Computer Science 2610
Summer 2000
Laboratory Assignment 8

Handed out: 7/11/00

This exercise uses arrays of characters, which are used to store strings of characters.

For this assignment, a "word" is any sequence of characters that does not contain a blank or newline character. For example, "r92#sm" is a word.

Step 1

Write a function that takes an array of characters holding a null-terminated string as a parameter. It should print the characters in that string backwards. For example, if given string "abcd", it should print dcba.

You should not have to tell this function how long the string is, since it can find out for itself. Function strlen computes the length (logical size) of a null-terminated string. Write strlen(s) for the length of the string stored in array s. Be sure to include system header file string.h to use strlen.

Write a small main program that tests this function. An easy way to test it is to give it constant strings. For example, if your function is called PrintBackwards, then try writing 

  PrintBackwards("abcde");

Test your function before moving on.

Step 2

Write a program that starts by reading the names of two files from the user: an input file name and an output file name.

Read each file name into an array of characters. If you read an array of characters using an ifstream object such as cin, a string will be read up to a blank or end-of-line. (But see footnote.) So if you write 

   char infilename[MAX_FILE_NAME_SIZE];
   cin >> infilename;
then the name of a file will be read from the user, and stored in array infilename. You will need to define constant MAX_FILE_NAME_SIZE.

Step 3

Next the program should open the input file, creating a new ifstream object, and also open the output file, creating a new ofstream object. Just use the array names as the names of the files to open. The array of characters will be treated as a string. For example, if you write 
  ifstream ins(infilename);
then object ins is tied to the file whose name is stored in array infilename. Anything that you read from ins comes from that file. Use ins like you would cin.

Note: When you open a file, it is a good idea to check whether the file was opened successfully. You can use the fail operation for that as well. So to open a file for reading, write 

    ifstream ins(infilename);
    if(ins.fail()) {
      ... what to do if filename could not be opened for reading.
    }

Step 4

The program should read words from the input file one at a time into another array of characters. After each word, that word should be printed backwards to the output file.

You can read the word using >>, just as you did for the file name.

Put a space between adjacent words.

The program should stop when it reaches the end of the input file. You can detect the end of file by asking the ifstream object whether it has failed on the previous attempt to read. Write 

    if(cin.fail()) {
      ...
    }
to ask the cin object whether it failed on the previous attempt to read. (Of course, you won't be asking cin if it failed. Who do you want to ask?)

When the program finishes, it should close both the input and the output file.

Test your program before moving on.

Step 5

As written, your program puts all of the words on one line. For this step, you will keep the lines reasonably short.

Keep track of the number of characters printed on each line. After each word, check whether you have printed at least 50 characters. If so, start a new line of output. That way each line will only have a little more than 50 characters on it.

Do not print an end-of-line character in the middle of a word. Break the lines between words.

For example, if the file contains 

T'was brillig and the slithy toves
Did gyre and gimbol in the wabe.
All mimsy were the borogoves,
And the mome raths outgrabe.
then the following would be printed. 
saw'T gillirb dna eht yhtils sevot diD eryg dna lobmig 
ni eht .ebaw lla ysmim erew eht ,sevogorob dnA eht emom
shtar .ebargtuo

Be sure to count the spaces as part of the 50 characters.

Test your program on a file that is long enough to force a few lines of output to be printed.

Turn in a printout of your program.

Footnote

When you read a string, normally the >> operation
  1. Skips any leading white space (spaces, tabs and newlines).
  2. Reads up to a white space character or end of file.
But if, after skipping leading white space, it sees a double quote mark ("), then the >> operator reads a string up to the next double quote mark, and puts the string that is between the quotes into the array. So if your program reads 
  Here are "some words"
it will print 
  ereH era sdrow emos
since it will treat "some words" as a single word.