Incorrect flag initialization

Oct 13, 2025 by pynkocean.com 110 views

Introduction

The C Programming Language, written by Brian Kernighan and Dennis Ritchie, is a comprehensive book that provides a thorough introduction to the C programming language. Exercise 1-9 of this book presents a problem that requires the reader to write a program that copies its input to its output, replacing each string of one or more blanks by a single blank. In this article, we will discuss the problem, provide a step-by-step solution, and highlight common mistakes that readers may encounter.

Understanding the Problem

The problem requires the reader to write a program that takes input from the user and copies it to the output, replacing each string of one or more blanks with a single blank. For example, if the user inputs "Hello World", the program should output "Hello World".

Step 1: Analyzing the Problem

To solve this problem, we need to analyze the requirements and identify the key components. The program should:

Take input from the user
Copy the input to the output
Replace each string of one or more blanks with a single blank

Step 2: Designing the Program

Based on the analysis, we can design a program that uses the following approach:

Read input from the user using a loop that continues until the user enters a blank line.
Use a flag variable to track whether we have encountered a blank character.
If we encounter a blank character, set the flag to true.
If we encounter a non-blank character, check if the flag is true. If it is, print a single blank character and reset the flag.
If the flag is false, print the non-blank character.

Step 3: Implementing the Program

Here is a sample implementation of the program in C:

#include <stdio.h>
int main() {
int c, flag = 0;
while ((c = getchar()) != EOF) {
    if (c == &#39; &#39;) {
        flag = 1;
    } else {
        if (flag) {
            printf(&quot; &quot;);
            flag = 0;
        }
        printf(&quot;%c&quot;, c);
    }
}

return 0;

}

Step 4: Testing the Program

To test the program, we can use a sample input file that contains a string of one or more blanks. For example, we can create a file called "input.txt" with the following contents:

Hello   World

We can then compile and run the program, passing the input file as an argument:

$ gcc program.c -o program
$ ./program < input.txt
Hello World

Common Mistakes

When implementing the program, readers may encounter the following common mistakes:

Incorrect flag initialization: The flag variable should be initialized to false, not true.
Incorrect flag reset: The flag should be reset to false when a non-blank character is encountered.
Incorrect printing: The program should print a single blank character when the flag is true, not multiple blank characters.

Conclusion

In this article, we discussed the C Programming Language Exercise 1-9 problem, provided a step-by-step solution, and highlighted common mistakes that readers may encounter. By following the solution and avoiding common mistakes, readers can successfully implement the program and gain a deeper understanding of the C programming language.