for (year = 0, total = 0; year < YEARS; year++) { // for each year, sum rainfall for each month for (month = 0, subtot = 0; month < MONTHS; month++) { subtot += rain[year][month]; }
printf("%5d %15.1f\n", 2010 + year, subtot); total += subtot; // tot al for all years }
As an exercise the book asks the reader to change the above code so it uses pointers instead of subscripts.
My attempt was to do the
[....]
float (* rainPointer)[2] rainPointer = rain;
[....]
for (year = 0, total = 0; year < YEARS; year++) { // for each year, sum rainfall for each month for (month = 0, subtot = 0; month < MONTHS; month++) { subtot += *(*(rainPointer + year) + month); } }
[....]
This would give the correct value for year 0, but year 1 (and subsequent) wouldn't. This confused me because the debugger was showing that rainPointer[year][month] held the correct values but it wasn't reading the correct float from the array.
The book states the following:
цитата:
[...] Hence,pz must point to an array of two ints, not to a single int. Here is what you can do: int (* pz)[2]; // pz points to an array of 2 ints
In my case I changed pz to rainPointer.
I learned that rainPointer is useless as I can just use rain. But if rainPointer points to rain, surely the following should be the same:
Отправлено: 06.04.15 16:28. Заголовок: It is not clear why ..
It is not clear why you use a pointer declared like
float (* rainPointer)[2];
and assign it by rain while the element of the array has type
float [MONTHS]
If you want to get equivalent results you have to use a pointer declared like
float (* rainPointer)[MONTHS];
Here is a demonstrative program. Also I appended it with a code block that shows the difference between these two pointer declarations. That is elements they point to have different sizes.
for ( ; year < YEARS; year++ ) { // for each year, sum rainfall for each month float subtot = 0.0f; int month = 0; for ( ; month < MONTHS; month++ ) { subtot += rain[year][month]; }
printf( "%5d %15.1f\n", 2010 + year, subtot ); total += subtot; // tot al for all years }
printf( "\n" );
rainPointer = rain;
for ( ; rainPointer != rain + YEARS; rainPointer++ ) { // for each year, sum rainfall for each month float subtot = 0.0f; const float *monthPointer = *rainPointer; for ( ; monthPointer != *rainPointer + MONTHS; monthPointer++ ) { subtot += *monthPointer; }
printf( "%5d %15.1f\n", 2010 + year, subtot ); total += subtot; // tot al for all years }
Отправлено: 07.04.15 01:56. Заголовок: I hate to be a pain ..
I hate to be a pain but could you point me in the right direction with this one?
Write a program that initializes an array-of-double and then copies the contents of the array into three other arrays. (All four arrays should be declared in the main program.) To make the first copy, use a function with array notation. To make the second copy, use a function with pointer notation and pointer incrementing. Have the first two functions take as arguments the name of the target array, the name of the source array, and the number of elements to be copied. Have the third function take as arguments the name of the target, the name of the source, and a pointer to the element following the last element of the source. That is, the function calls would look like this, given the following declarations:
Отправлено: 07.04.15 13:23. Заголовок: Trying to understand..
Trying to understand your first example, stepping through the code with the debugger I notice it's going backwards when putting things into target
I tried adding comments to help myself follow the logic here:
void copy_ptrs(double *target, double *source, double *lastElement) { // go to source get what's there // go to lastElement get what's there // if they're not the same continue
// (increment source)
for (; *source != *lastElement; ++source) {
// get whatever is at source // increase target location and get whatever is at target // assign whatever is at source to target
// (increment source + repeat until source and lastElement are equal)
*target++ = *source; }
}
how does target[4] end up being the first element to be filled? or am I reading my debugging window incorrectly?
how does target[4] end up being the first element to be filled?
I do not understand what you mean. The function sequantially fills elements pointed to by pointer target. For example after assigning the first element pointed to by target target is increased and so on.
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