which can lead to unexpected behavior and issues. Let me explain the problem and provide a solution.

  1. Mixing scanf() with fgets() or gets():
  • scanf() and fgets() or gets() have different ways of processing input and handling newline characters.
  • scanf() leaves the newline character (\n) in the input buffer, which can cause issues when you try to read input using fgets() or gets(). These functions might immediately read the leftover newline character, resulting in unexpected behavior.
  1. Mixing scanf() with C++ input functions like cin:
  • If you’re using C++ along with C input functions, mixing scanf() with cin can lead to synchronization problems between C and C++ input buffers.

Here’s a common solution to these problems:

C++
#include <iostream>
#include <cstdio>

int main() {
    int num;
    char buffer[100];

    // Read an integer using scanf
    if (scanf("%d", &num) != 1) {
        std::cerr << "Failed to read an integer." << std::endl;
        return 1;
    }

    // Consume the newline character left by scanf
    while (getchar() != '\n');

    // Read a line of text using fgets
    if (fgets(buffer, sizeof(buffer), stdin) == NULL) {
        std::cerr << "Failed to read a line of text." << std::endl;
        return 1;
    }

    // Use cin for C++ input
    std::string input;
    std::cout << "Enter a line of text: ";
    std::cin >> input;
    std::cout << "You entered: " << input << std::endl;

    return 0;
}

In this example:

  1. We use scanf() to read an integer and immediately consume the newline character left in the input buffer by scanf().
  2. We use fgets() to read a line of text and handle it separately.
  3. We use cin for C++ input, but be cautious when mixing C and C++ input functions like this. Ensure proper synchronization by handling newline characters when necessary.

By following this approach, you can avoid mixing C and C++ input functions and handle them separately to prevent unexpected behavior.

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