Introductory Programming Lecture Notes: C++ basic input

Basic input using cin

also in iostream library is the cin statement (accessed via std::cin or
using namespace std::cin;)
This allows the reading of user input from the keyboard
to read a value into a variable simply use
```
   cin >> variablename;
```
to read several values into a series of variables
```
   cin >> var1 >> var2 >> var3;
```
(these can be of different types, but be sure to prompt the user clearly as to the order in which the data should be entered)
For example, suppose we have the code fragment
```
   int Age;
   char Initial;
   float Income;

   cout << "Please enter your age, ";
   cout << "your middle initial, and your annual income, " << endl;
   cout << "e.g. 47 J 127,302.51" << endl;

   cin >> Age;
   cin >> Initial;
   cin >> Income;
```
This will work correctly as long as there is some whitespace (spaces, tabs, newlines, etc) between each pair of values, and as long as the user hits enter after the final value.
cin assumes the variables are separated, or delimited by whitespace and does not read a line of data until the user hits enter/return
Sometimes we want to read the whitespace the user is entering as well as the non-whitespace data, so to get the next character of input, including whitespace, declare a character variable to store it, then use cin.get()
```
   // my variable location
   char  mycharvar;

   // get the next character of data
   cin.get(mycharvar);   
```

Use of input

As stated above, the system won't read in a line of input until the user hits return
```
   int x, y, z;
   cout << "Please enter three integers" << endl;
   cin >> x >> y >> z;
```
If the user types in 3 7 21 but doesn't hit return afterward the system will just sit there

The data types the user types in must match the variables in the program, e.g.

   int i;  char c;  float f;
   cout << "Please enter a character, an integer,";
   cout << " and a floating point number" << endl; 
   cin >> c >> i >> f;

You should experiment with the impact of trying to enter the wrong kind of data into a variable (e.g. enter a char when a float or int is requested,
enter a float when an int or char is requested...)

Using files for input in Unix

Files for storing input

From the command line in Unix (i.e. when you are working in a terminal or dtterm window, or using ssh) you can use a file of data to provide input to a program instead of retyping the data each time.

This is handy if you have test cases you want to retry every time you alter the source code. Put the test cases in a file, and use the < symbol to direct the file data into your program:

csciun1.mala.bc.ca< myprogram < mydatafile

(Assuming your executable file is named myprogram and your test data is in file mydatafile) Note that when doing this the program doesn't take any input from the keyboard, it gets it all from the data file, so your data file must contain all the appropriate user responses in the correct order.

Also, the output is still printed to the monitor.

Files for storing output

Similarly, you can redirect the output from a program so that it is stored in a file instead of printed on the screen.

To do so, use the > symbol:

csciun1.mala.bc.ca< myprogram > resultsfile

If you wanted to use one file for input and store the results in another file, you can combine the two commands, e.g.

csciun1.mala.bc.ca< myprogram < mydatafile > resultsfile

Using programs to provide input for programs

Finally, if you want to directly use the output from one program as the input to another, you can use a pipe symbol | to connect the two:

csciun1.mala.bc.ca< generatorprogram | userprogram