I am shifting from Python to C so bit rusty on the semantics as well as coding habit. In Python everything is treated as an object and objects are passed to functions. This is not the case in C so I want to increment an integer using pointers. What is the correct assignment to do so. I want to do it the following way but have the assignments wrong:
#include <stdio.h>
int i = 24;
int increment(*i){
*i++;
return i;
}
int main() {
increment(&i);
printf("i = %d, i);
return 0;
}
I fixed your program:
#include <stdio.h>
int i = 24;
// changed from i to j in order to avoid confusion.
// note you could declare the return type as void instead
int increment(int *j){
(*j)++;
return *j;
}
int main() {
increment(&i);
printf("i = %d", i);
return 0;
}
Your main error was the missing int in the function's argument (also a missing " in the printf).
Also I would prefer using parentheses in expressions as *j++ and specify exactly the precedence like I did in (*j)++, because I want to increment the content of the variable in the 'j' location not to increment the pointer - meaning to point it on the next memory cell - and then use its content.
I basically wrote a code in which I take two command line arguments one being the type of file that I want to search in my directory and they other being the amount I want(which is not implemented yet, but I can fix that)
The code is like so:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define sizeFileName 500
#define filesMax 5000
int cmpfunc( const void *a, const void *b) {
return *(char*)a + *(char*)b;
}
int main( int argc, char ** argv) {
FILE * fp = popen( "find . -type f", "r");
char * type = argv[1];
char * extension = ".";
char* tExtension;
tExtension = malloc(strlen(type)+1+4);
strcpy(tExtension, extension);
strcat(tExtension, type);
// printf("%s\n",tExtension);
int amount = atoi(argv[2]);
//printf("%d\n",amount);
char buff[sizeFileName];
int nFiles = 0;
char * files[filesMax];
while(fgets(buff,sizeFileName,fp)) {
int leng = strlen(buff) - 1;
if (strncmp(buff + leng - 4, tExtension, 4) == 0){
files[nFiles] = strndup(buff,leng);
//printf("\t%s\n", files[nFiles]);
nFiles ++;
}
}
fclose(fp);
printf("Found %d files\n", nFiles);
long long totalBytes = 0;
struct stat st;
// sorting based on byte size from greatest to least
qsort(files, (size_t) strlen(files), (size_t) sizeof(char), cmpfunc);
for(int i = 0;i< nFiles; i ++) {
if(0!= stat(files[i],&st)){
perror("stat failed:");
exit(-1);
}
totalBytes += st.st_size;
printf("%s : %ld\n",files[i],st.st_size);
}
printf("Total size: %lld\n", totalBytes);
// clean up
for(int i = 0; i < nFiles ; i ++ ) {
free(files[i]);
}
return 0;
}
So far I have every section set up properly, upon running the code say $./find ini 5, it would print out all the ini files followed by their byte size(it's currently ignore the 5). However, for the qsort(), I'm not exactly sure how I would sort the contents of char * files as while it holds the pathnames, I had to use stat to get the byte sizes, how would I print out a sorted version of my print statements featuring the first statement being the most bytes and finishes at the least bytes?
If we suppose your input is valid, your question could be simplified with:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define filesMax 5000
int cmpfunc(const void const *a, const void *b) { return *(char *)a + *(char *)b; }
int main(void) {
int nFiles = 4;
char *files[filesMax] = {"amazing", "hello", "this is a file", "I'm a bad file"};
qsort(files, strlen(files), sizeof(char), cmpfunc);
for (int i = 0; i < nFiles;; i++) {
printf("%s\n", files[i]);
}
}
If you compile with warning that give you:
source_file.c:11:23: warning: incompatible pointer types passing 'char *[5000]' to parameter of type 'const char *' [-Wincompatible-pointer-types]
qsort(files, strlen(files), sizeof(char), cmpfunc);
^~~~~
qsort() expect the size of your array (or in your case a subsize) and it's also expect the size of one element of your array. In both you wrongly give it to it. Also, your compare function doesn't compare anything, you are currently adding the first bytes of both pointer of char, that doesn't make a lot of sense.
