Not using a loop to iterate over the input

Oct 13, 2025 by talynt.org 88 views

Introduction

The C programming language, as described in the book "The C Programming Language" by Brian Kernighan and Dennis Ritchie, is a fundamental language for computer programming. The book provides a comprehensive guide to the language, including exercises to help readers practice and reinforce their understanding of C. In this article, we will discuss Exercise 1-9, which requires writing a program to copy its input to its output, replacing each string of one or more blanks by a single blank.

Understanding the Problem

The problem statement is straightforward: 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. This means that if the input contains multiple consecutive spaces, the program should output a single space.

What Am I Doing Wrong?

If you're having trouble with this exercise, it's likely because you're not using the correct approach or logic. Here are some common mistakes to watch out for:

Not using a loop to iterate over the input: To replace each string of one or more blanks with a single blank, you need to iterate over the input using a loop.
Not checking for consecutive spaces: You need to check if the current character is a space and if the previous character was also a space. If both conditions are true, you should output a single space.
Not handling the first character correctly: When checking for consecutive spaces, you need to handle the first character separately, as it doesn't have a previous character to compare with.

A Step-by-Step Solution

Here's a step-by-step solution to Exercise 1-9:

Step 1: Initialize Variables

#include <stdio.h>
int main() {
int c; // variable to store the current character
int prev_space = 0; // variable to store whether the previous character was a space

Step 2: Read Input and Replace Consecutive Spaces

    while ((c = getchar()) != EOF) {
        if (c == ' ') {
            if (prev_space == 1) {
                putchar(' ');
                prev_space = 0;
            } else {
                prev_space = 1;
            }
        } else {
            prev_space = 0;
            putchar(c);
        }
    }
    return 0;
}

Step 3: Explanation

We use a while loop to iterate over the input until we reach the end of the file (EOF).
Inside the loop, we check if the current character is a space. If it is, we check if the previous character was also a space. If both conditions are true, we output a single space and set prev_space to 0. If the current character is not a space, we set prev_space to 0 and output the character.
We use getchar() to read the current character and putchar() to output the character.

Example Use Case

Here's an example use case for the program:

$ ./program
Hello   World!
Hello World!

In this example, the program takes the input "Hello World!" and outputs "Hello World!", replacing each string of one or more blanks with a single blank.

Conclusion

Introduction

In our previous article, we discussed Exercise 1-9 of the C programming language, which requires writing a program to copy its input to its output, replacing each string of one or more blanks by a single blank. In this article, we will provide a Q&A section to help you better understand the exercise and its solution.

Q&A Section

Q: What is the purpose of Exercise 1-9?

A: The purpose of Exercise 1-9 is to help you practice writing a program that replaces each string of one or more blanks with a single blank. This exercise is designed to help you understand the basics of C programming, including loops, conditional statements, and character manipulation.

Q: What is the main challenge of Exercise 1-9?

A: The main challenge of Exercise 1-9 is to correctly identify and replace consecutive spaces with a single space. This requires you to use a loop to iterate over the input, check for consecutive spaces, and handle the first character correctly.

Q: How do I handle the first character correctly?

A: To handle the first character correctly, you need to check if it is a space and output it if it is. If the first character is not a space, you can output it as is.

Q: What is the difference between `getchar()` and `putchar()`?

A: getchar() is used to read a character from the input, while putchar() is used to output a character to the output. In Exercise 1-9, you use getchar() to read the current character and putchar() to output the character.

Q: How do I check for consecutive spaces?

A: To check for consecutive spaces, you need to use a variable to store whether the previous character was a space. If the current character is a space and the previous character was also a space, you output a single space and set the variable to 0.

Q: What is the purpose of the `prev_space` variable?

A: The prev_space variable is used to store whether the previous character was a space. This variable is used to check for consecutive spaces and output a single space if necessary.

Q: How do I handle the end of the file (`EOF`)?

A: To handle the end of the file (EOF), you need to use a while loop that continues until the end of the file is reached. Inside the loop, you check for the end of the file using the EOF constant.

Example Use Cases

Here are some example use cases for the program:

Replacing multiple spaces with a single space: If the input is "Hello World!", the program outputs "Hello World!".
Handling the first character correctly: If the input is " Hello World!", the program outputs "Hello World!".
Handling the end of the file correctly: If the input is "Hello World!", the program outputs "Hello World!" and stops reading the input.

Conclusion

Exercise 1-9 is a simple yet effective exercise that helps you practice writing a program to replace consecutive spaces with a single space. By following the Q&A section provided above, you should be able to better understand the exercise and its solution. Remember to use a loop to iterate over the input, check for consecutive spaces, and handle the first character correctly. With practice and patience, you'll become proficient in writing C programs that solve real-world problems.