C++ 如何从递归函数返回指针,该函数将波兰语表示法转换为反向波兰语表示法?
我的一个朋友被指派编写一个函数,将波兰语表示法转换为反向波兰语表示法,因此在最后一个字符串“sin*-x4+83”应转换为“x4-83+*sin”。有一些约束,例如:在结束时,应该返回一个指针,并在堆上分配内存;函数原型应该只包含一个参数,并且应该是原始字符串。我知道使用静态变量是对人类的可怕犯罪,但尽管如此,我们得出的最好的结论是:C++ 如何从递归函数返回指针,该函数将波兰语表示法转换为反向波兰语表示法?,c++,c,pointers,recursion,postfix-notation,C++,C,Pointers,Recursion,Postfix Notation,我的一个朋友被指派编写一个函数,将波兰语表示法转换为反向波兰语表示法,因此在最后一个字符串“sin*-x4+83”应转换为“x4-83+*sin”。有一些约束,例如:在结束时,应该返回一个指针,并在堆上分配内存;函数原型应该只包含一个参数,并且应该是原始字符串。我知道使用静态变量是对人类的可怕犯罪,但尽管如此,我们得出的最好的结论是: char * prefix2postfix( char ppn_in[] ){ if (ppn_in[0] == '\0') { ret
char * prefix2postfix( char ppn_in[] ){
if (ppn_in[0] == '\0') {
return ppn_in;
}
int i = 0;
char *left = "",
*root = "";
while (ppn_in[i] == ' ') i++;
if (is_num(ppn_in[i]) || is_var(ppn_in[i])) {
char tmp[8];
memset(tmp, '\0', 7);
tmp[0] = ' ';
tmp[1] = ppn_in[i];
root = tmp;
left = prefix2postfix(ppn_in + i + 1);
} else if (is_op(ppn_in[i])) {
switch (ppn_in[i]) {
case '+':
root = " +";
break;
case '-':
root = " -";
break;
case '*':
root = " *";
break;
case '/':
root = " /";
break;
}
left = prefix2postfix(ppn_in + i + 1);
} else if (strncmp(ppn_in, "sqrt", 4) == 0) {
root = " sqrt";
left = prefix2postfix(ppn_in + i + 4);
} else if (strncmp(ppn_in, "sqr", 3) == 0) {
root = " sqr";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "sin", 3) == 0) {
root = " sin";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "cos", 3) == 0) {
root = " cos";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "tan", 3) == 0) {
root = " tan";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "log", 3) == 0) {
root = " log";
left = prefix2postfix(ppn_in + i + 3);
}
printf("reached end of the function\n");
static char buff[200];
buff[199] = '\0';
strcpy(buff, left);
strcat(buff, root);
return buff;
}
问题是(这可能不是唯一的一个),我不知道如何正确地结束函数,以使行printf(“到达函数的结尾\n”)
只被调用一次,在这种情况下,我将能够使用malloc
,否则我将导致内存泄漏。任何建议都将不胜感激
编辑
我试着自己来做一些事情,我最终会造成内存泄漏吗?
