[SOLVED] K&R Exercise 1-4: Help Checking My Math

anon033 · 05-22-2019, 06:59 PM

I am working through K&R as extra practice for my classes. I am on exercise 1-4 (converting Celsius to Fahrenheit). I believe my code should be solid, but I just need some help checking my math. My code is this:

Code:

#include <stdio.h>

/* printf Celsius-Fahrenheit table */

int main()
{
        float celsius, fahr;
        short lower, upper, step;

        lower = -17;
        upper = 148;
        step = 20;

        celsius = lower;
        printf("Celsius to Fahrenheit Table\n");

        while (celsius <= upper) {
                fahr = (9.0/5.0) * (celsius+32.0);
                celsius = celsius + step;
                printf("%3.0f %6.1f\n", celsius, fahr);
        }
}

which gives this output:

Code:

Celsius to Fahrenheit Table
  3   27.0
 23   63.0
 43   99.0
 63  135.0
 83  171.0
103  207.0
123  243.0
143  279.0
163  315.0

I am confident my code (in the sense of the actual C code) is right on track, however I would love it if someone could check my math. Thank you in advance!

# Solution:

Code:

#include <stdio.h>

/* printf Celsius-Fahrenheit table */

int main()
{
        float celsius, fahr;
        double lower, upper, step;

        lower = -17.78;
        upper = 148.889;
        step = 20;

        celsius = lower;
        printf("Celsius to Fahrenheit Table\n");

        while (celsius <= upper) {
                fahr = ((celsius) * (9.0/5.0)) + 32.0;
                printf("%3.2f %6.1f\n", celsius, fahr);
                celsius = celsius + step;
        }
}

The first issue is found here:

Code:

fahr = (9.0/5.0) * (celsius+32.0);

this is the wrong equation. The correct equation is

Code:

fahr = ((celsius) * (9.0/5.0)) + 32.0;

Then the issue is caused the line

Code:

celsius = celsius + step;

being executed before the print. You will then have the issue of the first number being 18 and thus the whole thing looks wrong and werid. To fix this change

Code:

printf("%3.0f %6.1f\n", celsius, fahr);

to

Code:

printf("%3.2f %6.1f\n", celsius, fahr);

Problem solved

Code:

Celsius to Fahrenheit Table
-17.78   -0.0
2.22   36.0
22.22   72.0
42.22  108.0
62.22  144.0
82.22  180.0
102.22  216.0
122.22  252.0
142.22  288.0

The resulting code should be as follows:

Code:

#include <stdio.h>

/* printf Celsius-Fahrenheit table */

int main()
{
        float celsius, fahr;
        double lower, upper, step;

        lower = -17.78;
        upper = 148.889;
        step = 20;

        celsius = lower;
        printf("Celsius to Fahrenheit Table\n");

        while (celsius <= upper) {
                fahr = ((celsius) * (9.0/5.0)) + 32.0;
                printf("%3.2f %6.1f\n", celsius, fahr);
                celsius = celsius + step;
        }
}

hydrurga · 05-22-2019, 07:19 PM

If you search for celsius to fahrenheit on DuckDuckGo, it shows a conversion box which states that 3C is 37.4F and 23C is 73.4F so something is wrong.

Your output (left-hand side) is rushing ahead of itself and upper is also not being respected. Check your code - the first line should have -17 in the first column.

Also, giving an answer as 135.0 gives the impression that the .0 is calculated with that level of precision. 27.0, for example, should be either 27 or e.g. 27.4 (it could of course be 27.0 but all of the results won't be y.0).

T(°F) = (T(°C) × 9/5) + 32

Using the above equation, work out why your formula is incorrect.

I'll leave the C technicalities to other people.

anon033 · 05-22-2019, 07:29 PM

I did some more searching and tried the following code:

Code:

#include <stdio.h>

/* printf Celsius-Fahrenheit table */

int main()
{
        float celsius, fahr;
        double lower, upper, step;

        lower = -17.78;
        upper = 148.889;
        step = 20;

        celsius = lower;
        printf("Celsius to Fahrenheit Table\n");

        while (celsius <= upper) {
                fahr = (9.0/5.0) * (celsius+32.0);
                celsius = celsius + step;
                printf("%3.0f %6.1f\n", celsius, fahr);
        }
}

however this is wrong as well. I can't seem to figure out exactly what is wrong with my formula... this seems like it should be the correct math to solve this problem. I looked here: https://www.metric-conversions.org/t...fahrenheit.htm even to check my numbers. I am very confused as to why this isn't working...

hydrurga · 05-22-2019, 07:32 PM

Spot the difference:

T(°F) = (T(°C) × 9/5) + 32

T(°F) = (T(°C) + 32) × 9/5

Which is correct?

astrogeek · 05-22-2019, 07:32 PM

As you asked for someone to check your math, you can easily see that it is not correct.

