Sort an array of structs in c by a specific parameter

Question

I have a structure for every user and I'm trying to sort them by users Lastname
Full code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>

struct phNum
{
    char name[30];
    char lastName[30];
    int phone;
};

typedef struct phNum PhNum;

PhNum phList[5] = {};
void showData() {
    for(int i = 0;i < 5;i++) {
        if(strcmp(phList[i].name,"empty") == 0 && phList[i].phone == 0){
            printf("%d : %s | %s | null\n", i+1,phList[i].name,phList[i].lastName);
        }
        else {
            printf("%d : %s | %s | %d\n", i+1,phList[i].name,phList[i].lastName,phList[i].phone);
        }
    }
}

The array maximum is 5 items and the minimum is 0
I want to sort them ascending by their Lastname and if there are 4 items don't show the fifth one. and I want to override my Array phList and rewrite the sorted one my showData function is supposed to sort and print I tried These answers but I didn't understand: How to sort an array of structs in C? and How do I sort an array of structs in C by name, age and id?

I also tried to make a bubble sort algorithm but I was struggling to put my sorted data into my phList

Thanks For Help

Even for such a small array I would recommend the standard qsort function. — Some programmer dude
– Some programmer dude, Commented Jan 16, 2023 at 13:57
I don't know that much about c I appreciate it if u write the code — BGOPC
– BGOPC, Commented Jan 16, 2023 at 13:58
Begin by writing a function that takes two arguments, of type "pointer to struct PhNum", and return an integer. The function should return a negative number if the first argument is "less" than the second, zero if both are equal, or a positive value if the first argument is "greater" than the second argument. Fortunately there already exists a standard function that return exactly that when comparing strings: strcmp. So call strcmp to compare the lastName of the two structures, and return the result. Now you have a function you can basically plug in directly for the qsort function. — Some programmer dude
– Some programmer dude, Commented Jan 16, 2023 at 14:04
There are also many examples, all over the Internet on how to use qsort, to sort any kind of arrays. Including arrays of structures. Experiment, and try something for yourself. That's a good way to learn. — Some programmer dude
– Some programmer dude, Commented Jan 16, 2023 at 14:05
First start by writing a code example that uses qsort and sorts a plain int array. — Lundin
– Lundin, Commented Jan 16, 2023 at 14:29

Vlad from Moscow · Accepted Answer · 2023-01-16 14:33:19Z

2

For starters this declaration

PhNum phList[5] = {};

is invalid in C. Opposite to C++ in C you may not specify empty braces.

You could define the array like for example

PhNum phList[5] = { 0 };

To sort the array you can use standard C function qsort declared in header <stdlib.h>.

You need yourself to track how many actually initialized elements in the array. You may not change the array size.

For example if the array contains n initialized elements where n is greater than 0 and less than or equal to 5 then you can write

int cmp( const void *a, const void *b )
{
    const PhNum *p1 = a;
    const PhNum *p2 = b;

    return strcmp( p1->lastName, p2->lastName );
}

and in main you can write

qsort( phList, n, sizeof( *phList ), cmp );

answered Jan 16, 2023 at 14:33

Vlad from Moscow

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4 Comments

Andrew Henle Over a year ago

{} will be standardized in C23. (Assuming it gets approved in 2023...)

BGOPC Over a year ago

Thanks, but what is n??

Vlad from Moscow Over a year ago

@BGOPC If you want to sort the whole array all elements of which are initialized with valid values then n is 5. Otherwise n is equal to the number of actually initialized elements of the array. If for example you set only 3 elements of the array and the last 2 are uninitialized then there is no sense to sort the whole array that is all its 5 elements. In this case you will need to set n equal to 3 to sort actually initialized elements that contain the initialized data member lastName

BGOPC Over a year ago

@VladfromMoscow Okay I understand, I make my code so if there is nothing in my list, There'll be some structures with default value not just an empty list so my n is always 5

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