Arrays em C

[51L3N7 1]

Pessoal, tenho a seguinte array:

 

char *meses[] = { "Janeiro", "Fevereiro", "Março", "Abril", "Maio", "Junho", "Julho", "Agosto", "Setembro", "Outubro", "Novembro", "Dezembro" };

 

E quero chamar-la a partir de uma variavel que contém um número, ficando assim:

 

printf( "%c", *meses[mes] ) //Supondo que int mes = 12

 

Mas a variável mes retorna 68, logo o programa vai printar na tela o caractere "D" (De "Dezembro"), pelo fato dele ser o 68º caractere da array. Mas por que isso acontece? E como resolver?

[guidjos 65]

Você está enganado. Se mes é 12, você está tentando acessar uma posição que não existe, e isso gera undefined behavior. Se mes for 11, e meses[mes] corresponder ao endereço do primeiro elemento de "Dezembro", então *meses[mes] tem valor 'd', que depende do character set da sua implementação. Se for ASCII, então é 68.

[51L3N7 1]

Entendi :natallaugh:

Mas como faço pra resolver isso e pegar todo o conteúdo ao invés de só a primeira letra?

[guidjos 65]

 

PRINTF(3) Linux Programmer's Manual PRINTF(3)

 

 

 

NAME

printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf,

vsnprintf - formatted output conversion

 

SYNOPSIS

#include <stdio.h>

 

int printf(const char *format, ...);

int fprintf(FILE *stream, const char *format, ...);

int sprintf(char *str, const char *format, ...);

int snprintf(char *str, size_t size, const char *format, ...);

 

#include <stdarg.h>

 

int vprintf(const char *format, va_list ap);

int vfprintf(FILE *stream, const char *format, va_list ap);

int vsprintf(char *str, const char *format, va_list ap);

int vsnprintf(char *str, size_t size, const char *format, va_list ap);

 

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

 

snprintf(), vsnprintf():

_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||

_POSIX_C_SOURCE >= 200112L;

or cc -std=c99

 

DESCRIPTION

The functions in the printf() family produce output according to a for-

mat as described below. The functions printf() and vprintf() write

output to stdout, the standard output stream; fprintf() and vfprintf()

write output to the given output stream; sprintf(), snprintf(),

vsprintf() and vsnprintf() write to the character string str.

 

The functions snprintf() and vsnprintf() write at most size bytes

(including the trailing null byte ('\0')) to str.

 

The functions vprintf(), vfprintf(), vsprintf(), vsnprintf() are equiv-

alent to the functions printf(), fprintf(), sprintf(), snprintf(),

respectively, except that they are called with a va_list instead of a

variable number of arguments. These functions do not call the va_end

macro. Because they invoke the va_arg macro, the value of ap is unde-

fined after the call. See stdarg(3).

 

These eight functions write the output under the control of a format

string that specifies how subsequent arguments (or arguments accessed

via the variable-length argument facilities of stdarg(3)) are converted

for output.

 

C99 and POSIX.1-2001 specify that the results are undefined if a call

to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause copy-

ing to take place between objects that overlap (e.g., if the target

string array and one of the supplied input arguments refer to the same

buffer). See NOTES.

 

Return value

Upon successful return, these functions return the number of characters

printed (not including the trailing '\0' used to end output to

strings).

 

The functions snprintf() and vsnprintf() do not write more than size

bytes (including the trailing '\0'). If the output was truncated due

to this limit then the return value is the number of characters (not

including the trailing '\0') which would have been written to the final

string if enough space had been available. Thus, a return value of

size or more means that the output was truncated. (See also below

under NOTES.)

 

If an output error is encountered, a negative value is returned.

 

Format of the format string

The format string is a character string, beginning and ending in its

initial shift state, if any. The format string is composed of zero or

more directives: ordinary characters (not %), which are copied

unchanged to the output stream; and conversion specifications, each of

which results in fetching zero or more subsequent arguments. Each con-

version specification is introduced by the character %, and ends with a

conversion specifier. In between there may be (in this order) zero or

more flags, an optional minimum field width, an optional precision and

an optional length modifier.

