Thread: Stumped on role of a particular statement in a program with Arrays & Nested For Loops

I can't seem to edit any post anymore.

Just to be clear, the statement I'm confused about is this one: value[i]= INITIAL_BALANCE;

I don't know what you mean by your note "gets expected output here". Is that the expected output? Only the header line of the table? Why is that "expected"? It's not what I expected. And where is the rest of the code in the second listing? Is it there but you just don't show it, or did you remove that too?

Anyway, the loop in question is doing two things at once.
It's printing out the top row of the table (incorrectly spaced!).
And it is also INITIALIZING the value array.
Without initialization, the value array will contain arbitrary values.

Your post is quite thorough ... a bit too much, in my opinion. I just glossed over to find the question at hand.

The statement you're asking about in the first "for()" loop initializes all elements in the "value" array.

This is because afterwards, the first operation on each element of the "value" array (in the third loop) depends upon the initial value of each element (value[i] = value[i] + ...). If the array is not initialized first, each element contains garbage, so the result of the subsequent operations are garbage.

Does that answer your question?

In the future, I would suggest avoiding images of code and output. Output can be posted in code tags, as well as the code itself. Also be sure the code is properly formatted before submitting (try "paste as text" to prevent unnecessary markup, so the formatting remains intact and the forum's code highlighting works properly), and preview your post to ensure the code looks right.

Note that neither Matticus or I are able to explain your output. Without the initialization line, I would expect the table to be printed with arbitrary values, whereas your output shows that only the header line was printed. That makes no sense to me.

Here's your code in a readable format:

Code:

#include <stdio.h>

#define NUM_RATES ((int)(sizeof(value)/sizeof(value[0])))
#define INITIAL_BALANCE 100.00

int main() {
    int low_rate;
    int num_years;
    int i;
    int year;
    float value[5];

    printf("Enter interest rate: ");
    scanf("%d", &low_rate);
    printf("Enter number of years: ");
    scanf("%d", &num_years);

    printf("\nYears  ");

    for(i = 0; i < NUM_RATES; i++) {
        printf("%7d", low_rate + i);
        //value[i] = INITIAL_BALANCE;   // with this commented out, it still prints the table body.
    }
    printf("\n");

    for (year = 1; year <= num_years; year++) {
        printf("%3d    ", year);
        for (i = 0; i < NUM_RATES; i++) {
            value[i] += (low_rate + i) / 100.0 *  value[i];
            printf("%7.2f", value[i]);
        }
        printf("\n");
    }

    return 0;
}

Originally Posted by algorism

I don't know what you mean by your note "gets expected output here". Is that the expected output? Only the header line of the table? Why is that "expected"? It's not what I expected. And where is the rest of the code in the second listing? Is it there but you just don't show it, or did you remove that too?

Anyway, the loop in question is doing two things at once.
It's printing out the top row of the table (incorrectly spaced!).
And it is also INITIALIZING the value array.
Without initialization, the value array will contain arbitrary values.

Thanks algorism. By the output, I mean that I get the same output for both fragments below when put with the declarations and directives at the top:

Code:

        for (i = 0; i < NUM_RATES; i++) {    
    
        printf("%6d", low_rate + i);         /* Prints out the low rate + i   according to the loop above */
                                                    
        value[i] = INITIAL_BALANCE;
    }

Same output as the following without the statement: value[i] = INITIAL_BALANCE;

Code:

        for (i = 0; i < NUM_RATES; i++) {    
    
        printf("%6d", low_rate + i);         /* Prints out the low rate + i   according to the loop above */
                                                    
    }

The output for both is displayed as:


But as you both explained, that statement in question is for the use of the other two loops that come later

Originally Posted by potomac

By the output, I mean that I get the same output for both fragments below

So only printing out the table header is "expected" by you? I can't see how that could happen with the code in my post above, for instance. Try copying/pasting/running it. It will print the table body with or without the line in question.

Whoops, I meant to type:

Code:

float value[5] = { 100, 100, 100, 100, 100 };

My honest advice is to rework the code, make those changes, and see if it still works. I can't think of a reason why it wouldn't work. It simply wasn't written that way. Perhaps the original author realized it wasn't necessary?

Either way, you're wrong about the macro not being useful here. It's fine.

Code:

float value[] = { INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE };

Originally Posted by potomac

Would it be reasonable to conclude that the author put the array initializer in the same loop as the interest rate labels for presentation purposes/to conserve space on the page?

Sort of - it looks like the author was trying to reduce the number of loops by having all that work done in the first loop.

Originally Posted by potomac

If this was production code, wouldn't it be better to put the array initializer and the interest rates in separate functions with For Loops?

There is nothing wrong with the current method, as in this case it's guaranteed that the number of array elements is the same as the number of column headers to print. In cases where quantities like this may differ, even if they happen to be the same at the time of writing, it is best to separate them (as a change to one later on would make those values different).

There is no hard and fast rule for whether to separate the header prints and the array initialization as in this case. This is where you can see the subjectivity of code writing. The more important factor is that, whichever way it is done, the code is clear and easy to understand.

Originally Posted by potomac

Code:

float value[5] = { 100, 100, 100, 100, 100 };

As you said, this would lead to more maintenance work. Using named constants allows values to easily be changed without hunting down and modifying the actual code that uses those constants, which would have to be done if the array were initialized as shown.

Also consider what you'd have to do to update the initializer list if you change the number of elements from 5 to 1000.

Originally Posted by whiteflags

My honest advice is to rework the code, make those changes, and see if it still works. I can't think of a reason why it wouldn't work. It simply wasn't written that way. Perhaps the original author realized it wasn't necessary?

Either way, you're wrong about the macro not being useful here. It's fine.

Code:

float value[] = { INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE, INITIAL_BALANCE };

I am assuming the OP was referring to the NUM_RATES macro, not INITIAL_BALANCE.