K&R C Exercise 1-17: Print Lines Longer Than 80 Characters

Exercise 1-17. Write a program to print all input lines that are longer than 80 characters.

Approach

This exercise is a direct application of the getline / main pattern introduced in K&R Section 1.9 — but now with a filter condition added to main. The key insight is that getline already returns the line length, so the filter is a single comparison: len > LONGLINE. There is no need to call strlen().

Two details are worth watching. First, getline is a name reserved by POSIX, so the function is renamed getline2 to avoid conflicts. Second, the condition is strictly greater than 80 — a line of exactly 80 characters is not printed. Defining the threshold as a named constant (LONGLINE 80) makes it trivial to adjust later.

Solution

/* K&R Exercise 1-17: print lines longer than 80 characters        */
/* Compile: gcc -ansi -Wall exercise1-17.c -o exercise1-17          */

#include <stdio.h>

#define MAXLINE  1000   /* maximum input line length to store        */
#define LONGLINE   80   /* threshold: lines longer than this printed */

int getline2(char line[], int maxline);

int main(void)
{
    int len;
    char line[MAXLINE];

    while ((len = getline2(line, MAXLINE)) > 0)
        if (len > LONGLINE)
            printf("%s", line);

    return 0;
}

/* getline2: read a line into line[], return its length */
int getline2(char line[], int maxline)
{
    int c, i;

    for (i = 0; i < maxline - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
        line[i] = c;

    if (c == '\n') {
        line[i] = '\n';
        ++i;
    }
    line[i] = '\0';
    return i;
}

Compile and Run

gcc -ansi -Wall exercise1-17.c -o exercise1-17
./exercise1-17

The program reads from standard input and writes matching lines to standard output — the classic Unix filter pattern. Type lines at the terminal (Ctrl-D to end), or pipe input from a file or command:

# Generate a test line of 85 'a' characters and pipe it in
python3 -c "print('a'*85)" | ./exercise1-17

# Or use a file
./exercise1-17 < somefile.txt

Sample Output

Input (four lines):
  Short line.                                        (11 chars  — not printed)
  Exactly eighty characters long, padded to fill it precisely here, yes it is.  (80 chars — not printed)
  This line is eighty-one characters long — one past the limit — so it appears.  (81 chars — PRINTED)
  Another very long line that clearly exceeds eighty characters and should appear in output.  (91 chars — PRINTED)

Output:
  This line is eighty-one characters long — one past the limit — so it appears.
  Another very long line that clearly exceeds eighty characters and should appear in output.

Edge Cases

• Exactly 80 characters: not printed — the condition is len > LONGLINE, not >=.
• Empty lines: getline2 returns 0 for an empty line (just a newline); the while loop exits cleanly.
• Lines longer than MAXLINE: characters beyond position 998 are silently discarded. For production code, consider reading in chunks; for this exercise the large buffer is fine.
• No trailing newline at end of file: the if (c == '\n') guard handles this — the final partial line is still returned and counted correctly.

What This Exercise Teaches

• The filter pattern: stdin → process → stdout. The program prints nothing unless a line qualifies. This is the foundation of Unix pipelines (grep, awk, sed).
• Reusing a function from the book: getline2 is lifted almost verbatim from Section 1.9. The exercise trains you to adapt existing building blocks rather than rewrite from scratch.
• Using the return value as metadata: getline2 returns the length as part of its contract. main exploits this directly — there is no separate strlen() call.
• Named constants for thresholds: #define LONGLINE 80 means changing the threshold to 120 requires editing exactly one line.

Set Up Your C Environment

To compile and run this solution you need GCC. If you haven’t set it up yet:

• Install GCC on Windows 11
• Install GCC on macOS
• Install GCC on Ubuntu / Linux
• VS Code for C Programming — recommended editor
• Complete C Development Environment Setup

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Chapter 1 Solutions  | 
Exercise 1-18 →

Book:

The C Programming Language, 2nd Ed — Kernighan & Ritchie