(我知道这是非常低效的,但我分配了200字节,将它们复制到一个新的缓冲区,使用该值后,我调用free)
您的解析并不是使用两个分支进行递归,但要回答您的问题,每次都需要malloc返回值,否则调用方将不知道何时释放。我把这个例子作为C代码来保存,因为我在你的问题中没有看到任何C++。
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int is_num( char v ) { return isdigit( v ); }
int is_var( char v ) { return isalpha( v ); }
int is_op( char v ) { return ispunct( v ); }
char * prefix2postfix( char ppn_in[] ){
if (ppn_in[0] == '\0') {
char * ptr = malloc( sizeof( char ) * 1024 );
ptr[0] = '\0';
return ptr;
}
int i = 0;
char tmp[16];
char *left = 0,
*root = "";
while (ppn_in[i] == ' ') i++;
if ( is_var(ppn_in[i])) {
tmp[0] = ' ';
tmp[1] = ppn_in[i];
char *tptr = &tmp[2];
while( is_var(ppn_in[i + 1] ) )
*tptr++ = ppn_in[++i];
*tptr = '\0';
root = tmp;
left = prefix2postfix(ppn_in + i + 1);
} else if (is_num(ppn_in[i]) || is_var(ppn_in[i])) {
char tmp[8];
memset(tmp, '\0', 7);
tmp[0] = ' ';
tmp[1] = ppn_in[i];
root = tmp;
left = prefix2postfix(ppn_in + i + 1);
} else if (is_op(ppn_in[i])) {
switch (ppn_in[i]) {
case '+':
root = " +";
break;
case '-':
root = " -";
break;
case '*':
root = " *";
break;
case '/':
root = " /";
break;
}
left = prefix2postfix(ppn_in + i + 1);
} else if (strncmp(ppn_in, "sqrt", 4) == 0) {
root = " sqrt";
left = prefix2postfix(ppn_in + i + 4);
} else if (strncmp(ppn_in, "sqr", 3) == 0) {
root = " sqr";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "sin", 3) == 0) {
root = " sin";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "cos", 3) == 0) {
root = " cos";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "tan", 3) == 0) {
root = " tan";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "log", 3) == 0) {
root = " log";
left = prefix2postfix(ppn_in + i + 3);
}
printf("reached end of the function '%s' '%s'\n", left ? left : "", root );
if( left )
{
strcat( left, root );
return left;
}
char * buff = malloc( sizeof( char ) * 1024 );
strcpy( buff, root );
return buff;
}
int main( int argc, char ** argv )
{
if( argc == 2 )
{
char * ptr = prefix2postfix( argv[1] );
printf( "Result: %s\n", ptr );
free( ptr );
}
}
您需要传递一个缓冲区来写入结果,或者让它从空闲存储区动态分配。调用者是如何使用它的?@GeorgeHoupis他获取结果并将其分配给一个char指针,使用后他调用“free”是的,你是对的,我从左和右开始,然后删除了右和其他变量。起初我认为这是一个树问题,但我无法解决它,所以我最终得到了我的结果。我说得对吗?在最后一步中为left获取一个缓冲区,并防止到达函数末尾,直到最后一个缓冲区结束?但是什么时候释放left?我在main中释放left,因为它最终由函数返回,或者我重用从递归的较低调用中分配的内容。您在某种程度上是正确的,但是您没有正确解析,因为您正在反转参数,使减法反转。您只想将操作符并置。@orustammanapov我给您提供了另一个您可以研究的解决方案。
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int is_num( char v ) { return isdigit( v ); }
int is_var( char v ) { return isalpha( v ); }
int is_op( char v ) { return ispunct( v ); }
char * prefix2postfix( char ppn_in[] ){
if (ppn_in[0] == '\0') {
char * ptr = malloc( sizeof( char ) * 1024 );
ptr[0] = '\0';
return ptr;
}
int i = 0;
char tmp[16];
char *left = 0,
*root = "";
while (ppn_in[i] == ' ') i++;
if ( is_var(ppn_in[i])) {
tmp[0] = ' ';
tmp[1] = ppn_in[i];
char *tptr = &tmp[2];
while( is_var(ppn_in[i + 1] ) )
*tptr++ = ppn_in[++i];
*tptr = '\0';
root = tmp;
left = prefix2postfix(ppn_in + i + 1);
} else if (is_num(ppn_in[i]) || is_var(ppn_in[i])) {
char tmp[8];
memset(tmp, '\0', 7);
tmp[0] = ' ';
tmp[1] = ppn_in[i];
root = tmp;
left = prefix2postfix(ppn_in + i + 1);
} else if (is_op(ppn_in[i])) {
switch (ppn_in[i]) {
case '+':
root = " +";
break;
case '-':
root = " -";
break;
case '*':
root = " *";
break;
case '/':
root = " /";
break;
}
left = prefix2postfix(ppn_in + i + 1);
} else if (strncmp(ppn_in, "sqrt", 4) == 0) {
root = " sqrt";
left = prefix2postfix(ppn_in + i + 4);
} else if (strncmp(ppn_in, "sqr", 3) == 0) {
root = " sqr";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "sin", 3) == 0) {
