Thread: Why does this string multiplier code crash after about 200 characters?

  1. #1
    Registered User
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    Oct 2021
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    12

    Why does this string multiplier code crash after about 200 characters?

    effective on small numbers.

    Code:
    #include<stdio.h>
    //#include<math.h>
    #include<stdlib.h>
    #include<string.h>
    
    
    int MAX=1000;
    
    
    char * multiply(char [],char[]);
    int main(){
        char a[MAX];
        char b[MAX];
        char *c;
        int la,lb;
        int i;
        printf("Enter the first number : ");
        scanf("%s",a);
        printf("Enter the second number : ");
        scanf("%s",b);
        printf("Multiplication of two numbers : ");
        c = multiply(a,b);
        printf("%s",c);
        return 0;
    }
    
    
    char * multiply(char a[],char b[]){
        char mul[MAX];
        char c[MAX];
        char temp[MAX];
        int la,lb;
        int i,j,k=0,x=0,y;
        long int r=0;
        long sum = 0;
        la=strlen(a)-1;
            lb=strlen(b)-1;
       
            for(i=0;i<=la;i++){
                    a[i] = a[i] - 48;
            }
    
    
            for(i=0;i<=lb;i++){
                    b[i] = b[i] - 48;
            }
    
    
        for(i=lb;i>=0;i--){
             r=0;
             for(j=la;j>=0;j--){
                 temp[k++] = (b[i]*a[j] + r)%10;
                 r = (b[i]*a[j]+r)/10;
             }
             temp[k++] = r;
             x++;
             for(y = 0;y<x;y++){
                 temp[k++] = 0;
             }
        }
       
        k=0;
        r=0;
        for(i=0;i<la+lb+2;i++){
             sum =0;
             y=0;
             for(j=1;j<=lb+1;j++){
                 if(i <= la+j){
                     sum = sum + temp[y+i];
                 }
                 y += j + la + 1;
             }
             c[k++] = (sum+r) %10;
             r = (sum+r)/10;
        }
        c[k] = r;
        j=0;
        for(i=k-1;i>=0;i--){
             mul[j++]=c[i] + 48;
        }
        mul[j]='\0';
        return mul;
    }

  2. #2
    Registered User
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    Did you try to run the program with your debugger? The debugger should be able to tell you exactly where the problem is detected.


    By the way returning a pointer to a local variable may be part of the problem.

  3. #3
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    Dec 2017
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    1,422
    For a quick fix to returning a local you could make mul static.
    Better still, return a malloc'ed string.
    MAX should be a define.

    In your for (i = lb; i >= 0; i--) loop, k is going way out of bounds.
    So it's not working correctly for small numbers, either, even though in that case it's not causing a segfault.
    For example, when multiplying 1234567890 by 1234567890, k reaches 165 although the result is only 19 digits long.
    A long time ago being crazy meant something. - Charles Manson

  4. #4
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    Oct 2021
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    12
    The entire point of adding strings is to be able to multiply large numbers.
    This method has multiple flaws because it integrates using integers at the wrong places.

    I would recommend not using this code.

    The author writes the numbers code backwards in temp[k++] and then leaves sum and r as long int. This complicates the entire point of using strings.


    I hate freemasonry.
    Last edited by cprogramnoob; 10-24-2021 at 04:25 PM.

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