K&R Exercise 1-16. Find the longest line in the input and print its length and first N characters

Question

I'm going through the K&R book (2nd edition, ANSI C ver.) and want to get the most from it. Note that, for the sake of exercise, I don't want to use techniques not introduced yet in the book and I'm compiling with the -ansi flag.

K&R Exercise 1-16

Revise the main routine of the longest-line program so it will correctly print the length of arbitrarily long input lines, and as much as possible of the text.

Solution

Note the explicit requirement to revise the main routine. I'm assuming that the author expect us to find a way to use the 2 functions as they are to solve the problem. I've seen solutions which change the getline function, but I would say they're wrong even though the resulting program does what it should. Even the solution book revised the getline function, how disappointing.

/* Exercise 1-16. Revise the main routine of the longest-line program so
 * it will correctly print the length of arbitrarily long input lines,
 * and as much as possible of the text. */

#include <stdio.h>
#define MAXLINE 10          /* buffer size */

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line length and the line itself or the first
 * MAXLINE-1 characters if the line couldn't fit in the buffer */
main()
{
    int len;                /* current line length */
    int lenx;               /* extra length to add for too long lines */
    int max;                /* maximum length seen so far */
    char line[MAXLINE];     /* full line or beginning of line */
    char linex[MAXLINE];    /* overflow buffer */
    char longest[MAXLINE];  /* longest line saved here */

    max = 0;
    while ((len = getline(line, MAXLINE)) > 0) {
        /* If we didn't reach the endl, consume the input until
         * the end is reached while keeping track of the length */
        if (len == MAXLINE-1 && line[MAXLINE-2] != '\n')
            while ((lenx = getline(linex, MAXLINE)) > 0 &&
            (len = len + lenx) && linex[lenx-1] != '\n')
                ;
        if (len > max) {
                max = len;
                copy(longest, line);
        }
    }

    if (max > 0) {  /* there was a line */
        if (max > MAXLINE-1) {
            printf("Longest line length: %d, first ", max);
            printf("%d characters: \n%s\n", MAXLINE-1, longest);
        }
        else
            printf("Longest line length: %d; longest line: \n%s\n", max,
            longest);
    }
    return 0;
}

/* getline:  read a line into s, return length */
int getline(char s[], int lim)
{
    int c, i;

    for (i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';
    return i;
}

/* copy:  copy 'from' into 'to'; assume to is big enough */
void copy(char to[], char from[])
{
    int i;

    i = 0;
    while ((to[i] = from[i]) != '\0')
        ++i;
}

Original program (from the book)

#include <stdio.h>
#define MAXLINE 1000 /* maximum input line length */

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print longest input line */
main()
{
    int len;            /* current line length */
    int max;            /* maximum length seen so far */
    char line[MAXLINE];     /* current input line */
    char longest[MAXLINE];  /* longest line saved here */

    max = 0;
    while ((len = getline(line, MAXLINE)) > 0)
        if (len > max) {
            max = len;
            copy(longest, line);
        }
    if (max > 0)    /* there was a line */
        printf("%s", longest);
    return 0;
}

/* getline:  read a line into s, return length */
int getline(char s[], int lim)
{
    int c, i;

    for (i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';
    return i;
}

/* copy:  copy 'from' into 'to'; assume to is big enough */
void copy(char to[], char from[])
{
    int i;

    i = 0;
    while ((to[i] = from[i]) != '\0')
        ++i;
}

Answer 1

lets start with the book you are using is VERY obsolete.

Regarding: main() there are only two valid signatures for main() they are int main( void ) and int main( int argc, char *argv[] )

the header file: stdio.h contains a prototype for the function: getline() that does not match the signature in the posted code, so suggest changing the name of that function to something unique, like myGetline()

when compiling, always enable the warnings,, then fix those warnings. (for gcc, at a minimum use: -Wall -Wextra -Wconversion -pedantic -std=gnu11 ) Note: other compilers will use different options to perform the same functionality.

the logic is copying all line parts, overlaying them. So the only item that will be printed is the last part of the line. Suggest only copying the first part of the line and ignoring the rest.

here is what compiling the posted code causes the compiler to output:

gcc -ggdb -Wall -Wextra -Wconversion -pedantic -std=gnu11 -c "untitled.c" 

untitled.c:8:5: error: conflicting types for ‘getline’
 int getline(char line[], int maxline);
     ^~~~~~~

In file included from untitled.c:5:0:
/usr/include/stdio.h:616:20: note: previous declaration of ‘getline’ was here
 extern _IO_ssize_t getline (char **__restrict __lineptr,
                    ^~~~~~~
untitled.c:13:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 main()
 ^~~~

untitled.c:49:5: error: conflicting types for ‘getline’
 int getline(char s[], int lim)
     ^~~~~~~

In file included from untitled.c:5:0:
/usr/include/stdio.h:616:20: note: previous declaration of ‘getline’ was here
 extern _IO_ssize_t getline (char **__restrict __lineptr,
                    ^~~~~~~

untitled.c: In function ‘getline’:
untitled.c:54:16: warning: conversion to ‘char’ from ‘int’ may alter its value [-Wconversion]
         s[i] = c;
                ^

untitled.c:56:16: warning: conversion to ‘char’ from ‘int’ may alter its value [-Wconversion]
         s[i] = c;
                ^

Compilation failed.

Answer 2

Program optimized after comments received :)

/* Exercise 1-16. Revise the main routine of the longest-line program so
 * it will correctly print the length of arbitrarily long input lines,
 * and as much as possible of the text. */

/* Some optimization after comments received:
 * https://codereview.stackexchange.com/a/205992/181968 */

#include <stdio.h>
#define MAXLINE 10          /* buffer size */

int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line length and the line itself or the first
 * MAXLINE-1 characters if the line couldn't fit in the buffer */
main()
{
    int len;                /* current line length */
    int max;                /* maximum length seen so far */
    char line[MAXLINE];     /* current [beginning of] line */
    char longest[MAXLINE];  /* [beginning of] longest line saved here */
    int c;

    max = 0;
    while ((len = getline(line, MAXLINE)) > 0) {
        /* If we didn't reach the endl, consume the input until
         * the end is reached while keeping track of the length */
        if (len == MAXLINE-1 && line[MAXLINE-2] != '\n')
            while ((c=getchar())!=EOF && ++len && c!='\n')
                ;
        if (len > max) {
                max = len;
                copy(longest, line);
        }
    }

    if (max > 0) {  /* there was a line */
        if (max > MAXLINE-1) {
            printf("Longest line length: %d, first ", max);
            printf("%d characters: \n%s\n", MAXLINE-1, longest);
        }
        else
            printf("Longest line length: %d; longest line: \n%s\n", max,
            longest);
    }
    return 0;
}

/* getline:  read a line into s, return length */
int getline(char s[], int lim)
{
    int c, i;

    for (i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';
    return i;
}

/* copy:  copy 'from' into 'to'; assume to is big enough */
void copy(char to[], char from[])
{
    int i;

    i = 0;
    while ((to[i] = from[i]) != '\0')
        ++i;
}