miniLesson on the printf() function

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printf
Christine S. Wolfe
Ohio University Lancaster
2008-Aug-01
This lesson describes the syntax and use of the printf library functions:
printf()
fprintf()
sprintf()
Vocabulary:
To use printf, fprintf, or sprintf, you must #include <stdio.h>Click
Tip
ASCII
conversion specifier
file name extension
FILE pointer
flag
fprintf()
placeholder
precision
printf()
result
sprintf()
stdout
text file
unicode
width

3
Christine S. Wolfe
2008-Aug-01
The printf functions are used to produce formatted output.
printf sends the output to stdout.
stdout is a predefined macro
that represents the standard
output and is typically the
computer monitor although
this can be changed by the
end user.
Click
Tip
Click
Tip
A text file is one in which
every byte is a character from
the ASCII, Unicode, or other
character set. The extension
can be anything the
programmer chooses.
sprintf sends the output to a string variable.
Click
Tip
fprintf sends the output to a text file.
/* VARIABLE DECLARATIONS */
char Message[81];
The target string must be
long enough to hold the
entire output + 1 byte for
the null.

4Christine S. Wolfe
2008-Aug-01
syntax diagrams for the printf cousins from Appendix B of the text
All 3 include, for each placeholder, an argument that specifies the value
to be used in place of the placeholder.
All 3 include an output string that specifies what to output expressed as:
literals + escape sequences + placeholders.
All 3 return the number of bytes (characters) written to output.
Although the square brackets indicate that the arguments at the end
are optional, that is a bit misleading. They are required if there are
any placeholders in the output string. There must be one argument
(value) provided for each placeholder.
Click
Tip
int printf (const char *format [, argument, …]);
int fprintf (FILE *stream, const char *format [, argument, …]);
int sprintf (char *buffer, const char *format [, argument, …]);

5Christine S. Wolfe
2008-Aug-01
The syntax of the cousins varies only in the required arguments.
sprintf has 2 required arguments. The first argument in sprintf identifies
the destination string variable and the 2nd argument is the output string.
fprintf has 2 required arguments. The first argument in fprintf identifies
the file pointer for the destination file and the 2nd argument is the output
string.
printf has 1 required argument. That required argument is the output
string.
stdout is treated like a file pointer in C so the following 2
constructions produce the same result:
printf("Hello");
fprintf(stdout, "Hello");
Click
Tip
int fprintf (FILE *stream, const char *format [, argument, …]);
int sprintf (char *buffer, const char *format [, argument, …]);

6
All 3 of the printf functions return a result with a data type of int.
The result is a count of the number of characters in the output.
Example:
NumLetters = printf(“Hello”);
NumLetters == 5
Christine S. Wolfe
2008-Aug-01

7
It is not necessary to assign the result to a value – but if the result is
assigned, then the variable on the left of the assignment operator
MUST be an int.
Example:
int NumLetters;
printf(“Goodbye”);
NumLetters = printf(“Hello”);
If assigned, the
result must be
stored in an int
It is OK to call a
function without
assigning its value
to a variable.
Christine S. Wolfe
2008-Aug-01

8
const char *format means that you must include an output string that is a set of
characters that are to be displayed on the screen along with escape
sequences and conversion specifiers (placeholders).
Christine S. Wolfe
2008-Aug-01
printf("First name: ");
printf("Thank you. Please visit again.");
printf("Please enter a number"); Please enter a number
First name:
Thank you. Please visit again.
printf("Report Menu"); Report Menu
What is the result of each of the following statements?
(Click each button to check your answer.)

9Christine S. Wolfe
2008-Aug-01
Escape sequences are used to express output that cannot be represented on
the keyboard or to express output that might be mistaken for C code.
Examples of output that cannot be represented on the keyboard:
r Carriage Return (move cursor to beginning of current line)
f Form Feed (new page)
a Audible Alert (bell)
Examples of output that might be mistaken for C code.
n Newline
t Horizontal Tab
v Vertical Tab
b Backspace
Backslash
? Question mark
' Single quote
" Double quote
%% Percent sign (honorary escape sequence)

10
printf("acd");
printf("abc");
printf("atbtc");
printf("acd");
printf("a"b");
printf("a"b");
printf("anbnc");
abc
a b c
a
b
c
syntax error!!!
a"b
acd
acd
printf("anbtc");
a
b c
c and d are not defined
escape sequences so the
is ignored.
Christine S. Wolfe
2008-Aug-01
printf("a%%cd"); a%cd

11
Conversion specifiers are placeholders for values are not known at the time the
code is written.
Some common conversion specifiers:
for data type use placeholder
int %d
float %f
char %c
string %s
int (base 10) %i
Christine S. Wolfe
2008-Aug-01
http://www.cplusplus.com/reference/clibrary/cstdio/printf.html
See the full list of conversion specifiers at:
type%

