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Submitters, please note:

1) On receiving a bug report which appears credible, I will annotate the relevant solution with the bug report, and email the originator of the solution.

2) If the originator of the solution has either provided a fix (in the form of a complete listing to replace the existing listing) or shown why the bug report is incorrect, within six weeks of being informed of the bug report, then I will update the relevant page accordingly. If not, I reserve the right to remove the solution from the page altogether (but I'll make every reasonable effort to contact the contributor again before taking this step).

COPYRIGHT

Any uncredited listings are supplied by me, so beware! Complaints to email, please...

Naming Conventions

The HTML page names are constructed as follows:

krx - prefix (Kernighan and Ritchie eXercises)

c - chapter number (1 to 8)

ee - exercise number

0v - ANSI/ISO C89 compliant. The example only uses the subset of C already covered at the point in the book at which the exercise appears.

1v - ANSI/ISO C89 compliant. The example uses aspects of C which may not have been covered at the point in the book at which the exercise appears.

2v - ANSI/ISO C99 compliant. The example only uses the subset of C already covered at the point in the book at which the exercise appears. It is compliant with the C99 standard (e.g. doesn't use implicit int ).

3v - ANSI/ISO C99 compliant. The example uses some aspects of C which exist only in the C99 release of the language.

The following solutions have not yet been addressed at any level. If you'd like to provide a solution, please do let me know. You know where to find me.

Chapter 1 - A Tutorial Introduction

Run the "hello world" program on your system. Experiment with leaving out parts of the program, to see what error messages you get.

Experiment to find out what happens when printf 's argument string contains \c, where c is some character not listed above.

Modify the temperature conversion program to print a heading above the table.

Write a program to print the corresponding Celsius to Fahrenheit table.

Modify the temperature conversion program to print the table in reverse order, that is, from 300 degrees to 0.

Category 0 Solutions by my good self and by Chris Sidi.

Verify that the expression getchar() != EOF is 0 or 1.

Write a program to print the value of EOF.

Write a program to count blanks, tabs, and newlines.

Write a program to copy its input to its output, replacing each string of one or more blanks by a single blank.

Category 0 Solutions by my good self and by Chris Sidi.

Write a program to copy its input to its output, replacing each tab by \t , each backspace by \b , and each backslash by \\ . This makes tabs and backspaces visible in an unambiguous way.

How would you test the word count program? What kinds of input are most likely to uncover bugs if there are any?

Write a program that prints its input one word per line.

Write a program to print a histogram of the lengths of words in its input. It is easy to draw the histogram with the bars horizontal; a vertical orientation is more challenging.

Write a program to print a histogram of the frequencies of different characters in its input.

Rewrite the temperature conversion program of Section 1.2 to use a function for conversion.

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.

Write a program to print all input lines that are longer than 80 characters.

Solution by "MJSR"

Write a program to remove all trailing blanks and tabs from each line of input, and to delete entirely blank lines.

Solution by Ben Pfaff, modification by Chris Sidi

Write a function reverse(s) that reverses the character string s . Use it to write a program that reverses its input a line at a time.

Write a program detab that replaces tabs in the input with the proper number of blanks to space to the next tab stop. Assume a fixed set of tab stops, say every n columns. Should n be a variable or a symbolic parameter?

Write a program entab that replaces strings of blanks with the minimum number of tabs and blanks to achieve the same spacing. Use the same stops as for detab . When either a tab or a single blank would suffice to reach a tab stop, which should be given preference?

Solution by Rick Dearman

Write a program to "fold" long input lines into two or more shorter lines after the last non-blank character that occurs before the n -th column of input. Make sure your program does something intelligent with very long lines, and if there are no blanks or tabs before the specified column.

Category 1 Solution by Rick Dearman

Write a program to remove all comments from a C program. Don't forget to handle quoted strings and character constants properly. C comments do not nest.

Write a program to check a C program for rudimentary syntax errors like unbalanced parentheses, brackets and braces. Don't forget about quotes, both single and double, escape sequences, and comments. (This program is hard if you do it in full generality.)

