What are the possible reasons an array will repeat an index multiple times?

Question

I am working on a program in C that will take the input of a temperature from a user and return a substance and boiling point if it is within 3% of that substance's BP.

I have currently run into 2 issues that I can't seem to solve.

The function int find_substance(double value) seems to skip and not print some iterations. I have it set to print the index to make sure the indexes are chosen properly. I checked each value on the table and the only ones that don't print are -78.5, -35.5, and 3,280.00.
Another issue at hand is when I reach the 8th index, 2,212.00, it prints index = 9. Following this, each iteration prints index = 9, until I reach the last index where it prints nothing.

So what could cause an error like this?

I have checked the arrays to make sure there weren't any errors I missed and still have yet to find what is causing the problem.

#include <stdio.h>
#include <stdlib.h>
const char* name[] = {"Carbon dioxide", "Ammonia", "Wax","Water", "Olive Oil", "Mercury", "Sulfur", "Talc", "Silver", "Copper", "Gold", "Iron", "Silicon"};
const double temp[] = {-78.5, -35.5, 45, 100.7, 300.00, 356.9, 444.6, 1,500.00, 2,212.00, 2,562.00, 2,700.00, 2,862.00, 3,280.00};
int is_substance_within_x_percent(double temp, double value);
int find_substance(double value);
int main()
{
    double obs_temp;
    printf("Enter the temperature: ");
    scanf("%lf", &obs_temp);
    double value = obs_temp;
    int index = find_substance(value);




}
int is_substance_within_x_percent(double temp, double value)
{
    temp = abs(temp);
    if((value>=temp - (0.03 * temp)) && (value <=temp + (0.03 * temp))){
        return 1;
    }
    else
        return 0;
}
int find_substance(double value)
{
    int index = 0;
    int i;
    for(i=0;i<13;i++)
    {
        if(is_substance_within_x_percent(temp[i], value) == 1){
            printf("index: %d", i);
            break;
        }

    }
    return i;
    if (is_substance_within_x_percent(temp[i], value)== -1){
        printf("No substance was found.\n");
        return 0;
    }
}

First of all you don't need the value variable in the main function, use abs_temp in the call directly. Secondly, the return statement returns immediately, no statements in the function will execute after it. Thirdly, the is_substance_within_x_percent function will never return -1. Fourthly, when the loop ends the value of i might be equal to 13 which means it will be out of bounds. — Some programmer dude
– Some programmer dude, Commented Jun 15, 2021 at 21:19
Lastly, if the loop ends with i == 13 then you know nothing was found, so don't need to call is_substance_within_x_percent again. Or you can just return from inside the loop instead of using break, in which case if the loop ends then you don't have to check anything, you know nothing was found. — Some programmer dude
– Some programmer dude, Commented Jun 15, 2021 at 21:22
is_substance_within_x_percent() is incorrect for negative temp arguments. — John Bollinger
– John Bollinger, Commented Jun 15, 2021 at 21:27
One of the biggest problems is the use of abs(), which is an integer function. By using it, you will lose any fractional portion of the temperature. Use fabs() instead. Be sure to #include <math.h>, and link with -lm. That may explain your observed problem, since you're using it to check the value, and if the check fails, you skip printing anything on that loop iteration. — Tom Karzes
– Tom Karzes, Commented Jun 15, 2021 at 21:47

Bryan · Accepted Answer · 2021-06-16 23:50:51Z

1

In the string temp[] change the commas for numbers in the thousands to straight numbers as well as remove any extra zeros since the array is type double, making them unnecessary (ex. 2,562.00 >>> 2562).

Also in is_substance_within_x_percent: add value = fabs(value) and change temp = abs(temp) into temp = fabs(temp) since both of these variables are double and you want the absolute value of each before calculating.

edited Jun 16, 2021 at 23:50

answered Jun 16, 2021 at 22:14

Bryan

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