Problems passing an array to a function

This is a discussion on Problems passing an array to a function within the C Programming forums, part of the General Programming Boards category; I am trying to pass a dynamically allocated array into a function to enter values. I have a function that ...

  1. #1
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    Problems passing an array to a function

    I am trying to pass a dynamically allocated array into a function to enter values. I have a function that allocates the array to the correct size and then that is passed into the function. My problem is that when the values are enter they don't seem to make it into the array. The program prints out a random number.
    Code:
    void readMatrix(double *mPtr, int size)
    {
    	int input = 0;
    	int count = 0;
    
    	printf("Please enter the values of your matrix row by row, left to right:\n");
    	do	//begin do/while
    	{
    		scanf("%f", &input);
    		mPtr[count] = input;
    		count++;
    	}while(count<size);	//end do/while
    
    }
    Whenever I try to print a value out of that do/while loop I get a number like 12486144843. I just want the inputs to make it into the array and be available outside of the function. This is passed by reference correct?

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    No, never.

    C ALWAYS passes by copying. It JUST HAPPENS however, that when you copy a memory address (like an array name or a pointer), that THAT particular kind of a copy is still perfectly good for appearing to pass by reference. So it's acting like a pass by reference, but it's actually a pass by copying, same as every other parameter passed in C.

    Try that!

    Adak

  3. #3
    cwr
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    You are trying to pass a float (%f) into an int (input), you don't want to do this.

    Other than that, at first glance, your array filling function appears to be reasonable. The code calling readMatrix(foo, size) will pass the address of your foo array to the function.

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    I changed the int input to double input, but now all the functions seems to do is spit out zeros. I check what the input is by printing it right after it is entered and it says that it is zero. It is like the values are never making it into the variable input.

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    cwr
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    Quote Originally Posted by ndenklau
    I changed the int input to double input, but now all the functions seems to do is spit out zeros. I check what the input is by printing it right after it is entered and it says that it is zero. It is like the values are never making it into the variable input.
    I said %f was float. A double is not the same as a float.

    In scanf, use %lf for a double, and %f for a float.

    In printf, use %f for both. It's inconsistent, but there's a reason for it.

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    That worked! Thanks for the help.

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