Selecting random line from array c - arrays

Question

So far I have read all my data into an array
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <stdlib.h>
int main(void) {
int i=0;
char* string[100];
char line[100];
FILE *file;
file = fopen("plates.txt", "r");
while(fgets(line, sizeof line, file)!=NULL) {
printf("%s",line);
string[i]=line;
i++;
}
fclose(file);
return 0;
}
but i want to now select a random line of my array and print it. All lines need to have an equal chance of being selected but they can only be selected once. Im not too sure how to do this...
Thank you in advance

Answer 1

Please be mindful of this line string[i]=line as it makes all the array entries in string that you set all point to the last line read which is not what you want and it's pretty important to understand that.
That said, here's a solution to the problem that assumes we can just store all the lines in memory and on the stack:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#define MAX_LINE_LENGTH 128
#define MAX_LINE_COUNT 1000
int main(int argc, char **argv) {
char lines[MAX_LINE_COUNT][MAX_LINE_LENGTH];
int numLines = 0;
if (argc < 2) {
fprintf(stderr, "missing file name\n");
return EXIT_FAILURE;
}
FILE *fp = fopen(argv[1], "r");
if (fp != NULL) {
while (fgets(lines[numLines++], MAX_LINE_LENGTH, fp)) {
printf("%03d> %s", numLines, lines[numLines-1]);
}
fclose(fp);
srand (time(NULL));
int randomIndex = rand() % numLines;
printf("Selected random line #%d> %s", randomIndex+1, lines[randomIndex]);
} else {
fprintf(stderr, "file '%s' not found\n", argv[1]);
return EXIT_FAILURE;
}
}
And the corresponding output:
➜ ~ gcc random-line.c && ./a.out random-line.c
001> #include <stdio.h>
002> #include <stdlib.h>
003> #include <string.h>
004> #include <time.h>
005>
006> #define MAX_LINE_LENGTH 128
007> #define MAX_LINE_COUNT 1000
008>
009> int main(int argc, char **argv) {
010> char lines[MAX_LINE_COUNT][MAX_LINE_LENGTH];
011> int numLines = 0;
012>
013> if (argc < 2) {
014> fprintf(stderr, "missing file name\n");
015> return EXIT_FAILURE;
016> }
017>
018> FILE *fp = fopen(argv[1], "r");
019> if (fp != NULL) {
020> while (fgets(lines[numLines++], MAX_LINE_LENGTH, fp)) {
021> printf("%03d> %s", numLines, lines[numLines-1]);
022> }
023> fclose(fp);
024> srand (time(NULL));
025> int randomIndex = rand() % numLines;
026> printf("Selected random line #%d> %s", randomIndex+1, lines[randomIndex]);
027> } else {
028> fprintf(stderr, "file '%s' not found\n", argv[1]);
029> return EXIT_FAILURE;
030> }
031> }
Selected random line #2> #include <stdlib.h>