Can anyone explain why this code prints garbage values?

int main(){
    int a, b, sum;
    sum = a b;
    scanf("%d%d",&a,&b);
    printf("addition is %d", sum);
    return 0;
}

But if I write code like this it prints the correct value?

int main(){
    int a, b, sum;
    scanf("%d%d",&a,&b);
    sum = a b;
    printf("addition is %d", sum);
    return 0;
}

CodePudding user response：

That's because C is an imperative language and each line is like a command, executed from top to bottom. So in this case:

int main(){
  int a, b, sum;        // create 3 variables: a, b and sum. Garbage on start
  sum = a b;            // add 'a' and 'b' and save it in 'sum'. Still garbage          
  scanf("%d%d",&a,&b);  // read 2 numbers and save them in 'a' and 'b'. Nowe they're valid, but 'sum' is still garbage 
  printf("addition is %d", sum); // print 'sum'. That's garbage
  return 0;
}

By simply reordering the operations (adding a and b after reading their values) we get valid result

CodePudding user response：

sum = a b;

is not a definition or a formula, it's a statement - it's executed as soon as it is encountered, with whatever values are in a and b (which are indeterminate).

C just isn't that high-level.