 

The arguments must correspond properly (after type promotion) with the

conversion specifier. By default, the arguments are used in the order

given, where each '*' and each conversion specifier asks for the next

argument (and it is an error if insufficiently many arguments are

given). One can also specify explicitly which argument is taken, at

each place where an argument is required, by writing "%m$" instead of

'%' and "*m$" instead of '*', where the decimal integer m denotes the

position in the argument list of the desired argument, indexed starting

from 1. Thus,

 

printf("%*d", width, num);

 

and

 

printf("%2$*1$d", width, num);

 

are equivalent. The second style allows repeated references to the

same argument. The C99 standard does not include the style using '$',

which comes from the Single UNIX Specification. If the style using '$'

is used, it must be used throughout for all conversions taking an argu-

ment and all width and precision arguments, but it may be mixed with

"%%" formats which do not consume an argument. There may be no gaps in

the numbers of arguments specified using '$'; for example, if arguments

1 and 3 are specified, argument 2 must also be specified somewhere in

the format string.

 

For some numeric conversions a radix character ("decimal point") or

thousands' grouping character is used. The actual character used

depends on the LC_NUMERIC part of the locale. The POSIX locale uses

'.' as radix character, and does not have a grouping character. Thus,

 

printf("%'.2f", 1234567.89);

 

results in "1234567.89" in the POSIX locale, in "1234567,89" in the

nl_NL locale, and in "1.234.567,89" in the da_DK locale.

 

The flag characters

The character % is followed by zero or more of the following flags:

 

# The value should be converted to an "alternate form". For o

conversions, the first character of the output string is made

zero (by prefixing a 0 if it was not zero already). For x and X

conversions, a nonzero result has the string "0x" (or "0X" for X

conversions) prepended to it. For a, A, e, E, f, F, g, and G

conversions, the result will always contain a decimal point,

even if no digits follow it (normally, a decimal point appears

in the results of those conversions only if a digit follows).

For g and G conversions, trailing zeros are not removed from the

result as they would otherwise be. For other conversions, the

result is undefined.

 

0 The value should be zero padded. For d, i, o, u, x, X, a, A, e,

E, f, F, g, and G conversions, the converted value is padded on

the left with zeros rather than blanks. If the 0 and - flags

both appear, the 0 flag is ignored. If a precision is given

with a numeric conversion (d, i, o, u, x, and X), the 0 flag is

ignored. For other conversions, the behavior is undefined.

 

- The converted value is to be left adjusted on the field bound-

ary. (The default is right justification.) Except for n con-

versions, the converted value is padded on the right with

blanks, rather than on the left with blanks or zeros. A - over-

rides a 0 if both are given.

 

' ' (a space) A blank should be left before a positive number (or

empty string) produced by a signed conversion.

 

+ A sign (+ or -) should always be placed before a number produced

by a signed conversion. By default a sign is used only for neg-

ative numbers. A + overrides a space if both are used.

 

The five flag characters above are defined in the C standard. The

SUSv2 specifies one further flag character.

 

' For decimal conversion (i, d, u, f, F, g, G) the output is to be

grouped with thousands' grouping characters if the locale infor-

mation indicates any. Note that many versions of gcc(1) cannot

parse this option and will issue a warning. SUSv2 does not

include %'F.

 

glibc 2.2 adds one further flag character.

 

I For decimal integer conversion (i, d, u) the output uses the

locale's alternative output digits, if any. For example, since

glibc 2.2.3 this will give Arabic-Indic digits in the Persian

("fa_IR") locale.

 

The field width

An optional decimal digit string (with nonzero first digit) specifying

a minimum field width. If the converted value has fewer characters

than the field width, it will be padded with spaces on the left (or

right, if the left-adjustment flag has been given). Instead of a deci-

mal digit string one may write "*" or "*m$" (for some decimal integer

m) to specify that the field width is given in the next argument, or in

the m-th argument, respectively, which must be of type int. A negative

field width is taken as a '-' flag followed by a positive field width.

In no case does a nonexistent or small field width cause truncation of

a field; if the result of a conversion is wider than the field width,

the field is expanded to contain the conversion result.

 

The precision

An optional precision, in the form of a period ('.') followed by an

optional decimal digit string. Instead of a decimal digit string one

may write "*" or "*m$" (for some decimal integer m) to specify that the

precision is given in the next argument, or in the m-th argument,

respectively, which must be of type int. If the precision is given as

just '.', or the precision is negative, the precision is taken to be

zero. This gives the minimum number of digits to appear for d, i, o,

u, x, and X conversions, the number of digits to appear after the radix

character for a, A, e, E, f, and F conversions, the maximum number of

significant digits for g and G conversions, or the maximum number of

characters to be printed from a string for s and S conversions.

 

The length modifier

Here, "integer conversion" stands for d, i, o, u, x, or X conversion.

 

hh A following integer conversion corresponds to a signed char or

unsigned char argument, or a following n conversion corresponds

to a pointer to a signed char argument.

 

h A following integer conversion corresponds to a short int or

unsigned short int argument, or a following n conversion corre-

sponds to a pointer to a short int argument.

