Weird behavior of a counter inside the for loop - c

Question

I've noticed that a counter(avariable) of the for loop, doesn't works well.
Indeed, the counter doesn't decrements correctly; I know this question seems stupid , but I can't understand why the a variable does this.
#include <stdio.h>
int main() {
int a,i,b,matrice[2][2];
printf("Put inside the matrix some numbers..\n");
for (a=2;a>=0;a--) {
for (b=2;b>=0;b--) {
matrice[a][b]=scanf("%d",&i);
}
}
return 0;
}

Answer 1

You are going out of bounds when you read into your array.
for (a=2;a>=0;a--) {
Array has only two elements, yet you try to read into the third on the first iteration.
matrice[2]
Is the third element. This happens for both dimensions.
scanf() call is not correct. It reads the value into i, but it returns the number of read items, not the value of i.

Answer 2

if you are trying to read the numbers into the matrix, then this is what you are supposed to do..
for (a=1;a>=0;a--) {
for (b=1;b>=0;b--) {
scanf("%d",&matrice[a][b]);
}
As the array matrice[2][2] has 2*2 elements, the valid indices are 0 and 1.
Thats why the for loop should start from 1 and not 2

Answer 3

1) The array index start from 0, as per your code it will access matrice[2][2] which will cause undefined behavior.
2) matrice[a][b]=scanf("%d",&i); will store the return value of scanf in matrice[a][b].
#include <stdio.h>
int main() {
int a,b,matrice[2][2];
printf("Put inside the matrix some numbers..\n");
for (a=1;a>=0;a--)
{
for (b=1;b>=0;b--)
{
scanf("%d",&matrice[a][b]);
}
}
//Then print or do other operations on matrice
return 0;
}

Answer 4

You are starting out of bounds.
For an array of size n you iterate from 0 to n-1
So what you want is
for (a=1;a>=0;a--){
for (b=1;b>=0;b--){
}
}
But one word of warning with running backwards with for loops. As is you are fine because you are using integers but if you were to do something like this
for (auto i = some_vector.size()-1;i>=0;i--){
}
You would be in a lot of trouble since i would not be able to take negative values as some_vector.size() is of type unsigned so the loop would never exit. I tend to always increment in a loop for this reason unless the logic dictates otherwise.

Answer 5

Your matrix has a size of [2][2] but you are using a loop that runs from 2 to 0 (inclusive). In an array of size 2 the maximum permissible index is 1.

Answer 6

I guess that you are just learning basic stuff.. so try to run this slightly version of your code:
#include <stdio.h>
int main() {
int a,i,b,matrice[2][2];
printf("Put inside the matrix some numbers..\n");
for (a=1;a>=0;a--) {
printf("a->%d\n", a);
for (b=1;b>=0;b--) {
printf(" b->%d\n", b);
scanf("%d",&matrice[a][b]);
}
}
printf("Check:\n");
for (a=1;a>=0;a--) {
for (b=1;b>=0;b--) {
printf(" [%d][%d]:%d\n", a, b, matrice[a][b]);
}
}
return 0;
}
In the first loop, the modified code run through the correct indices that for a 2x2 array are (0,0),(0,1),(1,0) and (1,1).
Please note that I have also modified the scanf part using the correct syntax: link.
The second loop is a simple test that what you've coded the data insertion correctly, by outputting the content of the matrix.