C++ for loop outputs different results with one array than multiple arrays

Question

I can't understand why the outputs are different when I put 1 array into a very simple for loop and when I put two arrays into it.

int arrR[100];
int arrN[100];

//function to run the simulation
void runsim(){
    for(int i=1;i<=100;i++){
        arrN[i] = i;
        arrR[i] = i;
    }
}


//function to print the array
void printarr(int x[100]){
    for(int i=0;i <= 100;i++){
        cout << x[i] << ", ";
    }
cout << endl;
}


int main(){
runsim();
printarr(arrR);
printarr(arrN);

return 0;
}

This outputs arrR as: 0,1,2,3,4,5,6,...,100 which is what I want, but it outputs arrN as: 100,1,2,3,4,5,6,...,100 which I do not understand.

If I remove arrR from the for loop arrN prints how I want

    int arrR[100];
int arrN[100];

//function to run the simulation
void runsim(){
    for(int i=1;i<=100;i++){
        arrN[i] = i;
    }
}


//function to print the array
void printarr(int x[100]){
    for(int i=0;i <= 100;i++){
        cout << x[i] << ", ";
    }
cout << endl;
}


int main(){
runsim();
printarr(arrN);

return 0;
}

This outputs arrN as 0,1,2,3,4,5,6,...,100

Interestingly, if I change all the 100's to 10's in the code this problem also goes away even if I keep both arrays in the for loop.

Can anyone help me understand what I'm missing? I'm sure it's simple because I'm pretty new to C++. Thank you!

`for(int i=1;i<=100;i++)` ... problem already: arrays are zero-based in C++. You are trying to access a region of memory which should not be accessible. — Tim Biegeleisen, Aug 29 '16 at 01:57

songyuanyao · Answer 1 · 2016-08-29T02:15:11.757

4

Note the condition of for is i<=100, equals sign means you will access the array by arrN[100], then get out of the bound. This is undefined behavior, anything is possible. The valid range should be [0, N) (N = 100), i.e. arrN[0], …, arrN[N - 1] (i.e. arrN[99]).

You might want to change all the conditions to i < 100.

edited Aug 29 '16 at 02:15

answered Aug 29 '16 at 01:58

songyuanyao

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Wow thank you! I knew it was going to be something simple. – Jimmy Aug 29 '16 at 02:02

score 2 · Answer 2 · edited May 23 '17 at 10:29

Please note that array indexing start from 0 instead of 1 ends at n-1 instead of n. Now in your function runsim() you are accessing the assigning value 100 to arrN[100] and arrR[100] which is not possible leads to undefined behavior.

You should change the for loop condition

for(int i = 0; i < 100; i++) {
 // Do something.
}

Please note that accessing element out bound will not produce any error as well. For more detail please refer Accessing an array out of bounds gives no error

C++ for loop outputs different results with one array than multiple arrays

2 Answers2