Shruts_me Shruts_me - 1 year ago 101
C Question

What will be the value of strlen(str)- 1 in a 'for' loop condition when str is empty?

I am analyzing a scenario:

char str[] = ""; // Understand

If I understand
, it comes out to be 0. This is OK.

printf(" %d, %ul, %u, %d, %ul, %u",
strlen(str) - 1,
strlen(str) - 1,
strlen(str) - 1);

Output is:

0, 0l, 0, -1, 4294967295l, 4294967295

I understand these as well.

for (int i = 0; i < strlen(str) - 1; i++)

Here I don't understand what the value of
strlen(str) - 1
would be in the
loop condition.

strlen(str) - 1
is giving the value 4294967295 in the for loop.
Why is that? Why not -1?

Answer Source

This statement

printf(" %d,  %ul,  %u, %d,  %ul, %u", strlen(str),strlen(str),strlen(str),strlen(str)-1,strlen(str)-1,strlen(str)-1);

shows that when strlen( str ) - 1 is outputed like unsigned integer for example using format specifier %ul its value is 4294967295l

In the condition of the loop

for (int i=0;i<strlen(str)-1;i++)

the compiler has to determine the common type of the left and right operands that to determine the type of the result of the condition


The right operand strlen(str)-1 has type size_t (the return type of function strlen is size_t). It is unsigned integer type usually that corresponds to unsigned long. It can not have negative values. Any value that stored in an object of this type is interpretated as a non-negative value and as the output shows the value of strlen(str)-1 is equal to 4294967295l. (The actual value you could get if you used type specifier %zu because it may not be excluded that size_t can correspond even to unsigned long long)

The right operand has type int. Its rank is at least not greater than the rank of size_type. So the both operands are converted to type size_t and have non-negative values.

This procedure of determining of the common type is called the usual arithmetic conversions. It is obvious that 4294967295l is greater than 0. Thus the loop will iterate 4294967295l times if it does not have a break statement.

You could get the expected result if you rewrite the condition in the loop the following way

for ( int i = 0; i < ( int )strlen( str ) - 1; i++ )
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