I was wondering if and how one would compare an array of char pointers to a string.
So say I have this array of char pointers:
char *input[20];
And each index of input contained a string, for example, input[0] contained hello. What would I use if I needed to do a comparison to find a keyword contained within the input array?
Not really clear what your problem is here. But something like:
for ( int i = 0; i < 20; i++ ) {
if ( strcmp( input[i], "keyword" ) == 0 ) {
// found - do something
}
}
But in C++ you would be better off using std::string and std::vector rather than messing around with C-style arrays and pointers.
I am doing an implementation of an expression evaluator where the user can type in a binary number with the 0b prefix. I want to be able to (using a string iterator), peek ahead to see if the next character in the expression after the 0 is a b, and if it is, to not pass by that b character and to go back a character to the 0 at the start of the prefix (something like ungetc). Is there a way to unget a character that has been passed over in a string?
What I've tried:
Token::pointer_type Tokenizer::_get_number( Tokenizer::string_type::const_iterator& currentChar, Tokenizer::string_type const& expression )
{
assert( isdigit( *currentChar ) && "currentChar must pointer to a digit" );
Integer::value_type const MAX_UNSIGNED_D10 = (std::numeric_limits<Integer::value_type>::max()-10)/10;
Integer::value_type accumulator = *currentChar++ - '0';
//Binary Numbers
if( *currentChar == '0' )
{
if( *currentChar++ == 'b' )
{
BinaryInteger::value_type binAccum = _get_binary( currentChar, expression );
return make<BinaryInteger>( binAccum );
}
}
}
Token::pointer_type Tokenizer::_get_number( Tokenizer::string_type::const_iterator& currentChar, Tokenizer::string_type const& expression )
{
assert( isdigit( *currentChar ) && "currentChar must pointer to a digit" );
Integer::value_type const MAX_UNSIGNED_D10 = (std::numeric_limits<Integer::value_type>::max()-10)/10;
Integer::value_type accumulator = *currentChar++ - '0';
std::stringstream iss( expression );
//Binary Numbers
if( iss.get() == '0' )
{
if( iss.get() == 'b' )
{
BinaryInteger::value_type binAccum = _get_binary( currentChar, expression );
return make<BinaryInteger>( binAccum );
}
}
}
Also have tried using [] access for the string expression's characters itself but that is extremely limiting to certain cases.
When you call the ++ operator on an iterator, it advances the iterator to the next element. To peek at the next element without advancing the iterator, you can use +1 instead, eg:
if( *currentChar == '0' )
{
if( *(currentChar+1) == 'b' )
{
BinaryInteger::value_type binAccum = _get_binary( currentChar+2, expression );
return make<BinaryInteger>( binAccum );
}
}
Just be careful if currentChar is already at the end of the string before you peek. The next element after the current one will be the string's end position, and you should not dereference that iterator value. You might want to consider adding an extra parameter to your tokenizer so it can detect when it reaches the end of the input string and does not iterate too far.
I believe stringstream peek is what you need (it's inherited from istream, just like unget. Stringstraems are streams working with string. They work the same as file streams and the default I/O streams like cin and cout.
Say I have a text file like this:
User: John
Device: 12345
Date: 12/12/12
EDIT:
I have my code to successfully search for a word, and display the info after that word. However when I try to edit the code to search for 2 or 3 words and display the info after them instead of just 1 word, I cannot get it to work. I have tried adding codes into the same while loop, and creating a new while loop for the other word, but both doesn't work. There must be something I am doing wrong/not doing.
Please advice, thanks!
Here is my code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main() {
char file[100];
char c[100];
printf ("Enter file name and directory:");
scanf ("%s",file);
FILE * fs = fopen (file, "r") ;
if ( fs == NULL )
{
puts ( "Cannot open source file" ) ;
exit( 1 ) ;
}
FILE * ft = fopen ( "book5.txt", "w" ) ;
if ( ft == NULL )
{
puts ( "Cannot open target file" ) ;
exit( 1 ) ;
}
while(!feof(fs)) {
char *Data;
char *Device;
char const * rc = fgets(c, 99, fs);
if(rc==NULL) { break; }
if((Data = strstr(rc, "Date:"))!= NULL)
printf(Data+5);
if((Data = strstr(rc, "Device:"))!=NULL)
printf(Device+6);
}
fclose ( fs ) ;
fclose ( ft ) ;
return 0;
}
Ok, hope I can clear it this time. Sorry if I get confusing sometimes but my english is not the best.
