将数组的值传递给修改值的函数，然后将其返回到新数组中

Question

I'm having trouble getting my code to return the correct arrays. void map takes in a function such as tiple and modifies the value that was passed into it from the array src and then returns the new value into dst which is then printed out. I can get the code to compile but it doesn't return the correct array. For example, when it takes in [1,2,3,4] it returns [0,3,6,9] instead of [3,6,9,12]

typedef int (*intModifier)(int);

int triple(int x){
    return 3*x;
}

void map(intModifier func, int src[], int dst[], int length){
    for(int *i = src; i < src + length; ++i){
    dst[*i] = func(*i);
    }
return;

}

void printIntArray(int arr[], int length){
cout << "{";
    if (length > 0){
    cout << arr[0];
    }
    for(int i = 1; i < length; ++i){
    cout << ", " << arr[i];
    }
    cout << "}";
    }

int main(){

int arr1[4] = {1, 2, 3, 4};
int arr2[4] = {0, 0, 0, 0};
int arr3[4] = {0, 0, 0, 0};
int arr4[4] = {0, 0, 0, 0};

cout << "Testing map." << endl;
cout << "  setting arr1 = {1,2,3,4}" << endl << endl;

cout << "  mapping from arr1 to arr2 using triple" << endl;
map(triple, arr1, arr2, 4);
cout << "  arr2 = ";
printIntArray(arr2, 4);  cout << endl << endl;
return 0;
}

Any help is appreciated. Thanks.

Answer 1

I am not sure why it is only the second element being modified. But the fact that is only one is almost definitely this:

void map(intModifier func, int src[], int dst[], int length){
    for(int *i = src; i < src + length; i++){
        dst[*i] = func(src[*i]);
        return; // You return here!!!
    }
}

You will always return from map during the first iteration of your for loop. See how beneficial it is to indent your code!

Answer 2

Your map function is what is causing most of the issues. Especially in the way you are trying to conduct the operations in the for loop.

void map(intModifier func, int src[], int dst[], int length){
    for(int *i = src; i < src + length; ++i){ // There are some major issues in how you are 
        dst[*i] = func(*i);                   // iterating through the source array.
    }
return;

}

You are using the value of the source array to increment and store data between your destination array and source array. Your arrays are only 4 int "blocks" yet you increment through source at the source base length plus 4 i < src + length which is going well past your array length.

It would be better if you iterate through using the length you pass in, that's essentially why you pass the length in correct? So you could use this:

void map(intModifier func, int src[], int dst[], int length){

    for (int i = 0; i < length; i++) // Iterate until we have met the length of the source
    {                                // array. Pass the source array off to our triple func
        dst[i] = func(src[i]);
    }

    return;
}

The function iterates through source and passes its contents to the triple func then stores it at the same location in our destination array.

EDIT: Credit given to 1201ProgramAlarm and Ben .