for循环未运行，尝试向后遍历数组元素

Question

To practice using pointers and arrays i'm trying to do a simple program capable of converting a binary input to denary.. i think i have a good idea for the logic but i haven't even got round to trying to implement it because im struggling to get my for loop running!

It seems silly but i know the code inside the for loop works fine outside of it, so it must be something wrong with the condition..? im trying to start at the back of the char array (navigating using pointers) and output each char(as an int) up to the first element.

So the desired output is "0 - 1 - 0 - 1 -"

#include <iostream>

using std::cout;
using std::endl;

//prototypes
void binaryToDenary(const char* input, int& inputLength);

int main(){

    const char binaryInput[] = {1,0,1,0};
    int inputLength = sizeof(binaryInput)/sizeof(binaryInput[0]);

    binaryToDenary(binaryInput, inputLength);

    return 0;
}
void binaryToDenary(const char* input, int& inputLength){
    //testing some stuff--- this all works as expected
    //cout << input[2] << " " << (int)*(input+2) << " " << inputLength <<endl;

    int i;
    for(i = inputLength; i < 0; i--){
        cout << (int)*(input+i) << " - ";
    }

}

Answer 1

Your for loop should be this:

for(i = inputLength -1 ; i  >= 0; i--)
{
    cout << (int)*(input+i) << " - ";
}

There are two problems in your code:

• i = inputLength which should be i = inputLength -1
• i < 0 which should be i >= 0

---

Also, change the second parameter type from int & to int :

void binaryToDenary(const char* input, int inputLength) //now its better!

The type int& reduces the use cases, and benefits almost nothing. If you use int & , then all of these would give compilation error:

const int inputLength = sizeof(binaryInput)/sizeof(binaryInput[0]);
^^^^ note this

binaryToDenary(binaryInput, inputLength); //compilation error
binaryToDenary(binaryInput, sizeof(binaryInput)/sizeof(binaryInput[0])); //error
binaryToDenary(binaryInput, 4); ////compilation error

So use int , and all of the above would compile fine!

Answer 2

Array indexes start from zero, so the last element is at inputLength - 1 . With i < 0 you exit from the loop immediately as that never be true...

for(i = inputLength - 1; i >= 0; i--){
    cout << (int)*(input+i) << " - ";
}

Answer 3

for(i = inputLength; i < 0; i--)

will run only if inputLength is less than 0, which is not possible?

You need:

for(i = (inputLength-1); i >= 0; i--)
         ^^^^^^^^^^^^^^    ^^

Answer 4

C arrays are 0 based so a valid index is given by

(0 <= i) && (i < array_length)

In your program, this means that the position of the last digit in your initialization should be inputLength - 1 and the loop condition should be i >= 0 .

(As for why you loop is not running, at the start you have i == inputLength , so i is positive, failing the i < 0 condition immediately).

Answer 5

You want to run as long as i is bigger (or maybe equal) to zero. You were trying to run the loop as long as i was less than zero, and beginning with a value greater than zero results to that you never enter the loop.

for(i = inputLength; i > 0; i--){
        cout << (int)*(input+i) << " - ";
}

Answer 6

You must check for iteration loop variable i to be positive..

However you should use a STL iterator on the binary input vector and not loop on it's content in ac fashion, if you want to practice C++, possible solution could be:

vector<char> binaryInput;

binaryInput.push_back(1);
binaryInput.push_back(0);
binaryInput.push_back(1);
binaryInput.push_back(0);

vector<char>::iterator it;

for ( it=myvector.begin() ; it < myvector.end(); it++ ){
    cout << " " << *it << endl; //or whatever you need to do with vector content
}