I'll explain the implementation inside comments:
#define BUFFSIZE 1024
int main()....
char buff[BUFFSIZE];
char delims[] = " "; /*Where your strtok will split the string*/
char *result = NULL;
char *device; /*To save your device - in your example: 12345*/
char *date; /*To save the date*/
int stop = 0;
fp = fopen("yourFile", "r");
while( fgets(buff, BUFFSIZE,fp) != NULL ) /*This returns null when the file is over*/
{
result = strtok( buff, delims ); /*You just need to do reference to buff here, after this, strtok uses delims to know where to do the next token*/
while(result != NULL){ /*Strtok returns null when finishes reading the given string*/
if(strcmp(result,"Device")==0){ /*strcmp returns 0 if the strings are equal*/
result = strtok(NULL, delims); /*this one gets the 12345*/
device = (char*)malloc((strlen(result)+1)*sizeof(char)); /*Alocate the right amount of memory for the variable device*/
strcpy(device, result); /*Now, device is "12345"*/
}
/*Here you do the same but for the string 'Date'*/
if(strcmp(result,"Date")==0){ /*strcmp returns 0 if the strings are equal*/
result = strtok(NULL, delims); /*this one gets the 12345*/
date = (char*)malloc((strlen(result)+1)*sizeof(char)); /*Alocate the right amount of memory for the variable device*/
strcpy(date, result); /*Now, device is "12/12/12"*/
}
/*And you can repeat the if statement for every string you're looking for*/
result = strtok(NULL,delims); /*Get the next token*/
}
}
/*No strtok necessary here */
...
Hope this helps.
fgetc returns an integer value, which is character, promoted to int.
I suppose you meant fgets which reads a whole line, but you need to reserve memory for it, for example:
#define BUF 100
...
char c[BUF];
fgets(c, BUF, fs);
Some helpful links.
There are a couple of problems in your code: basically it never compiled.
Here is a version with small cleanups - which at least compiles:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main() {
char file[100];
char c[100];
printf ("Enter file name and directory:");
scanf ("%s",file);
FILE * fs = fopen (file, "r") ;
if ( fs == NULL ) {
puts( "Cannot open source file" ) ;
exit(1 ) ;
}
while(!feof(fs)) {
char *Data;
char const * rc = fgets(c, 99, fs);
if(rc==NULL) { break; }
if((Data = strstr(rc, "Device"))!= NULL)
printf("%s", Data);
}
fclose ( fs ) ;
return 0;
}
Problems I found:
Missing include for exit()
Missing parameter for exit()
Missing while loop to run through the whole input file.
The output file was never used.
Missing return value of 'main'
Fancy Data[5]
Changed fgetc() to fgets()
I only did minimal edits - it's not perfect at all....
IMHO I would go for C++: many things are much simpler there.
If printf() isn't a hard/fast rule, and the input requirements are really this simple, I'd prefer a state-machine and a constant-memory input:
int c, x = 0; // c is character, x is state
while(EOF!=(c=getchar())){ // scanner entry point
if(c == '\n') x=0; // newline resets scanner
else if(x == -1) continue; // -1 is invalid state
else if (x < 7 && c=="Device:"[x])x++; // advance state
else if (x == 7 && isspace(c)) continue; // skip leading/trailing whitespace
else if (x == 7) putchar(c); // successful terminator (exits at \n)
else x = -1; // otherwise move to invalid state
}
I would do that with two loops: one to get a line from the file and other to make tokens from the line read.
something like:
#define BUFFSIZE 1024
int main()....
char buff[BUFFSIZE];
char delims[] = " ";
char *result = NULL;
int stop = 0;
fp = fopen("yourFile", "r");
while( fgets(buff, BUFFSIZE,fp) != NULL ) /*This returns null when the file is over*/
{
result = strtok( buff, delims ); /*You just need to do reference to buff here, after this, strtok uses delims to know where to do the next token*/
while(result != NULL){ /*Strtok returns null when finishes reading the given string*/
if(strcmp(result,"Device")==0){ /*strcmp returns 0 if the strings are equal*/
stop = 1; /*Update the flag*/
break; /*Is now possible to break the loop*/
}
result = strtok(NULL,delims); /*Get the next token*/
}
if(stop == 1) break; /*This uses the inside flag to stop the outer loop*/
}
result = strtok(NULL, delims); /*Result, now, has the string you want: 12345 */
...
this code is not very accurate and I didn't tested it, but thats how I would try to do it.
Hope this helps.
My suggestion is to use fread to read all the file.You could read it character by character, but IMHO (a personal taste here) it's simpler to get a string containing all the characters and then manipulating it.
This is the function prototype:
size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );
It returns the number of elements read.
For example:
char buffer[100];
size_t n= fread(buffer, 1,100, fs);
Then you can manipulate the string and divide it in tokens.
EDIT
There is a nice reference with also an example of how dividing a string into tokens here:
http://www.cplusplus.com/reference/cstring/strtok/
c and Data are char-pointers, pointers to (the start of a list of) character value(s).
fgetc's prototype is int fgetc ( FILE * stream ); meaning that it returns (one) integer value (an integer is convertible to a single char value).
If fgetc's prototype would've been int * fgetc ( FILE * stream ); the warning wouldn't have appeared.
#Dave Wang
My answer was too big to be a comment. So here it goes:
You're welcome. Glad to help.