root = " sin";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "cos", 3) == 0) {
root = " cos";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "tan", 3) == 0) {
root = " tan";
left = prefix2postfix(ppn_in + i + 3);
} else if (strncmp(ppn_in, "log", 3) == 0) {
root = " log";
left = prefix2postfix(ppn_in + i + 3);
}
printf("reached end of the function '%s' '%s'\n", left ? left : "", root );
if( left )
{
strcat( left, root );
return left;
}
char * buff = malloc( sizeof( char ) * 1024 );
strcpy( buff, root );
return buff;
}
int main( int argc, char ** argv )
{
if( argc == 2 )
{
char * ptr = prefix2postfix( argv[1] );
printf( "Result: %s\n", ptr );
free( ptr );
}
}
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int is_binary_operator( const char * token )
{
return ( ( *token == '+' ) ||
( *token == '*' ) ||
( *token == '-' ) ||
( *token == '/' ) );
}
int is_unary_operator( const char * token )
{
return ( !strcmp( token, "sin" ) ||
!strcmp( token, "cos" ) ||
!strcmp( token, "tan" ) ||
!strcmp( token, "exp" ) ||
!strcmp( token, "log" ) ||
!strcmp( token, "ln" ) ||
!strcmp( token, "sqrt" ) ||
!strcmp( token, "sqr" ) );
}
int reverse_polish_arranger( char ** tokens, size_t cnt, size_t iter/*, size_t depth*/ )
{
/*printf( "%*sPeeking at %s\n", depth << 1, "", tokens[iter] );*/
if( is_binary_operator( tokens[iter] ) )
{
int next_pos = reverse_polish_arranger( tokens, cnt, iter + 1 /*, depth + 1 */);
int end_pos = reverse_polish_arranger( tokens, cnt, next_pos + 1 /*, depth + 1 */);
char * ptr = tokens[iter];
memmove( tokens + iter, tokens + iter + 1, sizeof( char * ) * ( end_pos - iter ) );
tokens[end_pos] = ptr;
return end_pos;
}
else if( is_unary_operator( tokens[iter] ) )
{
int end_pos = reverse_polish_arranger( tokens, cnt, iter + 1 /*, depth + 1*/ );
char * ptr = tokens[iter];
memmove( tokens + iter, tokens + iter + 1, sizeof( char * ) * ( end_pos - iter ) );
tokens[end_pos] = ptr;
return end_pos;
}
else
return iter;
}
char * reverse_polish( const char * expr )
{
size_t index;
size_t push_index = 0;
size_t len = 0;
char * result = 0;
char * buffer = 0;
char ** pointers = 0;
/* Prepare the workspace. */
if( !expr )
return 0;
len = strlen( expr );
if( !( result = malloc( sizeof( char ) * len ) ) )
return 0;
if( !( buffer = malloc( sizeof( char ) * len ) ) )
{
free( result );
return 0;
}
if( !( pointers = malloc( sizeof( char * ) * len ) ) )
{
free( result );
free( buffer );
return 0;
}
strcpy( buffer, expr );
memset( pointers, 0, sizeof( char * ) * len );
/* Cheap tokenize using space a delimiter. */
pointers[push_index++] = buffer;
for( index = 0; index < len; ++index )
if( buffer[index] == ' ' )
buffer[index++] = '\0', pointers[push_index++] = &buffer[index];
/* printf( "Before:\n" );
* for( index = 0; index < push_index; ++index )
* printf( "Token %3d: %s\n", index, pointers[index] );
*/
/* Do the conversion. */
reverse_polish_arranger( pointers, push_index, 0/*, 0*/ );
/*
* printf( "After:\n" );
* for( index = 0; index < push_index; ++index )
* printf( "Token %3d: %s\n", index, pointers[index] );
*/
/* Prepare output buffer. */
result[0] = '\0';
for( index = 0; index < push_index; ++index )
strcat( strcat( result, pointers[index] ), " " );
/* Cleanup temporary workspace */
free( pointers );
free( buffer );
/* Return new string. */
return result;
}
int main( int argc, char ** argv )
{
if( argc == 2 )
{
char * reversed = reverse_polish( argv[1] );
if( reversed )
{
printf( "Reverse: %s\n", reversed );
free( reversed );
}
else
printf( "Failed to reverse: %s\n", argv[1] );
}
return 0;
}
./a.out "sin * - x 4 + 8 3"
Reverse: x 4 - 8 3 + * sin