Question about the first exercise

Hi All,

I’m fairly new to C and Objective-C and was wondering why I get an error (Passing ‘double’ to parameter of incompatible type ‘const char *’) When I try to print out just a variable, yet that error goes away when I add a string to the print out.

Thank you in advance for the input.

I am very pleased to see that you are doing Objective-C!

As to what you are experiencing.

When calling a function, always pass arguments of compatible types, as expected by the function.

A function call must always be preceded by its declaration. The declaration specifies the types of the parameters the function expects to receive.

Here is a complete example:

#import <Foundation/Foundation.h>

// Declare functions before use
void printCString (const char *);
void printDoubleValue (const double);

int main (int argc, const char * argv[]) {
    @autoreleasepool {
        const char * str = "PI ~= 22.0/7";
        printCString (str);
        printDoubleValue (strlen (str));
        printDoubleValue (M_PI);
        printDoubleValue (22.0/7);
    return 0;

void printCString (const char *str) {
    NSLog (@"%s: %s", __func__, str);

void printDoubleValue (const double v) {
    NSLog (@"%s: %f", __func__, v);

The above example produced the following output.

2018-09-02 21:59:17.289771+0930 Params[65720:7276979] printCString: PI ~= 22.0/7
2018-09-02 21:59:17.290167+0930 Params[65720:7276979] printDoubleValue: 12.000000
2018-09-02 21:59:17.290196+0930 Params[65720:7276979] printDoubleValue: 3.141593
2018-09-02 21:59:17.290218+0930 Params[65720:7276979] printDoubleValue: 3.142857