Solution by Rick Dearman

Chapter 2 - Types, Operators and Expressions

Write a program to determine the ranges of char , short , int , and long variables, both signed and unsigned , by printing appropriate values from standard headers and by direct computation. Harder if you compute them: determine the ranges of the various floating-point types.

Solution by Rick Dearman

Write a loop equivalent to the for loop above without using && or || .

Write the function htoi(s) , which converts a string of hexadecimal digits (including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0 through 9, a through f, and A through F .

Write an alternate version of squeeze(s1,s2) that deletes each character in the string s1 that matches any character in the string s2 .

Write the function any(s1,s2) , which returns the first location in the string s1 where any character from the string s2 occurs, or -1 if s1 contains no characters from s2 . (The standard library function strpbrk does the same job but returns a pointer to the location.)

Write a function setbits(x,p,n,y) that returns x with the n bits that begin at position p set to the rightmost n bits of y, leaving the other bits unchanged.

Write a function invert(x,p,n) that returns x with the n bits that begin at position p inverted (i.e., 1 changed into 0 and vice versa), leaving the others unchanged.

Solution by Gregory Pietsch

Write a function rightrot(x,n) that returns the value of the integer x rotated to the right by n bit positions.

Solution by Gregory Pietsch

In a two's complement number system, x &= (x-1) deletes the rightmost 1-bit in x . Explain why. Use this observation to write a faster version of bitcount .

Solution by Gregory Pietsch

Rewrite the function lower, which converts upper case letters to lower case, with a conditional expression instead of if-else .

Solution by Bryan Williams

Chapter 3 - Control Flow

Our binary search makes two tests inside the loop, when one would suffice (at the price of more tests outside). Write a version with only one test inside the loop and measure the difference in run-time.

Solutions by Paul Griffiths and Colin Barker

Write a function escape(s,t) that converts characters like newline and tab into visible escape sequences like \n and \t as it copies the string t to s . Use a switch . Write a function for the other direction as well, converting escape sequences into the real characters.

Solution by Paul Griffiths

Write a function expand(s1,s2) that expands shorthand notations like a-z in the string s1 into the equivalent complete list abc...xyz in s2 . Allow for letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-9 and -a-z . Arrange that a leading or trailing - is taken literally.

Solution by Paul Griffiths

In a two's complement number representation, our version of itoa does not handle the largest negative number, that is, the value of n equal to -(2 to the power (wordsize - 1)) . Explain why not. Modify it to print that value correctly regardless of the machine on which it runs.

Solution by Paul Griffiths

Write the function itob(n,s,b) that converts the integer n into a base b character representation in the string s . In particular, itob(n,s,16) formats n as a hexadecimal integer in s .

Solution by Paul Griffiths

Write a version of itoa that accepts three arguments instead of two. The third argument is a minimum field width; the converted number must be padded with blanks on the left if necessary to make it wide enough.

Solution by Paul Griffiths

Chapter 4 - Functions and Program Structure

Write the function strrindex(s,t) , which returns the position of the rightmost occurrence of t in s , or -1 if there is none.

Solution by Rick Dearman

Extend atof to handle scientific notation of the form 123.45e-6 where a floating-point number may be followed by e or E and an optionally signed exponent.

Solution by Dann Corbit

Given the basic framework, it's straightforward to extend the calculator. Add the modulus ( % ) operator and provisions for negative numbers.

Solution by Bob Wightman

Add commands to print the top element of the stack without popping, to duplicate it, and to swap the top two elements. Add a command to clear the stack.

Solution by Bob Wightman

Add access to library functions like sin , exp , and pow . See <math.h> in Appendix B, Section 4.

Solution by Bob Wightman

Modify getop so that it doesn't need to use ungetch. Hint: use an internal static variable.

Solution by Gregory Pietsch

Adapt the ideas of printd to write a recursive version of itoa ; that is, convert an integer into a string by calling a recursive routine.

Solution by Gregory Pietsch

Write a recursive version of the function reverse(s) , which reverses the string s in place.

Solution by Gregory Pietsch

Define a macro swap(t,x,y) that interchanges two arguments of type t . (Block structure will help.)