 

l (ell) A following integer conversion corresponds to a long int

or unsigned long int argument, or a following n conversion cor-

responds to a pointer to a long int argument, or a following c

conversion corresponds to a wint_t argument, or a following s

conversion corresponds to a pointer to wchar_t argument.

 

ll (ell-ell). A following integer conversion corresponds to a long

long int or unsigned long long int argument, or a following n

conversion corresponds to a pointer to a long long int argument.

 

L A following a, A, e, E, f, F, g, or G conversion corresponds to

a long double argument. (C99 allows %LF, but SUSv2 does not.)

 

q ("quad". 4.4BSD and Linux libc5 only. Don't use.) This is a

synonym for ll.

 

j A following integer conversion corresponds to an intmax_t or

uintmax_t argument.

 

z A following integer conversion corresponds to a size_t or

ssize_t argument. (Linux libc5 has Z with this meaning. Don't

use it.)

 

t A following integer conversion corresponds to a ptrdiff_t argu-

ment.

 

The SUSv2 only knows about the length modifiers h (in hd, hi, ho, hx,

hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf,

Lg, LG).

 

The conversion specifier

A character that specifies the type of conversion to be applied. The

conversion specifiers and their meanings are:

 

d, i The int argument is converted to signed decimal notation. The

precision, if any, gives the minimum number of digits that must

appear; if the converted value requires fewer digits, it is

padded on the left with zeros. The default precision is 1.

When 0 is printed with an explicit precision 0, the output is

empty.

 

o, u, x, X

The unsigned int argument is converted to unsigned octal (o),

unsigned decimal (u), or unsigned hexadecimal (x and X) nota-

tion. The letters abcdef are used for x conversions; the let-

ters ABCDEF are used for X conversions. The precision, if any,

gives the minimum number of digits that must appear; if the con-

verted value requires fewer digits, it is padded on the left

with zeros. The default precision is 1. When 0 is printed with

an explicit precision 0, the output is empty.

 

e, E The double argument is rounded and converted in the style

[-]d.ddde±dd where there is one digit before the decimal-point

character and the number of digits after it is equal to the pre-

cision; if the precision is missing, it is taken as 6; if the

precision is zero, no decimal-point character appears. An E

conversion uses the letter E (rather than e) to introduce the

exponent. The exponent always contains at least two digits; if

the value is zero, the exponent is 00.

 

f, F The double argument is rounded and converted to decimal notation

in the style [-]ddd.ddd, where the number of digits after the

decimal-point character is equal to the precision specification.

If the precision is missing, it is taken as 6; if the precision

is explicitly zero, no decimal-point character appears. If a

decimal point appears, at least one digit appears before it.

 

(The SUSv2 does not know about F and says that character string

representations for infinity and NaN may be made available. The

C99 standard specifies "[-]inf" or "[-]infinity" for infinity,

and a string starting with "nan" for NaN, in the case of f con-

version, and "[-]INF" or "[-]INFINITY" or "NAN*" in the case of

F conversion.)

 

g, G The double argument is converted in style f or e (or F or E for

G conversions). The precision specifies the number of signifi-

cant digits. If the precision is missing, 6 digits are given;

if the precision is zero, it is treated as 1. Style e is used

if the exponent from its conversion is less than -4 or greater

than or equal to the precision. Trailing zeros are removed from

the fractional part of the result; a decimal point appears only

if it is followed by at least one digit.

 

a, A (C99; not in SUSv2) For a conversion, the double argument is

converted to hexadecimal notation (using the letters abcdef) in

the style [-]0xh.hhhhp±d; for A conversion the prefix 0X, the

letters ABCDEF, and the exponent separator P is used. There is

one hexadecimal digit before the decimal point, and the number

of digits after it is equal to the precision. The default pre-

cision suffices for an exact representation of the value if an

exact representation in base 2 exists and otherwise is suffi-

ciently large to distinguish values of type double. The digit

before the decimal point is unspecified for nonnormalized num-

bers, and nonzero but otherwise unspecified for normalized num-

bers.

 

c If no l modifier is present, the int argument is converted to an

unsigned char, and the resulting character is written. If an l

modifier is present, the wint_t (wide character) argument is

converted to a multibyte sequence by a call to the wcrtomb(3)

function, with a conversion state starting in the initial state,

and the resulting multibyte string is written.

 

s If no l modifier is present: The const char * argument is

expected to be a pointer to an array of character type (pointer

to a string). Characters from the array are written up to (but

not including) a terminating null byte ('\0'); if a precision is

specified, no more than the number specified are written. If a

precision is given, no null byte need be present; if the preci-

sion is not specified, or is greater than the size of the array,

the array must contain a terminating null byte.

