C integers off, possible overflow

Question

I'm trying to make a C program that will count and print the number of tabs, spaces, and lines input by the user. The problem is that when it prints those numbers, they are wildly off. Here is my program code:

int c, b, t, nl;
b, t, nl = 0, 0, 0;
while ((c = getchar()) != EOF) 
{
    if (c == '\b') 
        b++;
    if (c == '\t') 
        t++;
    if (c == '\n') 
        nl++;
}
printf("b=%d t=%d nl=%d\n", b, t, nl);

When I input some data from the terminal (3 lines, one space, one tab), the result is b=1899313536, t=32768, and nl=3.

BTW: Good proper type of int c rather than char c.

chux
– chux

2015-01-01 22:59:35 +00:00
Commented Jan 1, 2015 at 22:59 — chux
– chux, Commented Jan 1, 2015 at 22:59

Grzegorz Szpetkowski · Accepted Answer · 2015-01-01 23:48:44Z

12

The problem is with this line:

b, t, nl = 0, 0, 0;

It uses comma operator on both sides of assignment, hence only nl is intialized with zero. There are no side-effects for evaluation of b, t on left side* and two trailing zeros on the right side of = operator (notice that assignment has higher precedence, than comma operator).

Change it to:

b = t = nl = 0;

which effectively means (as = operator has right associativity):

b = (t = (nl = 0));

^{*unless b or t are declared as volatile (as reading of such object is counted as side-effect by C Standard)}

edited Jan 1, 2015 at 23:48

answered Jan 1, 2015 at 22:31

Grzegorz Szpetkowski

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Comments

Iharob Al Asimi · Accepted Answer · 2015-01-01 22:39:57Z

3

b is uinintiaized this expression is wrong

b, t, nl = 0, 0, 0;

it only initializes nl, it should be

b = t = nl = 0;

edited Jan 1, 2015 at 22:39

answered Jan 1, 2015 at 22:29

Iharob Al Asimi

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Comments

Sergey Kalinichenko · Accepted Answer · 2015-01-01 22:31:22Z

2

Unlike Lua or several other languages that let you assign multiple variables in a single statement

v1, v2, v3 = c1, c2, c3; // <<== This is not a valid C syntax.

C requires you to assign them individually:

b = 0;
t = 0;
nl = 0;

Better yet, you could initialize the variables at the time of declaration, like this:

int c, b=0, t=0, nl=0;

The reason why your current code compiles, though, is a little curious: it treats commas in your code as comma operators, which are legal on both sides of the assignment.

answered Jan 1, 2015 at 22:31

Sergey Kalinichenko

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1 Comment

dmckee --- ex-moderator kitten Over a year ago

The comment about why this compiles is useful but might be slightly improved by exhibiting the equivalent line without them.

Jobs · Accepted Answer · 2015-01-01 22:31:54Z

2

Those are garbage values. You need to assign the variables b, t, and nl to 0 individually.

answered Jan 1, 2015 at 22:31

Jobs

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