Trying to get a function to work using pointers-CodePudding

This is a problem from my course lesson where we are learning C-language. Why do I get the error below? I tried to search it on Google and I couldn't find anything. The error is not especially helpful with the info provided as I can't change the line it references.

main.c:21:14: error: void value not ignored as it ought to be
   21 |    isChanged = AdjustState(waterTemperature, &waterState);
      |              ^

For the lesson I am not allowed to change the main function.

    #include <stdio.h>
    #include <stdbool.h>
  
void AdjustState(int waterTemperature, char* waterState) {
   if (waterTemperature < 100) {
      *waterState = 'L';
   }
   if (waterTemperature < 100) {
      *waterState = 'G';
   }
}

int main(void) {
   int waterTemperature;
    char waterState;
   bool isChanged;

   waterState = 'B';
   scanf("%d", &waterTemperature);

   isChanged = AdjustState(waterTemperature, &waterState);

   if (isChanged) {
      printf("Water state is changed to %c.\n", waterState);
   }
   else {
      printf("Water state %c is not changed.\n", waterState);
   }

   return 0;
}

CodePudding user response：

You should change the return type of the function AdjustState and have it return something that fit to the function main.

It looks like it should return if the function changed the value pointed at by waterState, it can be like this:

int AdjustState(int waterTemperature, char* waterState) {
   char originalWaterState = *waterState;
   if (waterTemperature < 100) {
      *waterState = 'L';
   }
   if (waterTemperature < 100) {
      *waterState = 'G';
   }
   return *waterState != originalWaterState;
}

CodePudding user response：

The function has the return type void

void AdjustState(int waterTemperature, char* waterState) {
   if (waterTemperature < 100) {
      *waterState = 'L';
   }
   if (waterTemperature < 100) {
      *waterState = 'G';
   }
}

Thus this statement

isChanged = AdjustState(waterTemperature, &waterState);

is invalid. A function with the return type void returns nothing.

If you may not change the function declaration then you should write for example

waterState = 'B';
char currentWaterState = waterState;

scanf("%d", &waterTemperature);

AdjustState(waterTemperature, &waterState);

isChanged = currentWaterState != waterState;

// ...

Also it seems that there is a typo within the function

   if (waterTemperature < 100) {
      *waterState = 'L';
   }
   if (waterTemperature < 100) {
      *waterState = 'G';
   }

The both if statements have the same condition expression. Check the conditions and change one or them.

Maybe you need to write something like the following

void AdjustState(int waterTemperature, char* waterState) {
   if (waterTemperature < 100) {
      *waterState = 'L';
   }
   else  {
      *waterState = 'G';
   }
}

Or alternatively

void AdjustState(int waterTemperature, char* waterState) {
   *waterState = waterTemperature < 100 ? 'L' : 'G'; 
}