C++ Seminar: 1094 Session 7

Handouts: "C++ Strings and Things" handout (with filename_test.cpp printout on the back)

Your goals for today: Finish up Session 7, do a bit more of Day 13 and some of Day 17, then an example of C++ strings used to create filenames.

Finish Up Session 7

If you didn't finish Session 7 (that may be just about everyone), go back and try to run through the rest of it. At this stage, getting exposure is more important than getting all the fine points.

Day 13: Arrays and Character Variables

This is a continuation of the discussion of arrays in Day 13. (If you haven't done that section from Session 6, go back and do it now.) A special type of array used for characters is called a "C-style string". It is designated with the keyword char (instead of int or double). You can declare a char array of length 80 like this:
char buffer[80];
or declare and initialize without specifying the length like this:
char Greeting[] = "Hello World";
In the second example, the array Greeting is 12 bytes long, one for each character you see and one more for a "null character" at the end, which marks the end of the array. [Note: In the last section below we'll look at using the C++ string class, which is also used for text.]

1. Look at List1310.cpp first. It declares a character array to read in a "string" (this just means a sequence of characters) from the user. Run it and enter your first name. Then run it again and enter your full name. What's the problem? (You're not expected to fix it here!)
   
   
   
   
2. A solution to the problem is given in List1311.cpp by using the get method (i.e., function) of cin. It takes two arguments (actually three in general!), the name of a char array and the maximum number of characters to read in. The unused third argument is an optional delimiter that signals where to stop (by default it is the newline character, which means it terminates with a return). Try it out with full sentences. Change the maximum number of characters to 10 and see what happens when you try a string of length 20 or so. What happens if the maximum number of characters allowed (and entered) is greater than the size of the char array?
   
   
   
   
3. There are various library functions that can be used to manipulate C-style strings. To access them, we need to include string.h. One example is strcpy, which is used to copy one string into another. Check out List1312.cpp for an example. Figure out from the example how it works and test it by getting a string from the user and copying it into String1. A similar function, strncpy, which lets you specify the maximum number of characters copied, is illustrated in List1313.cpp. Try changing MaxLength to something like 5.
   
   
   
   

Day 17: Streams

Day 17 is about streams, which we've been using all along for input from the keyboard and output to the screen. There is much to cover here, with many example programs available from the web page. We'll pick a subset of useful things to look at today; we recommend you explore the others!

1. List1701.cpp is a reminder that cin and cout are smart enough to figure out what type of variable is being input or output, and to store or print it appropriately. Just take a quick look.
   
   
   
   
2. We haven't tried to control what the output looks like; we've just taken what we're given. Let's see what we can adjust. First is List1712.cpp, which shows how to set the width of output. Try changing the values in the width to understand what it is doing. Then look at List1713.cpp to see how we can "fill" the result. Try different fill characters.
   
   
   
   
3. In List1716.cpp, we see an example of opening a file and writing to it. There are several new features here. Your task is to step through the code and figure out how to open a file with the same name as your own with a "_" between your names (e.g., Dick_Furnstahl) and write the numbers from 1 to 10 into it. An example of how to append to an existing file is in List1717.cpp. Try it out on your "name" file.
   
   
   
   

Extra: Strings and Things

The handout "C++ Strings and Things" gives an abbreviated summary of how to use the C++ string class with the goal of constructing filenames for input and output. C++ strings are an alternative to working with char arrays. On the back of the handout is a printout of the file filename_test.cpp, which we'll examine here.

1. Download filename_test.cpp and compile and run it. Then take a look at the printout of the code and the output files that are generated. Several things are illustrated one after another. Following the first example in the code and the table in the handout, add lines to declare a string called "my_string" and assign it a phrase. Print it out to check that it worked. Then add additional lines redefining my_string to a different word or phrase, and print it out. (Note that the size of the string adjusts automagically.)
   
   
   
   
2. Modify the code so that there is a loop running from 0 to 3 with index variable j. For each j, open a file with a name that includes the current value of j. Write "This is file j", where "j" here is the current value, into each file and then close it.
   
   
   
   
3. Modify the code to input a double named alpha and open a filename with 3 digits of alpha as part of the name. (E.g., something like output_alpha5.22_plot.dat if alpha = 5.21934.) Output something appropriate to the file as part of your test.