String variable assignment gives pointer error

Question

string reading_lev(int a, int b, int c)
{
    float L = (a / b) * 100;
    float S = (c / b) * 100;
    float index = 0.0588 * L - 0.296 * S - 15.8;
    if (round(index) <= 16 && round(index) >= 1)
    {
        string val = printf("Grade %f", index);
    }
    else if (round(index) > 16)
    {
        string val = printf("Grade 16+");
    }
    else
    {
        string val = printf("Before Grade 1");
    }
    return val
}

The error is in the first if block. There are cs50 libraries involved.

error: incompatible integer to pointer conversion initializing 'string' (aka 'char *') with an expression of type 'int' [-Werror,-Wint-conversion]

`string val = printf(...)`? `printf` returns an `int`, you need to allocate space and then use [snprintf](http://www.cplusplus.com/reference/cstdio/snprintf/) to return a string. — David Ranieri, Apr 08 '20 at 20:06
`val` only exists until the closing brace `}`, so the `return` should also be generating an error. — user3386109, Apr 08 '20 at 20:08

Sourav Ghosh · Answer 1 · 2020-04-08T20:30:41.677

4

The error message is self explanatory.

printf() returns an int, you cannot assign it to variable of type char*.

That said, you have multiple other issues:

The return statement uses a block scope variable, which is out of it's scope.
The return statement is missing a ; - syntax error.

To fix the code what you need to do is:

Allocate a buffer long enough to hold the final output. (Define a pointer and use allocated memory using malloc() or family, of sufficient size)
Use sprintf() to populate the memory with the output you need.
Return the pointer.
Once you're done using it, free() the returned pointer.

edited Apr 08 '20 at 20:30

answered Apr 08 '20 at 20:08

Sourav Ghosh

133,132
16
183
261

I'm sorry if this is really basic but I am new. How can printf() return an int? Isn't everything in " "? – Kushagra Sharma Apr 08 '20 at 20:10
2

@KushagraSharma Read the linked man page.What printf prints and what is returns are not the same. – Sourav Ghosh Apr 08 '20 at 20:11
@kaylum Updated. – Sourav Ghosh Apr 08 '20 at 20:11
1

@KushagraSharma One of the first things you need to do as a C programmer is find the documentation. On unix systems, you can open a terminal window and type `man 3 printf` to get the documentation for `printf`. On windows, the documentation is online by searching `msdn printf`. You might also want to invest in a [good reference book](https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list). – user3386109 Apr 08 '20 at 20:14
@user3386109 Its 2 in the morning where I live X( could you please briefly tell me how to fix my code? It would really help me. – Kushagra Sharma Apr 08 '20 at 20:23

score 1 · Answer 2 · edited Apr 09 '20 at 00:17

1

You can use sprintf to save the formatted data to a string. Be aware that you need a buffer big enough to save the string.

http://www.cplusplus.com/reference/cstdio/sprintf/

edited Apr 09 '20 at 00:17

cmp

35
6

answered Apr 08 '20 at 20:22

Guillermo Adrian Rodrguez Barr

81
5

Its 2 in the morning where I live X( could you please briefly tell me how to fix my code? It would really help me. – Kushagra Sharma Apr 08 '20 at 20:39

String variable assignment gives pointer error

2 Answers2