If you make a new loop, the fgets won't work because you are already 'down' in the text file. Imagine something like a pointer to the file, every time you 'fget it' from a file pointer, you advance that pointer. You have functions to reload the file or push that pointer up, but it is not efficient, you've already passed by the information you want, there must be a way to know when.
If you're using my implementation, that is done by using another string compare inside the loop:
if(strcmp(result,"date") == 0)
If you enter this if, you know that the next value in result token with strtok is the actual date.
Since you have now two conditions to be tested, you can't break the outer loop before having both of them. This can be accomplished by two ways:
1-Instead of a flag, use a counter that is incremented everytime you want an information. If that counter has the same number of information you want, you can break the outer loop.
2-Don't break the outer loop at all! :)
But in both, since there are 2 conditions, make sure you treat them inside the ifs so you know that you dealing with the right information.
Hope this helps. Anything, just ask.
I want the following code to remove a leading zero from a price (0.00 should be cut to .00)
QString price1 = "0.00";
if( price1.at( 0 ) == "0" ) price1.remove( 0 );
This gives me the following error: "error: conversion from ‘const char [2]’ to ‘QChar’ is ambiguous"
The main issue is that Qt is seeing "0" as a null-terminated ASCII string, hence the compiler message about const char[2].
Also, QString::remove() takes two arguments. So you code should be:
if( price1.at( 0 ) == '0' ) price1.remove( 0, 1 );
This builds and runs on my system (Qt 4.7.3, VS2005).
Try this:
price1.at( 0 ) == '0' ?
The problem is that the 'at' function returns a QChar which is an object that can't be compared to the native char/string "0". You have a few choices, but I'll just put two here:
if( price1.at(0).toAscii() == '0')
or
if( price1.at(0).digitValue() == 0)
digitValue returns -1 if the char is not a digit.
QString s("foobar");
if (s[0]=="f") {
return;
}
QChar QString::front() const Returns the first character in the
string. Same as at(0).
This function is provided for STL compatibility.
Warning: Calling this function on an empty string constitutes
undefined behavior.
http://doc.qt.io/qt-5/qstring.html#front
QString s("foobar");
/* If string is not empty or null, check to see if the first character equals f */
if (!s.isEmpty() && s.front()=="f") {
return;
}
I'm getting the text from editbox and I'd want to get each name separated by enter key like the character string below with NULL characters.
char *names = "Name1\0Name2\0Name3\0Name4\0Name5";
while(*names)
{
names += strlen(names)+1;
}
how would you do the same for enter key (i.e separated by /r/n) ? can you do that without using the std::string class?
Use strstr:
while (*names)
{
char *next = strstr(names, "\r\n");
if (next != NULL)
{
// If you want to use the key, the length is
size_t len = next - names;
// do something with a string here. The string is not 0 terminated
// so you need to use only 'len' bytes. How you do this depends on
// your need.
// Have names point to the first character after the \r\n
names = next + 2;
}
else
{
// do something with name here. This version is 0 terminated
// so it's easy to use
// Have names point to the terminating \0
names += strlen(names);
}
}
One thing to note is that this code also fixes an error in your code. Your string is terminated by a single \0, so the last iteration will have names point to the first byte after your string. To fix your existing code, you need to change the value of names to:
// The algorithm needs two \0's at the end (one so the final
// strlen will work and the second so that the while loop will
// terminate). Add one explicitly and allow the compiler to
// add a second one.
char *names = "Name1\0Name2\0Name3\0Name4\0Name5\0";
If you want to start and finish with a C string, it's not really C++.
This is a job for strsep.
#include <stdlib.h>
void split_string( char *multiline ) {
do strsep( &multiline, "\r\n" );
while ( multiline );
}
Each call to strsep zeroes out either a \r or a \n. Since only the string \r\n appears, every other call will return an argument. If you wanted, you could build an array of char*s by recording multiline as it advances or the return value of strsep.
void split_string( char *multiline ) {
vector< char* > args;
do {
char *arg = strsep( &multiline, "\r\n" );
if ( * arg != 0 ) {
args.push_back( arg );
}
} while ( multiline );
}
This second example is at least not specific to Windows.
Here's a pure pointer solution
char * names = "name1\r\nName2\r\nName3";
char * plast = names;
while (*names)
{
if (names[0] == '\r' && names[1] == '\n')
{
if (plast < names)
{
size_t cch = names - plast;
// plast points to a name of length cch, not null terminated.
// to extract the name use
// strncpy(pout, plast, cch);
// pout[cch] = '\0';
}
plast = names+2;
}
++names;
}
// plast now points to the start of the last name, it is null terminated.
// extract it with
// strcpy(pout, plast);
Since this has the C++ tag, the easiest would probably using the C++ standard library, especially strings and string streams. Why do you want to avoid std::string when you're doing C++?
std::istringstream iss(names);
std::string line;
while( std::getline(iss,line) )
process(line); // do process(line.c_str()) instead if you need to