All issues corrected

issue1.c

@@ -8,7 +8,7 @@ char* reverse(char* str){
     char* rev = (char*)malloc(sizeof(str));
     int size = strlen(str);
     for(int i=0;i<size;i++){
-        rev[i] = str[i];
+        rev[i] = str[size-i-1];
     }
     return rev;
 }

issue2.c

@@ -4,7 +4,7 @@
 
 int main(){
     int even = 0;
-    for(int i = 0; i <= 20; i++){
+    for(int even = 0; even<= 20; even++){
         even *= 2;
         printf("%d\t", even);
     }

issue3.c

@@ -9,7 +9,7 @@ int main(){
     char new[50] = ""; 
 
     for(int i = 0; i < 6; i++){
-        new[i] =  z;
+        new[i] =  z[i];
     }
 
     for(int i = 0; i < 6; i++){

issue4.c

@@ -4,7 +4,7 @@
 
 void sort(int a[], int b){
     for(int i = 0; i < b; i++){
-        for(int j = 1; j < b; j++){
+        for(int j = i+1; j < b; j++){
             if(a[i] > a[j]){
                 int temp = a[i];
                 a[i] = a[j];
@@ -20,6 +20,6 @@ int main(){
     sort(numbers, 6);
 
     for(int i = 0; i < 6; i++){
-        printf("%d", numbers[i]);
+        printf("%d ", numbers[i]);
     }
 }

issue5.c

@@ -2,7 +2,7 @@
 
 #include<stdio.h>
 int main(void){
-    int flag = 0;
+    int flag ;
 
     printf("Choose the operation:-\n1. Multiples of 2\n2. Multiples of 3\n3. Multiples of 4\n");
     scanf(" %d", &flag);
@@ -12,16 +12,20 @@ int main(void){
             for(int i=1;i<6;i++){
                 printf("%d\t", 2*i);
             }
+            break;
+
             printf("\n");
         case 2 : 
             for(int i=1;i<6;i++){
                 printf("%d\t", 3*i);
             }
+            break;
             printf("\n");
         case 3 : 
             for(int i=1;i<6;i++){
                 printf("%d\t", 4*i);
             }
+            break;
             printf("\n");
         default :
                 printf("Choose a valid option!!!\n");

issue6.c

@@ -6,7 +6,7 @@ int main(void){
     char* str1 = "This is a string";
     char* str2 = "This is another string";
 
-    if(strcmp(str1, str2))
+    if(!strcmp(str1, str2))
         printf("Strings are same.\n");
     else
         printf("Strings are not same.\n");

issue7.c

@@ -7,5 +7,5 @@ int main(void){
     int a = 5;
     int* b = (int*)malloc(sizeof(int));
     *b = 7;
-    printf("%d\n", (a+b));
+    printf("%d\n", (a+*b));
 }

issue8.c

@@ -5,7 +5,7 @@
 #include<string.h>
 int main(void){
     char* str = "Welcome to IEEE\n";
-    for(int i=0;i<100;i++){
+    for(int i=0;str[i]!='\0';i++){
         printf("%c", str[i]);
     }
     printf("\n");

issue9.c

@@ -2,7 +2,7 @@
 
 #include<stdio.h>
 int factorial(int n){
-    if(n == 2)
+    if(n == 0)
         return 1;
     else
         return n*factorial(n-1);