Printf: Reading User Values

The previous program is good, but it would be better if it read in the values 5 and 7 from the user instead of using constants. Try this program instead: 

#include 

int main()
{
   int a, b, c;
   printf("Enter the first value:");
   scanf("%d", &a);
   printf("Enter the second value:");
   scanf("%d", &b);
   c = a + b;
   printf("%d + %d = %d\n", a, b, c);
   return 0;
}

Here's how this program works when you execute it:


Make the changes, then compile and run the program to make sure it works. Note that scanf uses the same sort of format string as printf (type man scanf for more info). Also note the & in front of a and b. This is the address operator in C: It returns the address of the variable (this will not make sense until we discuss pointers). You must use the & operator in scanf on any variable of type char, int, or float, as well as structure types (which we will get to shortly). If you leave out the & operator, you will get an error when you run the program. Try it so that you can see what that sort of run-time error looks like.

Let's look at some variations to understand printf completely. Here is the simplest printf statement: 

    printf("Hello");

This call to printf has a format string that tells printf to send the word "Hello" to standard out. Contrast it with this: 

    printf("Hello\n");

The difference between the two is that the second version sends the word "Hello" followed by a carriage return to standard out.

The following line shows how to output the value of a variable using printf. 

    printf("%d", b);

The %d is a placeholder that will be replaced by the value of the variable b when the printf statement is executed. Often, you will want to embed the value within some other words. One way to accomplish that is like this: 

    printf("The temperature is ");
   printf("%d", b);

canf

The scanf function allows you to accept input from standard in, which for us is generally the keyboard. The scanf function can do a lot of different things, but it is generally unreliable unless used in the simplest ways. It is unreliable because it does not handle human errors very well. But for simple programs it is good enough and easy-to-use.

The simplest application of scanf looks like this: 

    scanf("%d", &b);

The program will read in an integer value that the user enters on the keyboard (%d is for integers, as is printf, so b must be declared as an int) and place that value into b.

The scanf function uses the same placeholders as printf:

  • int uses %d
  • float uses %f
  • char uses %c
  • character strings (discussed later) use %s
You MUST put & in front of the variable used in scanf. The reason why will become clear once you learn about pointers. It is easy to forget the & sign, and when you forget it your program will almost always crash when you run it.

In general, it is best to use scanf as shown here -- to read a single value from the keyboard. Use multiple calls to scanf to read multiple values. In any real program, you will use the gets or fgets functions instead to read text a line at a time. Then you will "parse" the line to read its values. The reason that you do that is so you can detect errors in the input and handle them as you see fit.

The printf and scanf functions will take a bit of practice to be completely understood, but once mastered they are extremely useful.

Try This!
  • Modify this program so that it accepts three values instead of two and adds all three together: 

    #include 

    int main()
    {
       int a, b, c;
       printf("Enter the first value:");
       scanf("%d", &a);
       printf("Enter the second value:");
       scanf("%d", &b);
       c = a + b;
       printf("%d + %d = %d\n", a, b, c);
       return 0;
    }

  • Try deleting or adding random characters or words in one of the previous programs and watch how the compiler reacts to these errors.

    For example, delete the b variable in the first line of the above program and see what the compiler does when you forget to declare a variable. Delete a semicolon and see what happens. Leave out one of the braces. Remove one of the parentheses next to the main function. Make each error by itself and then run the program through the compiler to see what happens. By simulating errors like these, you can learn about different compiler errors, and that will make your typos easier to find when you make them for real.

C Errors to Avoid
  • Using the wrong character case - Case matters in C, so you cannot type Printf or PRINTF. It must be printf.
  • Forgetting to use the & in scanf
  • Too many or too few parameters following the format statement in printf or scanf
  • Forgetting to declare a variable name before using it 


   printf(" degrees\n");

An easier way is to say this: 

    printf("The temperature is %d degrees\n", b);

You can also use multiple %d placeholders in one printf statement: 

    printf("%d + %d = %d\n", a, b, c);

In the printf statement, it is extremely important that the number of operators in the format string corresponds exactly with the number and type of the variables following it. For example, if the format string contains three %d operators, then it must be followed by exactly three parameters and they must have the same types in the same order as those specified by the operators.

You can print all of the normal C types with printf by using different placeholders:

  • int (integer values) uses %d
  • float (floating point values) uses %f
  • char (single character values) uses %c
  • character strings (arrays of characters, discussed later) use %s
You can learn more about the nuances of printf on a UNIX machine by typing man 3 printf. Any other C compiler you are using will probably come with a manual or a help file that contains a description of printf.