To fix your code you must write:
qsort(files, nFiles, sizeof *files, &cmpfunc);
and also fix your compare function:
int cmpfunc_aux(char * const *a, char * const *b) { return strcmp(*a, *b); }
int cmpfunc(void const *a, void const *b) { return cmpfunc_aux(a, b); }
also size should be of type size_t:
size_t nFiles = 0;
Don't forget that all informations about how to use a function are write in their doc.
how would I print out a sorted version of my print statements featuring the first statement being the most bytes and finishes at the least bytes?
Your code don't show any clue that your are trying to do that, you are currently storing name file and only that. How do you expect sort your file with an information you didn't acquired ?
However, that simple create a struct that contain both file name and size, acquire information needed to sort it and sort it:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <inttypes.h>
struct file {
off_t size;
char *name;
};
int cmpfunc_aux(struct file const *a, struct file const *b) {
if (a->size > b->size) {
return -1;
} else if (a->size < b->size) {
return 1;
} else {
return 0;
}
}
int cmpfunc(void const *a, void const *b) { return cmpfunc_aux(a, b); }
#define filesMax 5000
int main(void) {
size_t nFiles = 4;
struct file files[filesMax] = {{42, "amazing"},
{21, "hello"},
{168, "this is a file"},
{84, "I'm a bad file"}};
qsort(files, nFiles, sizeof *files, &cmpfunc);
for (size_t i = 0; i < nFiles; i++) {
printf("%s, %" PRId64 "\n", files[i].name, (intmax_t)files[i].size);
}
}
The function cmpfunc() provided adds the first character of each string, and that's not a proper comparison function (it should give a opposite sign value when you switch the parameters, e.g. if "a" and "b" are the strings to compare, it adds the first two characters of both strings, giving 97+98 == 195, which is positive on unsigned chars, then calling with "b" and "a" should give a negative number (and it again gives you 98 + 97 == 195), more on, it always gives the same result ---even with signed chars--- so it cannot be used as a sorting comparator)
As you are comparing strings, why not to use the standard library function strcmp(3) which is a valid comparison function? It gives a negative number if first string is less lexicographically than the second, 0 if both are equal, and positive if first is greater lexicographically than the second.
if your function has to check (and sort) by the lenght of the filenames, then you can define it as:
int cmpfunc(char *a, char *b) /* yes, you can define parameters as char * */
{
return strlen(a) - strlen(b);
}
or, first based on file length, then lexicographically:
int cmpfunc(char *a, char *b)
{
int la = strlen(a), lb = strlen(b);
if (la != lb) return la - lb;
/* la == lb, so we must check lexicographycally */
return strcmp(a, b);
}
Now, to continue helping you, I need to know why do you need to sort anything, as you say that you want to search a directory for a file, where does the sorting take place in the problem?
I am learning to implement graph using c++. I came across to see the follow code. Could anyone explain what is the function of symbols * and & behide the data type "vertex" and "string"?
#include <iostream>
#include <vector>
#include <map>
#include <string>
using namespace std;
struct vertex {
typedef pair<int, vertex*> ve;
vector <ve> adj; //cost of edge, distination to vertex
string name;
vertex (string s) : name(s) {}
};
class gragh
{
public:
typedef map<string, vertex *> vmap;
vmap work;
void addvertex (const string&);
void addedge (const string& from, const string&, double cost);
};
void gragh::addvertex (const string &name)
{
vmap::iterator itr = work.find(name);
if (itr == work.end())
{
vertex *v;
v = new vertex(name);
work[name] = v;
return;
}
cout << "Vertex alreay exist";
}
int main()
{
return 0;
}
'*' means to de-reference something i.e. go to the address of some variable whose address lies in the pointer.
int x=*p;
This means x will have the value of memory address to whom p is pointing.
x=&p;
This means x will have the address of that memory location where p resides.
In the below code scanf() is working for getting the name from the user but fgets() is not working pls someone help me to understand why it's not working
#include <stdio.h>
#include <stdlib.h>
typedef struct university{
int roll_no;
char name[16];
}uni;
int main()
{
uni *ptr[5],soome;char i,j=0;
for(i=0;i<5;i++)
{
ptr[i]=(uni*)calloc(1,20);
if(ptr[i]==NULL)
{
printf("memory allocation failure");
}
printf("enter the roll no and name \n");
printf("ur going to enter at the address%u \n",ptr[i]);
scanf("%d",&ptr[i]->roll_no);
//scanf("%s",&ptr[i]->name);
fgets(&ptr[i]->name,16,stdin);
}
while(*(ptr+j))
{
printf("%d %s\n",ptr[j]->roll_no,ptr[j]->name);
j++;
}
return 0;
}
First of all, fgets(char *s, int n, FILE *stream) takes three argument: a pointer s to the beginning of a character array, a count n, and an input stream.
