Can you help me to find the logical error with this C recursive function?

Question

I have this C recursive function

#include<stdio.h> 
int main(){ 
    int entier; 
    int rlt; 
    printf("\nSaisir un entier : ");  
    scanf("%d",&entier); 
    rlt=loga(5); 
    printf("Le logarithme base 2 de %d est %d:",entier,rlt); 
} 

int loga(int x){ 
    if(x==1){
        return 0;
    } 
    else {
        return (loga(x)=1+loga(x/2));
    } 
}

You make it sound as if a recursive function was some sort of disease. — jogojapan, Jan 01 '13 at 12:00
It would also be nice to translate the texts into English. Although we all love and speak French, some don't. — , Jan 01 '13 at 12:03
Welcome to Stack Overflow. Ignore the chauvinists who can't read anything but English. The question should be English; AFAIAC, the code does not have to be. However, your question should identify what the problem is. For example, it should show the error message from the compiler complaining about the problems in the second `return` statement in `loga()`. Since you're using the C99 feature to return 0 from `main()`, you should also follow C99 rules and declare `loga()` before using it. — Jonathan Leffler, Jan 01 '13 at 12:15
@JonathanLeffler: Yes I agree with you for technicality we can check the syntax, but many a times(not in this question) some logic comes from the variablenames, method names, and it becomes quite difficult to understand it. Then we have to discuss in comments. Other way round, few people from non-english region cant even ask question in english that is very difficult to understand. Can we have a translate to english kind of thing in SO? — Anoop Vaidya, Jan 01 '13 at 12:28

score 2 · Answer 1 · answered Jan 01 '13 at 12:02

2

You need to declare your function before you use it.

#include<stdio.h> 

int loga(int x); //declaration

int main(){ 
    //... 
} 

int loga(int x){ 
    if(x==1){return 0;} else {return (loga(x)=1+loga(x/2));} 
}

answered Jan 01 '13 at 12:02

Luchian Grigore

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a bit nit-picky on the terminology wise - it's called a "forward declaration" – Jon Clements Jan 01 '13 at 12:04
thx very much , am kinda new to this stuff ^_^ – user1940935 Jan 01 '13 at 12:06
2

`loga(x)=` also wont work. – TrueBlue Jan 01 '13 at 12:07
1

@JonClements I doubt that, can you provide a reference? AFAIK, forward declaration only refers to classes, and even in that case, it's equivalent to a declaration. – Luchian Grigore Jan 01 '13 at 12:09
yes it won't work I kinda was trying to make a logarithme function – user1940935 Jan 01 '13 at 12:09
1

@user1940935 Since you didn't say what was wrong, I tackled the most evident issue. – Luchian Grigore Jan 01 '13 at 12:10
The code you show can't compile! – Jonathan Leffler Jan 01 '13 at 12:17
@LuchianGrigore - C supports it, and I AFAIK C didn't have classes - ref: http://www.learncpp.com/cpp-tutorial/17-forward-declarations/ – Jon Clements Jan 01 '13 at 12:23
@JonClements The C standard doesn't contain the term "forward declaration". From the language's standpoint, it's just a declaration. Calling it a forward declaration may be useful to convey that it's a declaration that's not part of a definition, but that's a human convention, not part of the language definition. – Daniel Fischer Jan 01 '13 at 15:21
@JonClements C != C++ (the question was tagged C++ when I answered it). But it's irrelevant anyway, Daniel already explained why. – Luchian Grigore Jan 01 '13 at 16:21

score 1 · Answer 2 · answered Jan 01 '13 at 12:05

1

You're assigning into non-lvalue in loga(x)=1+loga(x/2). This is meaningless and invalid.

answered Jan 01 '13 at 12:05

SomeWittyUsername

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any one have a solution ? – user1940935 Jan 01 '13 at 12:10
@user1940935: yes, user [timrau](http://stackoverflow.com/users/718379/timrau) has a [solution](http://stackoverflow.com/a/14110270/15168). – Jonathan Leffler Jan 01 '13 at 12:11
@JonathanLeffler assuming that's the *real* intent of the program... – SomeWittyUsername Jan 01 '13 at 12:13
10x everyone , Is their any good source to learn C++ , am a java developper , but I've foud C++ more cool :P – user1940935 Jan 01 '13 at 12:15
1

@user1940935 See here: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list . BTW, your code is essentially C, not C++ – SomeWittyUsername Jan 01 '13 at 12:16
@user1940935: the code you wrote was not using any distinctive feature of C++ (it was C; that's why it got retagged to C). In C++, every function must be declared before it is used. – Jonathan Leffler Jan 01 '13 at 12:16

score 0 · Accepted Answer · answered Jan 01 '13 at 12:05

0

The recursive part seems strange.

int loga(int x){ 
    if(x==1){
        return 0;
    } else {
        return (1+loga(x/2));
//             ^^^^^^^^^^^^^ Changed here
    } 
}

answered Jan 01 '13 at 12:05

timrau

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I like that Clang-style spot-of-error identification with `^^^^^^`. – Jan 01 '13 at 12:14
Please see that the function's prototype is not defined. It will also generate an error. See my answer posted below please.. – Praveen Vinny Jan 01 '13 at 12:19

score -1 · Answer 4 · answered Jan 01 '13 at 12:17

-1

I think, the following code will solve your issue.

#include<stdio.h> 
int loga(int);

int main()
{ 
    int var; 
    int result; 
    printf("\nPlease enter a value : ");  
    scanf("%d",&var); 
    result=loga(5); 
    printf("The logarithm to the base 2 for %d is %d:",var,result); 
} 

int loga(int x)
    { 
    if(x==1)
        {
        return 0;
        } 
    else
        {
        return (1+loga(x/2));
        } 
      }

This code should execute correctly.

answered Jan 01 '13 at 12:17

Praveen Vinny

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I tried to find log(10) and your program printed the answer as 2. – user93353 Jan 07 '13 at 18:14
@user93353 - I had corrected the error with the SO question only. I had not made any changes to the formula he had used. – Praveen Vinny Jan 08 '13 at 02:43

Can you help me to find the logical error with this C recursive function?

4 Answers4