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6B. Writing

Printf

The <cstdio> library is a popular one for writing information. To use it, #include <cstdio>.

To print string s, use printf(s);. For example, statement

  printf("Have a nice day.\n");

writes Have a nice day on stdout. Notice character sequence \n in the string. It is an end-of-line indicator, usually abbreviated to newline. A line ends only when you explicitly end it. For example,

  printf("Have a ");
  printf("nice day.\n");

writes

Have a nice day.

Don't forget to end lines, or your output will be unreadable.

Printf can actually take any number of parameters. Printf is short for "print formatted", and the string is called a format. The format can contain value designators, each starting with %. Printf substitutes the value of the n-th parameter after the initial string for the n-th value designator. For example, suppose that x is 25 and y is 500. Then

  printf("x = %i and y = %i\n", x, y)

writes string "x = 25 and y = 500\n" on the standard output.

Value designators

Value designators include the following.

%i %d

The value has type int. Show it as a (decimal) integer. (%i is the same as %d. The d stands for decimal.)

%5i %5d

These are like %i and %d, but use at least 5 characters by adding spaces on the left-hand side as necessary. (If the number needs more than 5 characters then it will not be shortened.)

For example,

  printf("%6i%4i\n", 45, 300);
  printf("%6i%4i\n", 25000, 2);
  printf("%6i%4i\n", 500, 27);

writes

    45 300
 25000   2
   500  27

Number 5 in %5i is called a field width. All of the value designators allow a field width.

Designator %0d is equivalent to %d.

%−5i

Like %5i, but put the spaces on the right side instead of on the left side. All value designators allow a negative field width.

%li

Like %i, but the value has type long. Notice that this is %li, with a lower-case ell.

%lf %10.2lf

Show a number of type double. %10.2lf shows the number using a total of at least 10 characters, with 2 digits shown to the right of the decimal point. For example, if variable stddev has value 450.346, then

  printf("The standard deviation is %10.2lf\n", stddev)

prints

The standard deviation is     450.35.

Large numbers will not be cut off to force them to fit into the designated field width. So

  printf("The standard deviation is %0.2lf\n", stddev

prints

The standard deviation is 450.35.

with no padding.

In %lf, the lower-case ℓ stands for 'long', since type double is sometimes referred to as 'long float'.

%10.2le

Like %lf, but use E format, showing the value using scientific notation. Use this for showing very large or very small values.

%*i

If the field width is given by an asterisk, the width is taken from a parameter. So this actually uses two of the parameters after the format, the field width then the integer to show. For example,

  printf("%*i", n, x);

shows the value of integer x with a field width of n.

%s

Use %s to show a string.

Examples

Statement

  printf("My sister is named %s and she is %i years old\n", "Laura", 10);

writes

 My sister is named Laura and she is 10 years old

Be sure not to forget to add \n if you want to end the line. Statements

  printf("I am ");
  printf("anxious to finish\n");

write

  I am anxious to finish

Also do not forget spaces. Statement

  printf("%i%i\n", 25, 32);

writes

Putchar

Statement

  putchar(c);

writes character c on the standard output. For example,

  putchar('R');

writes an upper-case R. Putchar writes one character, not a whole string.

Standards

See the following standards for output.

Output destination. Follow the assignment concerning where to write output.
Output format. Follow the assignment concerning what the output should look like.
End of output. End textual output with an end-of-line character. (This does not apply to binary output.)

Exercises

Suppose that variable mass is an integer (type int). Write a printf statement that writes
```
This thing's mass is ... kilograms
```
where the ... has been replaced by the value of mass. Write an end-of-line at the end of it. Answer
Repeat the previous question, but this time assume that variable mass has type double. Answer
Write a definition of procedure writeSpaces(n), which writes n spaces. Assume that n is at least 1. Answer

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