Having a hard time adding results together in C (For Loop) - c

Question

I wrote this code in C. What it basically do is it takes an input (number) for the user and, depending the number the user chose, it will run a for loop. The for loop will asks the user to input more numbers depending on the statement that it is asking. The code will then take those input from the user and apply it to the formula.
Let say the user inputs 2 at first. What it is supposed to do is go to through the loop twice and take the sum from the first and second time and added them together. Unfortunately, it doesn't do that and will only give back the first result from the loop separated by the second result.
This is the code:
#include <stdio.h>
int main(){
int i = 0;
int robotMany = 0;
int enginePower = 0;
int resistance = 0;
int weight = 0;
int height = 0;
printf("Depending the numbers of robot that you enter.You will need to do the following\n");
printf("Engine power\nResistance\nWeight\nHeight\n");
printf("Enter the amount of robots\n");
scanf("%d",&robotMany);
printf("You chose %d robot\n", robotMany);
//Depending the user input, it will run # of times.
for(i = 0; i < robotMany; i++){
//for loop begins
printf("Enter the engine power.(1-20)\n");
scanf("%d", &enginePower);
printf("Enter the resistance.(1-3)\n");
scanf("%d", &resistance);
printf("Enter the weight of the robot\n");
scanf("%d", &weight);
printf("Enter the height of Robot.\n");
scanf("%d", &height);
int product;
//take the user inputs and apply it to the formual.
product = (enginePower + resistance) * (weight - height);
printf("This is your total power\n");
printf("%d\n", product);
}
return 0;
}

Answer 1

You have a fundamental logic error in failing to keep a running total for the product calculated within each loop iteration.
However you have a larger problem in failing to validate the return of scanf (or any function you use for user-input). It is critical you validate EVERY user input, and even more so when using scanf to avoid invoking Undefined Behavior in your code. Why? If the user enters a stray character that does not match the conversion specifier in the format string, a matching failure occurs and character extraction from stdin ceases at that point leaving the offending character in the input buffer unread. In your case this would result in the same matching failure occurring for every call to scanf throughout the remainder of your program.
Take for example the user reaches for the '2' key but accidentally taps the 'w' key instead when entering robotMany. Not only does the input for robotMany fail, but since the 'w' is left in stdin unread, it causes input for enginePower, resistance, weight & height to fail as well. (if this occurs within a loop, your loop will likely go spinning out of control until the process is manually terminated)
That is why it is critical to always validate the return of any input function, but especially the scanf family of functions. To catch and handle any input failure (matching failure, input failure, or manually generated EOF), just check the return, e.g.
if (scanf ("%d", &robotMany) != 1) { /* VALIDATE EVERY USER INPUT! */
fputs ("error: invalid input, or user canceled - robotMany\n", stderr);
return 1;
}
On to your logic errors. First, you need a variable to track the sum of each product calculated on each trip through the loop. Since it appears this will be a total power, just add a totalpwr variable and initialize it to zero (as you have done a good job doing with all your variables), e.g.
int i = 0,
robotMany = 0,
enginePower = 0,
resistance = 0,
weight = 0,
height = 0,
totalpwr = 0; /* sum of product from each iteration */
Now within your loop, you want to add product to totalpwr each time it is computed, e.g.
/* Depending the user input, it will run # of times. */
for (i = 0; i < robotMany; i++) {
...
/* take the user inputs and apply it to the formula. */
int product = (enginePower + resistance) * (weight - height);
totalpwr += product; /* add product to total power */
}
To output the totalpwr, you will output the value after you exit the loop, e.g.
...
}
/* you output the total power required outside the loop */
printf ("\ntotal power required: %d\n", totalpwr);
return 0;
}
Putting it altogether in an example (without passing on the correctness of the product calculation or the range of values needed) you can do something similar to the following:
#include <stdio.h>
int main (void) {
int i = 0,
robotMany = 0,
enginePower = 0,
resistance = 0,
weight = 0,
height = 0,
totalpwr = 0; /* sum of product from each iteration */
/* you only need a single printf (fputs would do due to no conversion) */
printf ("Depending the numbers of robot that you enter, you will need "
"to enter:\n\n"
" Engine power | Resistance | Weight | Height\n\n"
"Enter the amount of robots: ");
if (scanf ("%d", &robotMany) != 1) { /* VALIDATE EVERY USER INPUT! */
fputs ("error: invalid input, or user canceled - robotMany\n", stderr);
return 1;
}
printf ("\nYou chose: %d robots\n\n", robotMany);
/* Depending the user input, it will run # of times. */
for (i = 0; i < robotMany; i++) {
printf ("Enter the engine power.(1-20): ");
if (scanf("%d", &enginePower) != 1) { /* validate! */
fputs ("error: invalid input - enginePower\n", stderr);
return 1;
}
printf ("Enter the resistance.(1-3): ");
if (scanf("%d", &resistance) != 1) { /* validate! */
fputs ("error: invalid input - resistance\n", stderr);
return 1;
}
printf ("Enter the weight of the robot: ");
if (scanf("%d", &weight) != 1) { /* validate! */
fputs ("error: invalid input - weight\n", stderr);
return 1;
}
printf ("Enter the height of Robot: ");
if (scanf("%d", &height) != 1) { /* validate! */
fputs ("error: invalid input - height\n", stderr);
return 1;
}
/* take the user inputs and apply it to the formula. */
int product = (enginePower + resistance) * (weight - height);
/* the following outputs the per-loop/per-robot product (you can remove) */
printf (" iteration %d product: %d\n\n", i+1, product);
totalpwr += product; /* add product to total power */
}
/* you output the total power required outside the loop */
printf ("\ntotal power required: %d\n", totalpwr);
return 0;
}
(note: only one printf is required to output multiple lines of output. There is no need to make repeated calls one after the other just to output separate lines. All sequential strings will be concatenated by the compiler)
Example Use/Output
$ ./bin/robotmany
Depending the numbers of robot that you enter, you will need to enter:
Engine power | Resistance | Weight | Height
Enter the amount of robots: 3
You chose: 3 robots
Enter the engine power.(1-20): 4
Enter the resistance.(1-3): 2
Enter the weight of the robot: 10
Enter the height of Robot: 31
iteration 1 product: -126
Enter the engine power.(1-20): 5
Enter the resistance.(1-3): 3
Enter the weight of the robot: 9
Enter the height of Robot: 31
iteration 2 product: -176
Enter the engine power.(1-20): 3
Enter the resistance.(1-3): 1
Enter the weight of the robot: 8
Enter the height of Robot: 31
iteration 3 product: -92
total power required: -394
You may want to add constraints on the weight and height entry to prevent a negative power-required result -- but that is left to you. Look things over and let me know if you have further questions.