 

If an l modifier is present: The const wchar_t * argument is

expected to be a pointer to an array of wide characters. Wide

characters from the array are converted to multibyte characters

(each by a call to the wcrtomb(3) function, with a conversion

state starting in the initial state before the first wide char-

acter), up to and including a terminating null wide character.

The resulting multibyte characters are written up to (but not

including) the terminating null byte. If a precision is speci-

fied, no more bytes than the number specified are written, but

no partial multibyte characters are written. Note that the pre-

cision determines the number of bytes written, not the number of

wide characters or screen positions. The array must contain a

terminating null wide character, unless a precision is given and

it is so small that the number of bytes written exceeds it

before the end of the array is reached.

 

C (Not in C99, but in SUSv2.) Synonym for lc. Don't use.

 

S (Not in C99, but in SUSv2.) Synonym for ls. Don't use.

 

p The void * pointer argument is printed in hexadecimal (as if by

%#x or %#lx).

 

n The number of characters written so far is stored into the inte-

ger indicated by the int * (or variant) pointer argument. No

argument is converted.

 

m (Glibc extension.) Print output of strerror(errno). No argu-

ment is required.

 

% A '%' is written. No argument is converted. The complete con-

version specification is '%%'.

 

CONFORMING TO

The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and

vsprintf() functions conform to C89 and C99. The snprintf() and

vsnprintf() functions conform to C99.

 

Concerning the return value of snprintf(), SUSv2 and C99 contradict

each other: when snprintf() is called with size=0 then SUSv2 stipulates

an unspecified return value less than 1, while C99 allows str to be

NULL in this case, and gives the return value (as always) as the number

of characters that would have been written in case the output string

has been large enough.

 

Linux libc4 knows about the five C standard flags. It knows about the

length modifiers h, l, L, and the conversions c, d, e, E, f, F, g, G,

i, n, o, p, s, u, x, and X, where F is a synonym for f. Additionally,

it accepts D, O, and U as synonyms for ld, lo, and lu. (This is bad,

and caused serious bugs later, when support for %D disappeared.) No

locale-dependent radix character, no thousands' separator, no NaN or

infinity, no "%m$" and "*m$".

 

Linux libc5 knows about the five C standard flags and the ' flag,

locale, "%m$" and "*m$". It knows about the length modifiers h, l, L,

Z, and q, but accepts L and q both for long double and for long long

int (this is a bug). It no longer recognizes F, D, O, and U, but adds

the conversion character m, which outputs strerror(errno).

 

glibc 2.0 adds conversion characters C and S.

 

glibc 2.1 adds length modifiers hh, j, t, and z and conversion charac-

ters a and A.

 

glibc 2.2 adds the conversion character F with C99 semantics, and the

flag character I.

 

NOTES

Some programs imprudently rely on code such as the following

 

sprintf(buf, "%s some further text", buf);

 

to append text to buf. However, the standards explicitly note that the

results are undefined if source and destination buffers overlap when

calling sprintf(), snprintf(), vsprintf(), and vsnprintf(). Depending

on the version of gcc(1) used, and the compiler options employed, calls

such as the above will not produce the expected results.

 

The glibc implementation of the functions snprintf() and vsnprintf()

conforms to the C99 standard, that is, behaves as described above,

since glibc version 2.1. Until glibc 2.0.6 they would return -1 when

the output was truncated.

 

BUGS

Because sprintf() and vsprintf() assume an arbitrarily long string,

callers must be careful not to overflow the actual space; this is often

impossible to assure. Note that the length of the strings produced is

locale-dependent and difficult to predict. Use snprintf() and

vsnprintf() instead (or asprintf(3) and vasprintf(3)).

 

Linux libc4.[45] does not have a snprintf(), but provides a libbsd that

contains an snprintf() equivalent to sprintf(), that is, one that

ignores the size argument. Thus, the use of snprintf() with early

libc4 leads to serious security problems.

 

Code such as printf(foo); often indicates a bug, since foo may contain

a % character. If foo comes from untrusted user input, it may contain

%n, causing the printf() call to write to memory and creating a secu-

rity hole.

 

EXAMPLE

To print pi to five decimal places:

 

#include <math.h>

#include <stdio.h>

fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

 

To print a date and time in the form "Sunday, July 3, 10:02", where

weekday and month are pointers to strings:

 

#include <stdio.h>

fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",

weekday, month, day, hour, min);

 

Many countries use the day-month-year order. Hence, an international-

ized version must be able to print the arguments in an order specified

by the format:

 

#include <stdio.h>

fprintf(stdout, format,

weekday, month, day, hour, min);

 

where format depends on locale, and may permute the arguments. With

the value:

 

"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

 

one might obtain "Sonntag, 3. Juli, 10:02".

