C - Sorting struct element based on another element [duplicate] - c++

Question

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How to sort an array of structs in C?
(1 answer)
Closed 6 years ago.
I have a C struct declared as below (just a simple example):
typedef struct
{
float score;
char* name;
}
person;
So, along the program, I changed the value of the person's score (i gave them an initial value). So, i plan to put all of the scores inside a vector and sort them. Then, I would like to print a list of the name of all the persons, starting from biggest score to smallest score. Any idea on this last part? I'm not sure how to code it. Thanks.

Answer 1

Similar to the C++ approach above, there is a library function in the C standard library called qsort. It is based upon a comparision function, that creates the ordering on the array members.
For your case a minimal example could look like this:
#include <stdlib.h>
#include <stdio.h>
// Definitiion of a person
typedef struct person
{
float score;
char *name;
} person;
// Some small epsilon
#define EPSILON 1.0e-3f
// Comaprision function. Returns <0 for a<b =0 for a=b and >0 for a>b
static int compare_people( const void *a, const void *b )
{
// Cast to the correct pointer type
const person *p1 = (const person*) a;
const person *p2 = (const person*) b;
// There are different ways of comparing floats to each other. In this case we use |a - b| < epsilon for some small epsilon
float difference = p2->score - p1->score;
if( difference <= -EPSILON )
{
return -1;
}
else if( difference >= +EPSILON )
{
return +1;
}
return 0;
}
int main()
{
// Initialize list of people
size_t num_people = 5;
person people[num_people];
people[0].score = 0.3f;
people[0].name = "Homer";
people[1].score = 1.4f;
people[1].name = "Marge";
people[2].score = 0.02f;
people[2].name = "Bart";
people[3].score = 13.2f;
people[3].name = "Lisa";
people[4].score = 1.0f;
people[4].name = "Maggie";
// Print unsorted
printf( "Unsorted:\n" );
for( size_t i = 0; i < num_people; ++i )
{
printf( " %s - %2.2f\n", people[i].name, people[i].score );
}
printf( "\n" );
// Sort
qsort( people, num_people, sizeof(person), &compare_people );
// Print sorted
printf( "Sorted:\n" ) ;
for( size_t i = 0; i < num_people; ++i )
{
printf( " %s - %2.2f\n", people[i].name, people[i].score );
}
printf( "\n" );
// Done
return EXIT_SUCCESS;
}
Note the comment about comparing floating point values.
If you are using Linux, you can investigate system calls and functions of the C standard library by looking up the corresponding man page, e.g.
man qsort

Answer 2

For example you ll have
vector<person> vec;
Now you want sort this using sort from STL
You first must create method for operator " < "
bool operator<(const person& a, const person& b){
return a.score<b.score;
}
I hope this helps you I m sorry for my bad grammar :)
Example of usage
sort(vec.begin(),vec.end(),[](const person& a, const person&b){return b<a});
Now your vector of person will be sorted upside - down order.