12
printf(“The answer is %d”, 3);
printf(“She is %d years old.”, age);
printf(“%d + %d = %d”, numA, numB, numA + numB);
Christine S. Wolfe
2008-Aug-01
For each placeholder, there must be a value in the argument list.
The data type of the value must match or be cast as the value indicated by the
format specifier.
The order in which the placeholders appear in the output string must exactly
match the order of the values in the argument list.
1 placeholder 1 value
1 placeholder 1 value
3 placeholder 3 values

13
Assume the following code appears before the statements below:
int answer;
int age;
int numA;
int numB;
answer = 3;
age = 22;
numA = 14;
numB = 12;
Christine S. Wolfe
2008-Aug-01
printf("%d + %d = %d", numA, numB, numA + numB);
printf("She is %d years old.", age);
printf(“The answer is %d”, 3); The answer is 3
14 + 12 = 26
She is 22 years old.
printf("%d + %d = %d", numA, numB, numA + numA); 14 + 12 = 28
Understand why the computer displays a falsehood.

14
Assume the following code appears before the statements below:
double classAvg = 78.5;
char AvgLetter = 'B';
Christine S. Wolfe
2008-Aug-01
printf(“pi can be expressed as %c, %d, or %f”, PI, 3, 22/7.0);
printf(“The average letter grade was %c”, AvgLetter);
printf(“The class average was %f”, classAvg);
The class average was 78.500000
pi can be expressed as π, 3, or 3.142857
The average letter grade was B
PI is a predefined keyword in C.
Click
Tip

15
Notice that the arguments can be literals, variables, or any expression that
returns a value with the appropriate data type.
printf(“%d %d %d”, 5, NumClients, 6 + 3);
literal variable expression
Christine S. Wolfe
2008-Aug-01

16
As in all C code – you must sweat the details!
Sweat it that there is a comma between every pair of arguments - including the
output string if it is followed by another argument.
Christine S. Wolfe
2008-Aug-01
Sweat it that the closing double quote is after the output string - NOT after the
last argument!
Sweat it that there is a semicolon at the end of the statement.
printf(“pi can be expressed as %c, %d, or %f” , PI, 3, 22/7.0);
printf("Hello World");
printf(“The average letter grade was %c”, AvgLetter);

17
Sweat it that printf does not automatically add spaces around the inserted
arguments. The programmer must include the spaces in the output
string.
Christine S. Wolfe
2008-Aug-01
printf("She is%dyears old.", 23); She is23years old.
printf("She is %d years old.", 23); She is 23 years old.

18
Sweat it that the programmer must code every character that should appear on
the screen,
… to get commas in a list of values, include the commas at the
appropriate place inside the format string
Christine S. Wolfe
2008-Aug-01
printf(“%d %d %d”, 5, NumClients, 6 + 3); 5 123 9
printf(“%d, %d, %d”, 5, NumClients, 6 + 3); 5, 123, 9
The DOS window uses a non-proportional font. Each
character uses the exact same amount of horizontal space.
Courier New is an example of a non-proportional font.
Click
Tip

19
The printf placeholders may include formatting options.
Required
type%
Optional
Christine S. Wolfe
2008-Aug-01
[flags] [width] [.prec] [hlL] type%

20
http://www.cplusplus.com/reference/clibrary/cstdio/printf.html
In your textbook, see Table 12.2 "Placeholders for printf format strings"
The following link is also very helpful.
Christine S. Wolfe
2008-Aug-01
[flags] [width] [.prec] [hlL]% type
[flags] The most common flag is the minus sign, -. It reverses the normal
left/right alignment. Normal for text is left aligned, normal for numeric
data is right aligned.
[width] The width specifies the MINIMUM number of characters to be
displayed. If the value requires fewer characters, the width is
padded with blank spaces. If the value requires more characters, all
the characters are output using a greater width. The specified width
is a MINIMUM.
For numeric types, the width specifer includes space for the digits
(both right and left of the decimal point) and the decimal point itself.
[.prec] The precision option specifies the exact number of digits to display to
the right of the decimal point. These digits count as part of the width.
[hlL] Also referred to as the length option, this indicates a modification of
the data type as short or long.

21
Christine S. Wolfe
2008-Aug-01
printf(“Average cost: %-8.3f !” , 3.4);
printf(“ ***%-10.4f***”, 12.333);
printf(“ ***%10.1f***”, 12.333); *** 12.3***
Average cost: 3.400 !
***12.3330 ***
printf(“%5s%10sn”, "ID","Amount");
printf(“n%5d%10.2f”, 1,12.15);
printf(“n%5d%10.2f”, 2,1.6);
printf(“n%5d%10.2f”, 3,4500.5);
printf(“n%5d%10.2f”, 4,1234567890.0123);
printf(“n%5d%10.2f”, 5,16.367);
ID Amount
1 12.15
2 1.60
3 4500.50
41234567890.01
5 16.37
Make sure you understand why the row for ID 4
is out of line with the others.
Send me an email telling me how you would
correct the problem.

miniLesson on the printf() function

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