In the original application you used the address operator & to get the pointer not to the first element of the name[16] array, but to something else (to use the address operator, you should have referenced the first char in the array: name[0]).
You use a lot of magic numbers in your application (e.g. 20 as the size of the uni struct). In my sample I'm using sizeof as much as possible.
Given that you use calloc, I've used the fact that the first parameter is the number of elements of size equal to the second parameter to preallocate all the five uni struct at once.
Final result is:
#include <stdio.h>
#include <stdlib.h>
#define NUM_ITEMS (5)
#define NAME_LENGTH (16)
typedef struct university{
int roll_no;
char name[NAME_LENGTH];
} uni;
int main()
{
uni *ptr;
int i;
ptr = (uni*)calloc(NUM_ITEMS, sizeof(uni));
if(NULL == ptr) {
printf("memory allocation failure");
return -1;
}
for(i=0; i<NUM_ITEMS; i++) {
printf("enter the roll no and name \n");
printf("You're going to enter at the address: 0x%X \n",(unsigned int)&ptr[i]);
scanf("%d",&ptr[i].roll_no);
fgets(ptr[i].name, NAME_LENGTH, stdin);
}
for(i=0; i<NUM_ITEMS; i++) {
printf("%d - %s",ptr[i].roll_no,ptr[i].name);
}
free(ptr);
return 0;
}
Note: I've added a call to free(ptr); to free the memory allocated by calloc at the end of the application and a different return code if it's not possible to allocate the memory.
I have a QList consist of QVector3D. A QVector3D represents a vertex or a point. This List holds also all vertices of a STL-File. The problem is that a vertex exist multiple times in the list. In need a list of the unique vertices of a STL-File. How can i implement it with Qt 5.0.2?
QSet uses a hash-function for ensuring the uniqueness of the value (QMap uses operator <)
There is no qHash implementation for QVector3D in Qt.
You could implement your own one e.g. as in example:
//place anywhere in Qt-code
#include <QSet>
#include <QVector3D>
#include <QList>
uint qHash(const QVector3D &v)
{
return qHash( QString( "%1x%2x%3" ).arg(v.x()).arg(v.y()).arg(v.z()) ) ;
}
int foo()
{
QList<QVector3D> uvector3D_1;
QSet<QVector3D> uvector3D_2;
uvector3D_2 = QSet<QVector3D>::fromList(uvector3D_1);
return 0;
}
static int testFoo = foo();
Of cause it is not the fastest one, it relies on Qt's function qHash for QString. But I think it's good for demonstration.
QList<QVector3D> originalVector = ...;
then either:
QSet<QVector3D> noDublicatesSet = QSet<QVector3D>::fromList(originalVector);
or
QSet<QVector3D> noDublicatesSet = originalVector.toSet();
also you can add something like if you need QList back..
QList<QVector3D> destinationVector = QList<QVector3D>::fromSet(noDublicatesSet);
you also will need those things (sorry has them in my code for ages.. forgot that they are external).. you might want to change hash function:
#define ROTL10(x) (((x) << 10) | (((x) >> 22) & 0x000000ff))
#define ROTL20(x) (((x) << 20) | (((x) >> 12) & 0x0000ffff))
uint qHash(double data)
{
union U {
quint64 n;
double f;
};
U u;
u.f = data;
return u.f;
}
inline uint qHash(const QVector3D &v, uint seed)
{
return qHash(v.x()) ^ ROTL10(qHash(v.y())) ^ ROTL20(qHash(v.z()));
}
P.S. that's a code for Qt 5.0, actually to add missing qHash() for vectors, that's why they dont fit in QSet/QHash by default
Starting from Qt 5.14, you can use the new constructor:
template <typename InputIterator> QSet::QSet(InputIterator first, InputIterator last
Here is an example taken from the docs:
// For example, if you have code like
QStringList list;
QSet<QString> set = QSet<QString>::fromList(list);
// you can rewrite it as
QStringList list;
QSet<QString> set(list.begin(), list.end());