Water freezes at 0 Celsius and 32 Farenheit, so 3 C cannot correspond to 27 F!

Similarly for boiling, 100 C and 212 F, so 103 C cannot correspond to 207 F!

The problem is in order of operations (i.e., grouping). You are adding 32 to C before scaling to F. It should be the other way around.

anon033 · 05-22-2019, 07:34 PM

I have this:

T(°F) = (T(°C) × 9/5) + 32

eg

fahr = (9.0/5.0) * (celsius+32.0);

I am not sure how this wrong. I clearly can't spot the difference. I need some help understanding how this is wrong.

hydrurga · 05-22-2019, 07:36 PM

Quote:

Originally Posted by FOSSilized_Daemon

I have this:

T(°F) = (T(°C) × 9/5) + 32

eg

fahr = (9.0/5.0) * (celsius+32.0);

I am not sure how this wrong. I clearly can't spot the difference. I need some help understanding how this is wrong.

Those two are not the same. What are you multiplying by 9/5 in the first? Right, what are you multiplying 9/5 by in the second?

astrogeek · 05-22-2019, 07:40 PM

Quote:

Originally Posted by FOSSilized_Daemon

I have this:

T(°F) = (T(°C) × 9/5) + 32

eg

fahr = (9.0/5.0) * (celsius+32.0);

I am not sure how this wrong. I clearly can't spot the difference. I need some help understanding how this is wrong.

No, you have this...

T(°F) = 9/5 * (T(°C) + 32))

but you need this...

T(°F) = (T(°C) × 9/5) + 32

Remember, operations inside parenthesis are performed first!

anon033 · 05-22-2019, 07:40 PM

I am making the changes I believe I am supposed to make based on the advice given. My code is as follows:

Code:

#include <stdio.h>

/* printf Celsius-Fahrenheit table */

int main()
{
        float celsius, fahr;
        double lower, upper, step;

        lower = -17.78;
        upper = 148.889;
        step = 20;

        celsius = lower;
        printf("Celsius to Fahrenheit Table\n");

        while (celsius <= upper) {
                fahr = ((celsius) * (9.0/5.0)) + 32.0;
                celsius = celsius + step;
                printf("%3.0f %6.1f\n", celsius, fahr);
        }
}

however the output is still wrong:

Code:

Celsius to Fahrenheit Table
  2   -0.0
 22   36.0
 42   72.0
 62  108.0
 82  144.0
102  180.0
122  216.0
142  252.0
162  288.0

I am confused about what is causing the output to be wrong. If I am correct this should give the desired output, I don't understand why the output is as it is.

astrogeek · 05-22-2019, 07:43 PM

You are changing the celsius value before printing the output of the calculation. Another kind of order of operations error!

hydrurga · 05-22-2019, 07:45 PM

With the greatest of respect, you're not carefully reading the answers you have been given. You've got the equation right now but both astrogeek and I have told you why column 1 is incorrect.

For a start, can you please return lower and upper to -17 and 148 so that the analysis of your results is easier.

anon033 · 05-22-2019, 07:47 PM

Aha! I am almost there!

Code:

Celsius to Fahrenheit Table
-18   -0.0
  2   36.0
 22   72.0
 42  108.0
 62  144.0
 82  180.0
102  216.0
122  252.0
142  288.0

I stepped to soon haha. My numbers appear to be off, I am not wanting the answer; however what is the cause? Is it operational, data type or wrong numbers?

hydrurga · 05-22-2019, 07:51 PM

Incorrect display of results in both columns due to excessive rounding.

Edit: a hint: -17.78 is not -18.

Make it easier on yourself to verify your data and read the second part of post #11.

astrogeek · 05-22-2019, 07:58 PM

On quick inspection those look pretty good to me.

2 C should be about 35.6 F (2 * 1.8 + 32).

102 C should be about 215.6 F (212 + (2 * 1.8)).

So your math looks right now, but the precision of your results as printed looks to be wrong.

hydrurga · 05-22-2019, 07:58 PM

I see you've marked the thread as "Solved". Any chance you could post your final code and output to help others in the future?