Friday, 21 January 2011

The C Programming Language (K&R) 01x13—Char Arrays & Functions—Section 1.9

Section 1.9 of “The C Programming Language” by Brian Kernighan and Dennis Ritchie aka K&R discusses character arrays and how to use them as an argument for a function. The general rule is that a function receives its argument as a value and not a reference to a variable. That means the called function can manipulate the argument without affecting the variable in the calling function. That’s not the case with a character array. The function can change the array even though the variable is declared in the calling function. Note: there are excepts for variables declared outside of a function (i.e., global scope) and where pointers are used.

The sample code in section 1.9 inputs characters from the keyboard one line at a time and finds the longest line from all entered.

Two lines of code of interest.

for (i=0; i < lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)

The loop will continue provided three conditions are true:

1. The current character count (i) is less than the limit less 1,
2. The current character is not EOF, AND
3. The current character is not a newline.

The first condition is required because of the maximum number of elements the array can hold. Without it, you could exceed the limit and write data to memory that is being used elsewhere. This leads to corrupted data and crashed programs.

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

C results in code you wouldn’t expect but work. A single equal sign is the assignment operator and not the equal to test. The condition is true until it reaches the end of the array as marked with the null character.

Sample Code.

I am using Visual C++ 2010 and created the sample code as a console application.

// Function prototype.
int getline(char line[], int maxline);
void copy(char to[], char from[]);

// The standard library includes the system function.
#include <cstdlib>

// Standard I/O library.
#include <cstdio>

#define MAXLINE 1000 /* maximum input line length */

int 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);

     // Keep console window open.

     // Return some value.
     return 0;

} // end main

/* 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;

     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')

The longest line is…

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