複製特定的結構成員到另一個結構

Question

所以我有一個文件，我必須加載一些曲目，找到超過3:30的曲目，並找到這些曲目的平均bpm。

這是我的代碼。

主文件：

#include <stdio.h> 
#include "functions.h" 
int main(void) 
{ 
    Playlist *all, *long_tracks; 
    float avg_bpm; 
    all = load_tracks("tracks.txt"); 
    long_tracks = get_tracks_longer_than(3,30,all); 
    print_playlist(long_tracks); 
    avg_bpm = get_avg_bpm(long_tracks); 

    printf("Average BPM of the playlist is: %.2f\n", avg_bpm); 

    return 0; 
} 

頁眉：

#ifndef FUNCTIONS_H 
#define FUNCTIONS_H 

typedef struct track { 
char track[250]; 
float bpm; 
int mm, ss; 
} Track; 

typedef struct playlist { 
int count; 
Track **tracks; 
} Playlist; 


void print_playlist(Playlist *list); 
Playlist* load_tracks(char *filename); 
Playlist* get_tracks_longer_than(int mm, int ss, Playlist* all); 
float get_avg_bpm(Playlist *list); 

#endif 

功能：

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include "functions.h" 


void print_playlist(Playlist *list) { 
    int i; 
    for (i = 0; i < list->count; i++) { 
    printf("%s (%d:%d) [%f]\n", list->tracks[i]->track, list->tracks[i]->mm, list->tracks[i]->ss, list->tracks[i]->bpm); 
    } 
} 

Playlist* load_tracks(char *filename) { 
    int n,i; 
    Playlist *list = (Playlist*) malloc(sizeof(Playlist)); 
    Track *tracks; 

    FILE *f = fopen(filename, "r"); 
    fscanf(f, "%d", &n); 
    printf("Tracks: %d\n", n); 

    tracks = (Track*) malloc(n*sizeof(Track)); 
    list->tracks = (Track**) malloc(n*sizeof(Track*)); 

    for (i = 0; i < n; i++) { 
    fscanf(f, "%d:%d", &tracks[i].mm, &tracks[i].ss); 
    fscanf(f, "%f", &tracks[i].bpm); 
    fgetc(f); 
    fgets(tracks[i].track, 250, f); 
    tracks[i].track[strlen(tracks[i].track)-1] = '\0'; 
    list->tracks[i] = &tracks[i]; 
    } 


    list->count = n; 
    return list; 
} 

Playlist* get_tracks_longer_than(int mm, int ss, Playlist* all) { 
    Playlist *longSongs = (Playlist*) malloc(sizeof(Playlist)); 

    Track *tracks; 
    tracks = (Track*) malloc(all->count*sizeof(Track)); 
    longSongs->tracks = (Track**) malloc(all->count*sizeof(Track*)); 

    int i, n = 0; 

    for(i=0;i<all->count;i++) 
    { 

     if(all->tracks[i]->mm>mm) 
     { 
      n++; 
      memcpy (&longSongs, &all, sizeof(all)); //I also tried longSongs[i]=all[i]; 
     } 
     else if(all->tracks[i]->mm==mm) 
     { 
      if(all->tracks[i]->ss>ss) 
      { 
      n++; 
      memcpy (&longSongs, &all, sizeof(all)); 
      } 
     } 
    } 

    longSongs->count=n; 
    return longSongs; 
} 

float get_avg_bpm(Playlist *list) { 
    float avg_bpm=0; 
    int i; 

    for(i=0;i<list->count;i++) 
    { 
     avg_bpm+=list->tracks[i]->bpm; 
    } 

    return avg_bpm/=list->count; 

} 

問題是我get_tracks_longer_than功能，我真的不知道如何複製結構成員我想從源結構到目標結構，我只設法複製所有成員f從一個結構到另一個結構。 我的n計數器正確計數磁道數，而我的計算bpm的函數似乎可行，但我不知道如何獲得我想要的確切數據。 我嘗試使用memcpy函數，試圖像這樣分配的：longSongs[i]=all[i]，我試圖手動分配值，e.g：

longSongs->tracks[i]->mm = all->tracks[i]->mm; 
longSongs->tracks[i]->ss = all->tracks[i]->ss; 
longSongs->tracks[i]->bpm = all->tracks[i]->bpm; 

但沒有任何工程，它總是將源結構的所有成員，無視我的if語句。

Answer 1

您不需要複製成員，只需複製指針即可。

longSongs->tracks[n] = all->tracks[i]; 

注意，索引longSongs->tracks是n，不i。否則，您將在longSongs->tracks陣列中爲所有未被選中的歌曲留下間隙。

Playlist* get_tracks_longer_than(int mm, int ss, Playlist* all) { 

    int min_secs = mm*60 + ss; 

    Playlist *longSongs = malloc(sizeof(Playlist)); 
    longSongs->tracks = malloc(all->count*sizeof(Track*)); 

    int i, n = 0; 

    for(i=0;i<all->count;i++) 
    { 
     if(all->tracks[i]->mm*60 + all->tracks[i]->ss > min_secs) 
     { 
      longSongs->tracks[n++] = all->tracks[i]; 
     } 
    } 

    longSongs->count=n; 
    return longSongs; 
} 

其他變更：我刪除未使用的tracks變量，我通過簡化分秒組合成只是總秒數比較運行時間。