string changling

#include<stdio.h>
#include<string.h>
char x[5],y[5],str[110];
void value(int length)
{
    int i,j,a,b,res;
    for(i=0;i<length;i++){
        if(str[i]=='+'){
            a=str[i-1]-'0';
            b=str[i+1]-'0';
            res=a+b;
            printf("%d\n",res);
            return;
        }
        if(str[i]=='-'){
            a=str[i-1]-'0';
            b=str[i+1]-'0';
            res=a-b;
            printf("%d\n",res);
            return;
        }
        if(str[i]=='*'){
            a=str[i-1]-'0';
            b=str[i+1]-'0';
            res=a*b;
            printf("%d\n",res);
            return;
        }
        if(str[i]=='/'){
            a=str[i-1]-'0';
            b=str[i+1]-'0';
            res=a/b;
            printf("%d\n",res);
            return;
        }
        if(str[i]=='^'){
            a=str[i-1]-'0';
            b=str[i+1]-'0';
            res=1;
            for(j=1;j<=b;j++){
                res*=a;
            }
            printf("%d\n",res);
            return;
        }
    }
}
int main()
{
    int i,length;
    gets(str);
    length=strlen(str);
    printf("Letter to change : ");
    scanf("%s",&x);
    printf("New letter : ");
    scanf("%s",&y);
    for(i=0;i<length;i++){
        if(x[0]==str[i]){
            str[i]=y[0];
        }
    }
    if(y[0]>='0'&&y[0]<='9'){
        value(length);
    }
    else{
        printf("%s\n",str);
    }
    return 0;
}