 

To allocate a sufficiently large string and print into it (code correct

for both glibc 2.0 and glibc 2.1):

 

#include <stdio.h>

#include <stdlib.h>

#include <stdarg.h>

 

char *

make_message(const char *fmt, ...)

{

int n;

int size = 100; /* Guess we need no more than 100 bytes. */

char *p, *np;

va_list ap;

 

if ((p = malloc(size)) == NULL)

return NULL;

 

while (1) {

 

/* Try to print in the allocated space. */

 

va_start(ap, fmt);

n = vsnprintf(p, size, fmt, ap);

va_end(ap);

 

/* If that worked, return the string. */

 

if (n > -1 && n < size)

return p;

 

/* Else try again with more space. */

 

if (n > -1) /* glibc 2.1 */

size = n+1; /* precisely what is needed */

else /* glibc 2.0 */

size *= 2; /* twice the old size */

 

if ((np = realloc (p, size)) == NULL) {

free(p);

return NULL;

} else {

p = np;

}

}

}

 

SEE ALSO

printf(1), asprintf(3), dprintf(3), scanf(3), setlocale(3), wcrtomb(3),

wprintf(3), locale(5)

 

COLOPHON

This page is part of release 3.32 of the Linux man-pages project. A

description of the project, and information about reporting bugs, can

be found at http://www.kernel.org/doc/man-pages/.

 

 

 

GNU 2010-09-20 PRINTF(3)

[51L3N7 1]

Isso não fala nada do que eu quero.

[guidjos 65]

Claro que fala. Leia a parte sobre s.

[Shurd 0]

Tem que ser matriz e não vetor não?

[Renato Utsch 24]

Olá!

 

Simplificando, "%c" printa caracteres, enquanto "%s" printa strings. Quando você usa "%c", você está pedindo para o printf() mostrar o caractere que está no lugar que você pediu (D). Tente mudar par "%s" e veja o que acontece: o printf() vai printar a string.

 

É isso o que deve estar escrito nas páginas do man que o guidjos mandou...

 

Sugiro que leia um livro sobre C. Por exemplo, o "C Programming: a Modern Approach, 2nd edition" do K.N.King é um livro MUITO bom, que cobre toda a C89/C99, muito bem escrito, etc.

 

Por exemplo? Qual a diferença entre vetor e matriz? O que você quis dizer na frase acima? Não fez o menor sentido com relação ao problema que você teve, ainda mais que você está, nas entrelinhas, usando uma matriz.

 

Esse livro (ou outros do mesmo nível) te ajudarão a acabar com sua confusão e te farão um melhor programador ^^

 

 

 

Abraços :D

[guidjos 65]

Não existem matrizes em C. Elas podem ser modeladas usando arrays de arrays (vetores de vetores).

 

O problema aqui é que ele não sabe utilizar %s, que espera um argumento de tipo (char *) ao invés de (char), e se recusa a ler o manual para aprender. Simplesmente substituir não vai 'dar certo'.

[51L3N7 1]

@Renato Utsch

Funcionou perfeitamente. :natalbiggrin: Valeu!

Mas com relação ao livro, ele só tem em inglês certo? Eu não domino tanto o inglês assim.

[Renato Utsch 24]

Olá!

 

Tá, mas qualquer livro bom basta. Só não compre livros do Schildt...

 

Quanto a matrizes, o conceito de matrizes que eu aprendi é "vetor bidimensional", que, nas entrelinhas, pode ser interpretado como um vetor de vetores...

 

Pelo o menos foi o conceito que eu aprendi ^^

 

 

Abraços :D

[guidjos 65]

Não existem matrizes em C. Elas podem ser modeladas usando arrays de arrays (vetores de vetores). O problema aqui é que ele não sabe utilizar %s, que espera um argumento de tipo (char *) ao invés de (char), e se recusa a ler o manual para aprender.

 

Além disso, a definição de 'matriz' impede que o número de linhas varie de acordo com colunas, ou vice-versa. Portanto, o que foi modelado com o seguinte código não é uma matriz:

 

 

char *meses[] = { "Janeiro", "Fevereiro", "Março", "Abril", "Maio", "Junho", "Julho", "Agosto", "Setembro", "Outubro", "Novembro", "Dezembro" }; // um vetor de ponteiros para char.