CSCI-203 Lab 8 Fall 2002
- More emacs; Input from Files; vector Class -

Purpose

1. Learn more about emacs:Identation, Filename Completion, Search, Search-and-Replace.
2. Practice getting input from files
3. Work with the vector class
4. Combine input from files with vector operations
5. Practice the use of: ifstream, for, while, vector and vector::push_back().

More Details on the emacs Editor

Create a Lab8 directory and do your work there.

Coloring and Indentation

Select Open File and create a new file called lab8. Type in the following code:

#include <iostream>
#include <string>

int getValue(const string & inPrompt) {

    int val;
    cout << inPrompt;
    cin  >> val;
    return val;
}

int main() {

    int value;
    value = getValue("Type in a quiz grade: ");
    cout <<  "value = " << value << endl;

    return 0;
}

But, the TAB key does not work properly and no colors. Why? Because the file name did not end in .cc. emacs uses the file name to determine if a particular ``mode'' should be used when editing the file. The basic no-frills mode is the `Fundamental' mode, which you should see indicated on the status line (the darkened one) at the bottom of the emacs window. To get emacs into the proper mode `C++ Fill' mode you need to rename the file you are editing. To do this select `Save Buffer As...' from the File menu and use the name lab8.cc. As an alternative, you can use the command M-x rename-buffer. Notice that the file name (at the left end of the status line) is the new name and that the mode indicator says (C++ Fill). Also notice that the text in the window is now colorized properly. If you put the cursor on the top line you can do the trick with the tab key to get all the lines properly indented.

There are many emacs modes for different languages and situations, for example, C mode, Fortran mode, Lisp mode. and L^ATEX mode.

File Name Completion

Copy the files Fraction.h and Fraction.cc from ~cs203/include directory into your Lab8 directory.

cp ~cs203/include/Fraction.*  .

(Yes! That command will copy both.)

Now in emacs, select Open File and type only the character `F'. Now hit the space bar. What happened? emacs searched all the file names in the directory and completed as much of the name as possible. If there is only one file starting with `F' then the entire file name will be filled in (completed). Then when you hit return the file will be opened.

But in this case, you have two files starting with `F' so the file name is only completed as far as the common part goes -- i.e., up to the dot. Press the space bar again emacs splits your screen into two windows and displays the file choices which match your request so far. Move the mouse to the window with the file names and click (once) on Fraction.h -- it will be highlighted. Now press return and the file you selected is opened.

Hitting the space bar to complete a file name is a feature which many emacs users use heavily!

Search and Replace

Search

If you ever need to search for some text in a program, e.g., an object or function name, you can use the incremental search feature of emacs. Here's a demonstration. Press C-s and type `i' or select Search in the Search menu (Don't press the RET key). Now, press `n'. Now press `t'. Notice that emacs finds the first `i', then from there the first `in', then the first `int'. Continuing to press C-s will move the pointer to the next occurrence.

But what do you do once you find the thing (or don't find the thing) you are looking for. If you want to stay where you find the last occurrence, then click the left mouse button near the string you found. This will quit the search process. If you want to quit searching you can also press C-g, but that will take you back to where you originally began the search.

After you have quit the search, if you press C-s C-s, the same search will be repeated. C-r does the same as C-s except it searches backwards through the buffer.

Replace

Suppose, in the text of the lab8.cc program, you want to replace all of the object names value with the name cost. Move the filled rectangle anywhere before the first occurrence of `value' (to the beginning of file is often a good idea). From the Search menu select Query Replace.... When emacs prompts you at the bottom of the window, type in the string to be replaced, in this case, value, then press RET. Now type in the new text, in this case, cost followed by a RET. emacs will move the filled rectangle to the first occurrence of value and wait. You either type `y' to replace it or `n' to not replace it. Then emacs goes to the next occurrence of the text and again you type `y' or `n' and so on. To get out prematurely, just press RET. Change all occurrences of value to cost in the file by using query replace.

Input from Files

For the first part of the lab, you will write a program that counts the number of words in a message in a file, then displays the count.

• Open a file message.txt and type a message of your choice into it.

• Open a file wordCount.cc. In it, write a program that opens the file message.txt, then reads the words one at a time, counting each one as it is read. A few things to note:
  • You will need to use the directives

    #include <fstream>
    #include <string>
    

    to make your program work.

  • Words can be read by using a loop. Use a while loop for your counting, with the usual loop condition -- i.e., while(inFile » words).

  After the words are read, print out each on a line by itself and display the message

  There are <k> words in my message.
  

  where <k> is replaced by the count of the words in the file.

• Run your program. Now edit your file message.txt to make the message longer. Run your program again (you do not need to recompile!) to see the changed count.

• Create a handin.txt file, and include your your original message and the output of running wordCount.exe on it, and your revised message and the result of running wordCount.exe on it. Print handin.txt and wordCount.cc.

The vector Class

To use the vector class in your programs, you need to include the following header file (interface file):

#include <vector>

The general form of defining a vector object is the following:

vector<class-name> object-name(number-of-elements);

For example, to define an object x with 100 elements of the class int, we would type:

vector<int> x(100);

It's important to remember that vector indices always start at 0 in .

Part A

Write a program that creates a vector of 25 integers and with a definite loop assigns the values 1 to 25 to the elements. Output the vector on a line with a space between each value. Add the listing of the program and run to your lab handin.txt file.

The class of the elements in a vector is not restricted to only built-in simple classes like int and double. It may be any class that has a default constructor (no parameters). For example, one can use a vector of Fraction.

Part B

For this part of the lab, you are to write a program that computes the average of ten user-input values of type double. Your program should do the following:

1. Create a vector of ten double values.

2. Prompt the reader to enter a value, then read that value into the next unassigned entry on the vector. This should be repeated 10 times, which will require the use of the repetition control structure. Remember that vectors use indexing just as strings do; the first entry is in position 0, and the last will be in position 9.

3. After input is complete use a second loop to compute the average of the entries in the vector and output the average.

4. Print your program. Save a sample run in another file (such as output.txt) and print it too.

Combining Vectors and File Input

In this part of the lab you will combine reading data from a file with vectors. You will also practice using push_back() when you don't know in advance how big the vector should be.

Copy the program file fracs.cc and the data file fractions.dat from ~cs203/Labs/Lab8. This program is meant to read fraction data from a file and then display the fractional sum of that data. The data file contains a sequence of lines, each containing two integer values representing the numerator and denominator (in that order) of a fraction. You should read them from the file using a while(inFile » num » denom) then use num and denom to insert another fraction into the Fraction vector.

The program file you have copied is incomplete. You should do the following with the file.

1. Put the appropriate declaration in main() for the Fraction vector. Notice that the rest of main() assumes the vector is named `fracs'. Don't declare the vector to have any particular size.

2. Complete the two calls in main() by making sure the parameter lists are correct and complete.

3. Complete the function getFractions. Use push_back to add elements to your vector so that it will grow as necessary.

4. Complete the function sumFractions. This should look familiar. Use the vector method size() to determine the size of the vector.

Compile and execute the program -- remember to build the name of the data file into the program. You will also have to compile Fraction.cc in the ~cs203/include directory along with your modified fracs.cc. You may do this step by either referring to the file directly on the compile line or copying over Fraction.cc into your space. It is preferable if you refer directly to the file. Also, prepare a Makefile so that you will not need to retype the compile line each time. An added benefit of a Makefile is that it is a record of what you need to do to build an executable for when you return to this project at a later date.

Save a sample run of your program in an output file and then print your program and the sample run.