tokenizing a string with strtok and saving it

I am trying to write a program that receives the name of the currency and its value from the input in each line. When the program is run and we input the first line, the name and value are correctly entered into the coins structure, but when we input the second line, the name of the previous currency is changed and becomes incorrect!

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <math.h>

int main() {
    int number_coin, index; 
    union coin {
        char name[20]; 
        float value; 
        float amount;
    };
    union coin coins[1000];
    scanf("%d", &number_coin);
    getchar();
    for (index = 0; index < number_coin; index++) {
        char input[40];
        fgets(input, 40, stdin);
        input[strcspn(input, "\n")] = 0; // delete \n in end of input string
        char *token = strtok(input, " ");
        while (token != NULL) {
            strcpy(coins[index].name, token);
            token = strtok(NULL, " ");
            coins[index].value = atof(token);
            token = strtok(NULL, " ");
        }
    }
    return 0;
}

input like this:

2
jibcoin 2
bitcoin 5

output:
just save name and value in union

This isn't how unions work.

When you have a variable of a union type, it has space for the largest datatype of the union and could be any of the datatypes within the union. You assign the name, this is fine, but when you assign the float you are overwriting the same variable with a float value type in the same memory area.

I think the behavior you are looking for is a struct.

See below for a demonstration of your problem:

$ cat union.c 
#include <stdio.h>
#include <string.h>

int main(int argc,char **argv) {
  union coin {
    char name[20];
    float value;
  };
  union coin coins[1000];
  strcpy(coins[0].name,"name 1");
  printf("coins[0].name = %s\n",coins[0].name);
  coins[0].value = 123.456;
  printf("coins[0].value = %f\n",coins[0].value);
  printf("coins[0].name changed to -> %s\n",coins[0].name);
  printf("resetting coins[0].name . . .\n");
  strncpy(coins[0].name,"name 1",20);
  printf("coins[0].name = %s\n",coins[0].name);
  printf("coins[0].value changed to -> %f\n",coins[0].value);
  return 0;
}
$ gcc union.c 
$ ./a.out 
coins[0].name = name 1
coins[0].value = 123.456001
coins[0].name changed to -> y��B 1
resetting coins[0].name . . .
coins[0].name = name 1
coins[0].value changed to -> 70062382309412494639104.000000

This trait of unions can also be used to determine the endianness of a system.

For each line, you want to store the name and the value, and probably the resulting amount later, so you should be using a struct instead of a union that can only store one of the fields at a time.

Note also that your code has undefined behavior if any of the lines after the first one does not contain two space separated tokens.

Here is a modified version:

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

int main() {
    int number_coin, index; 
    struct coin {
        char name[40]; 
        float value; 
        float amount;
    };
    struct coin coins[1000];

    scanf("%d", &number_coin);
    getchar();
    for (index = 0; index < number_coin; index++) {
        char input[40];
        fgets(input, sizeof input, stdin);
        char *token = strtok(input, " \t\n");
        if (token != NULL)
            strcpy(coins[index].name, token);
            token = strtok(NULL, " \t\n");
            if (token) {
                coins[index].value = atof(token);
                continue;
                token = strtok(NULL, " \t\n");
                if (token == NULL)
                    continue;
            }
        }
        printf("invalid line\n");
        index--;
    }
    return 0;
}

Here is an alternative using sscanf() with better error reporting:

#include <stdio.h>

int main() {
    char input[40];
    char newline;
    int number_coin, index; 
    struct coin {
        char name[20]; 
        float value; 
        float amount;
    };
    struct coin coins[1000];

    if (!fgets(input, sizeof input, stdin)) {
        fprintf(stderr, "empty input file\n");
        return 1;
    }
    if (sscanf(input, "%d%1[\n]", &number_coin, &newline) != 2
    ||  number_coins < 0 || number_coins > 1000) {
        fprintf(stderr, "invalid number of coins: %s", input);
        return 1;
    }
    for (index = 0; index < number_coin; index++) {
        if (!fgets(input, sizeof input, stdin)) {
            fprintf(stderr, "missing input lines\n");
            return 1;
        }
        if (sscanf(input, "%19s%f%1[\n]", coins[index].name,
                   coins[index].value, &newline) != 3) {
            fprintf(stderr, "invalid format: %s\n", input);
            return 1;
        }
        // initialize amount...
        coins[index].amount = 0;
    }
    return 0;
}

it get modified becouse you are using [*token=strtok()], so in this way you will modify the value/string previously stored in the memory address of "token". Also I think could be better to cast coin as a struct instead of union, so in this way you create (as you already did with union), an array of 1000 structures iterable with a for loop that includes the already existing for loop. Another suggestion is to store the return value of [gets] in the [input] variable without using fgets. hope to be useful!