The C Programming Language Solutions

Provide high quality solutions to the exercises in "The C Programming Language" by Kernighan and Ritchie (2nd edition), also referred to as "K&R C Bible".

Table of Contents

• Chapter 1. A Tutorial Introduction
• Chapter 2. Types, Operators, and Expressions
• Chapter 3. Control Flow
• Chapter 4. Functions and Program Structure
• Chapter 5. Pointers and Arrays

Chapter 1. A Tutorial Introduction

• Exercise 1-9. Write a program to copy its input to its output, replacing each string of one or more blanks by a single blank. solution
• Exercise 1-10. 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. solution
• Exercise 1-12. Write a program that prints its input one word per line. solution
• Exercise 1-13. 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 orientations is more challenging. solution
• Exercise 1-14. Write a program to print a histogram of the frequencies of different characters in its input. solution
• Exercise 1-15. Rewrite the temperature conversion program of Section 1.2 to use a function for conversion. solution
• Exercise 1-16. Revise the main routine of the longest-line program so it will correctly print the lenght of arbitrarily long input lines, and as much as possilbe of the text. solution
• Exercise 1-17. Write a program to print all input lines that are longer than 80 characters. solution
• Exercise 1-18. Write a program to remove trailing blanks and tabs from each line of input, and to delete entriely blank lines. solution
• Exercise 1-19. 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. solution
• Exercise 1-20. 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 symbolic parameter? solution
• Exercise 1-21. Write a program entab that replaces strings of blanks by the minimum number of tabs and blanks to achieve the same spacing. Use the same tab 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
• Exercise 1-22. 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. solution
• Exercise 1-23. 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. solution
• Exercise 1-24. 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

Chapter 2. Types, Operators, and Expressions

• Exercise 2-1. Write a program to determine the ranges of char, short, int, and long variables, both signed and unsigned, by printing appropriate values from standard hearders and by direct computation. Harder if you compute them: determine the ranges of the various floating-point types. solution
• Exercise 2-2. Write a loop equivalent to the for loop above without using && or ||. solution
• Exercise 2-3. 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. solution
• Exercise 2-4. Write an alternate version of squeeze(s1, s2) that deletes each character in s1 that matches any character in the string s2. solution
• Exercise 2-5. Write the function any(s1, s1), 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.) solution
• Exercise 2-6. 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. solution
• Exercise 2-7. Write a function invert(x, p, n) that return 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
• Exercise 2-8. Write a function rightrot(x, n) that returns the value of the integer x rotated to the right by n bit positions.
• Exercise 2-9. 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
• Exercise 2-10. Rewrite the function lower, which converts the upper case letters to lower case, with a conditional expression instead of if-else. solution

Chapter 3. Control Flow

• Exercise 3-1. 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. solution
• Exercise 3-2. 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, coverting escape sequences into the real characters. solution
• Exercise 3-3. Write a function expand(s1, s2) that expands shorhand 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
• Exercise 3-4. 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^{wordsize-1}). Explain why not. Modify it to print that value correctly, regardless of the machine on which it runs. solution
• Exercise 3-5. Write the function itob(n, s, b) that converts the integer n into a base b character representation in the string s. In particalar, itob(n, s, 16) formats n as a hexadecimal integer in s. solution
• Exercise 3-6. 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

Chapter 4. Functions and Program Structure

• Exercise 4-1. 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
• Exercise 4-2. 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
• Exercise 4-3. Given the basic framework, it's straightforward to extend the calculator. Add the modulus (%) and provisions for negative numbers. solution
• Exercise 4-4. Add commands to print the top element of the stack without poping, to duplicate it, and to swap the top two elements. Add a command to clear the stack. solution
• Exercise 4-5. Add access to library functions like sin, exp and pow. See <math.h> in Appendix B, Section 4. solution
• Exercise 4-6. Add commands for handling variables. (It's easy to provide twenty-six variables with single-letter names.) Add a variable for the most recently printed value.
• Exercise 4-7. Write a routine ungets that will push back an entire string onto the input. Should ungets know about buf and bufp, or should it just use ungetch?
• Exercise 4-8. Supppose that there will never be more than one character of pushback. Modify getch and ungetch accordingly.
• Exercise 4-9. Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties ougth to be if an EOF is pushed back, then implement your design.
• Exercise 4-10. An alternative organization uses getline to read an entire input line; this makes getch and ungetch unnecessary. Revise the calculator to use this appproach.
• Exercise 4-11. Modify getop so that it doesn't need to use ungetch. Hint: use an internal static variable. solution
• Exercise 4-12. 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
• Exercise 4-13. Write a recursive version of the function reverse(s), which reverse the string s in place. solution
• Exercise 4-14. Define a macro swap(t, x, y) that interchanges two arguments of type t. (Block structure will help.) solution

Chapter 5. Pointers and Arrays

• Exercise 5-1. 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
• Exercise 5-2. Write getfloat, the floating-point analog of getint. What type does getfloat return as its function value? solution
• Exercise 5-3. 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.
• Exercise 5-4. Write the function strend(s, t), which returns 1 if the string t occurs at the end of the string s, and zero otherwise.
• Exercise 5-5. Write the version of the library functions strncpy, strncat, and strncmp, which operate on at most the first n characters of their arguement strings. For example, strncpy(s, t, n) copies at most n characters of t to s. Full descriptions are in Appendix B.
• Exercise 5-6. Rewrite appropriate programs from earlier chapters and exercises with ponters instead of array indexing. Good possibilites include getline (Chapter 1 and 4), atoi, itoa, and their variants (Chapter 2, 3, and 4), reverse (Chapter 3), and strindex and getop (Chapter 4).
• Exercise 5-7. Rewrite readlines to store lines in an array supplied by main, rather than calling alloc to maintain storage. How much faster is the program? solution
• Exercise 5-8. There is no error checking in day_of_year or month_day. Remedy this defect. solution
• Exercise 5-9. Rewrite the routines day_of_year and month_day with pointers instead of indexing. solution