CODEWITHSUNDEEP
cfor-loopwhile-loop
Michi
Michi

Reputation: 5307

WHILE loop shows a different output than FOR loop

As far as I know the difference between for loop and while loop looks like this:

for:

fist initialization takes place then conditional expression is checked and if it results in TRUE then only statement part gets executed this loop is continuous till conditional expression results FALSE.

While:

conditional expression is checked first and if result TRUE then statement part gets executed otherwise not, this loop is continuous till conditional expression results FALSE.

Today I wrote an Algorithm which check if a string has duplicates, and if so to print only those which aren't:

#include<stdio.h>
#include<string.h>

int main(void){
    const char *str = "Mississippi";
    char tmp[15] = {0};
    size_t i=0,j=0,k=1;
    int found=0;

    tmp[0] = str[0];

    printf("Before = %s\n",str);
    while(str[i] != '\0'){
        for(j=0;tmp[j] != '\0';j++){
            if(tmp[j] == str[i]){
                found++;
            }
        }

        if(found==0){
            tmp[k]=str[i];
            k++;
        }

        found=0;
        i++;
    }

    tmp[strlen(tmp)] = '\0';
    printf("After  = %s\n",tmp);
    return 0;
}

Output:

Before = Mississippi
After  = Misp

Now watch what happens if replace the for loop:

for(j=0;tmp[j] != '\0';j++){
    if(tmp[j] == str[i]){
        found++;
    }
}

With a `while loop:

while(tmp[j] != '\0'){
    if(tmp[j] == str[i]){
        found++;
    }
    j++;
}

I get:

#include<stdio.h>
#include<string.h>

int main(void){
    const char *str = "Mississippi";
    char tmp[15] = {0};
    size_t i=0,j=0,k=1;
    int found=0;

    tmp[0] = str[0];

    printf("Before = %s\n",str);
    while(str[i] != '\0'){
        while(tmp[j] != '\0'){
            if(tmp[j] == str[i]){
                found++;
            }
            j++;
        }

        if(found==0){
            tmp[k]=str[i];
            k++;
        }

        found=0;
        i++;
    }

    tmp[strlen(tmp)] = '\0';
    printf("After  = %s\n",tmp);
    return 0;
}

But the output is not as expected:

Before = Mississippi
After  = Misp

But:

Before = Mississippi
After  = Misisipi

Why does this happen?

Upvotes: 0

Views: 120

Answers (5)

bolov
bolov

Reputation: 75854

for loop

for ( init-statement(optional) ; condition(optional) ;
      iteration_expression(optional) )
  statement

The above syntax produces code equivalent to:

{
  init_statement 
  while ( condition ) { 
    statement 
    iteration_expression ; 
  }
}

Except that

  1. Names declared by the init-statement (if init-statement is a declaration) and names declared by condition (if condition is a declaration) are in the same scope (which is also the scope of statement).
  2. continue in the statement will execute iteration_expression
  3. Empty condition is equivalent to while(true)

Upvotes: 1

dbush
dbush

Reputation: 224852

What's missing is that the for loop is initializing j to 0 when it first enters.

Although j is initialized to 0 when it declared, the while loop is called within another while loop, so it doesn't get reinitialized each time.

Add the initialization:

    j=0;
    while(tmp[j] != '\0'){
        if(tmp[j] == str[i]){
            found++;
        }
        j++;
    }

And you get:

Before = Mississippi
After  = Misp

Upvotes: 2

Draco18s no longer trusts SE
Draco18s no longer trusts SE

Reputation: 15951

You never reset j to 0 with the while loop.

This bit:

for(j=0;

Upvotes: 1

cadaniluk
cadaniluk

Reputation: 15229

Note that the while loop doesn't initialize j to 0.

Remove the initialization of j to 0 and add

j = 0;

in front of your nested while loop.

Upvotes: 1

simpel01
simpel01

Reputation: 1782

Because the while loop does not initialize j=0 at every iteration of the outermost while loop!

Upvotes: 1

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