Solutions by Gregory Pietsch and Lars Wirzenius

Chapter 5 - Pointers and Arrays

As written, getint treats a + or - not followed by a digit as a valid representation of zero. Fix it to push such a character back on the input.

Solution by Gregory Pietsch

Write getfloat , the floating-point analog of getint . What type does getfloat return as its function value?

Solutions by Chris Mears and Gregory Pietsch

Write a pointer version of the function strcat that we showed in Chapter 2: strcat(s,t) copies the string t to the end of s .

Write the function strend(s,t) , which returns 1 if the string t occurs at the end of the string s , and zero otherwise.

Solution by Bryan Williams

Write versions of the library functions strncpy , strncat , and strncmp , which operate on at most the first n characters of their argument strings. For example, strncpy(s,t,n) copies at most n characters of t to s . Full descriptions are in Appendix B.

Solution by Lars Wirzenius

Rewrite appropriate programs from earlier chapters and exercises with pointers instead of array indexing. Good possibilities include getline (Chapters 1 and 4), atoi , itoa , and their variants (Chapters 2, 3, and 4), reverse (Chapter 3), and strindex and getop (Chapter 4).

Solution by Gregory Pietsch

There is no error-checking in day_of_year or month_day. Remedy this defect.

Solution by Lars Wirzenius

Rewrite the routines day_of_year and month_day with pointers instead of indexing.

Solutions by Lars Wirzenius and Gregory Pietsch

Write the program expr , which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument. For example,
expr 2 3 4 + *
evaluates 2 X (3+4).

Category 1 Solution by Lars Wirzenius

Modify the programs entab and detab (written as exercises in Chapter 1) to accept a list of tab stops as arguments. Use the default tab settings if there are no arguments.

Solution by Gregory Pietsch

Write the program tail, which prints the last n lines of its input. By default, n is 10, say, but it can be changed by an optional argument, so that

  tail -n

Solution by Gregory Pietsch

Chapter 6 - Structures

Our version of getword does not properly handle underscores, string constants, comments, or preprocessor control lines. Write a better version.

Solution by Ben Pfaff

Write a cross-referencer that prints a list of all words in a document, and, for each word, a list of the line numbers on which it occurs. Remove noise words like "the," "and," and so on.

Write a program that prints the distinct words in its input sorted into decreasing order of frequency of occurrence. Precede each word by its count.

Solution by Bryan Williams

Write a function undef that will remove a name and definition from the table maintained by lookup and install .

Solution by Paul Griffiths

Chapter 7 - Input and Output

Write a program that converts upper case to lower or lower case to upper, depending on the name it is invoked with, as found in argv[0].

Write a program that will print arbitrary input in a sensible way. As a minimum, it should print non-graphic characters in octal or hexadecimal according to local custom, and break long text lines.

Write a program to compare two files, printing the first line where they differ.

Solution by "flippant_squirrel" (!)

Listing krx706



Exercise 7-9, page 168

Functions like isupper can be implemented to save space or to save time. Explore both possibilities.

Solution by Gregory Pietsch

Chapter 8 - The UNIX System Interface

Please note: these solutions use the Unix system interface, so they're not portable. Nevertheless, they're in K&R, so they appear here despite their inherent non-portability. In any case, they will become portable as soon as everyone's using Unix, as God intended.

Rewrite the program cat from Chapter 7 using read , write , open and close instead of their standard library equivalents. Perform experiments to determine the relative speeds of the two versions.

Solution by Andrew Tesker

Design and write _flushbuf , fflush and fclose .

Solution by Gregory Pietsch

The standard library function
int fseek(FILE *fp, long offset, int origin)
is identical to lseek except that fp is a file pointer instead of a file descriptor and the return value is an int status, not a position. Write fseek . Make sure that your fseek coordinates properly with the buffering done for the other functions of the library.

Solution by Gregory Pietsch

The standard library function calloc(n,size) returns a pointer to n objects of size size , with the storage initialized to zero. Write calloc , by calling malloc or by modifying it.

Solution by Bryan Williams

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