They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:
Missing header
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
To fix it, we must include the header:
#include <iostream>
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
If you wrote the header and included it correctly, the header may contain the wrong include guard.
To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.
Misspelled variable
Another common source of beginner's error occur when you misspelled a variable:
int main() {
int aComplicatedName;
AComplicatedName = 1; /* mind the uppercase A */
return 0;
}
Incorrect scope
For example, this code would give an error, because you need to use std::string:
#include <string>
int main() {
std::string s1 = "Hello"; // Correct.
string s2 = "world"; // WRONG - would give error.
}
Use before declaration
void f() { g(); }
void g() { }
g has not been declared before its first use. To fix it, either move the definition of g before f:
void g() { }
void f() { g(); }
Or add a declaration of g before f:
void g(); // declaration
void f() { g(); }
void g() { } // definition
stdafx.h not on top (VS-specific)
This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:
#include <iostream>
#include "stdafx.h"
The #include <iostream> would be ignored. You need to move it below:
#include "stdafx.h"
#include <iostream>
Feel free to edit this answer.
Hello, I'm new to programming and while coding my example program I ran into an issue that I've been trying to figure out how to resolve. When print_Menu_Info(); executes and it tries to execute print_Customer_Table(&c); I encounter the error in the title.
sales.c:101:24: error: use of undeclared identifier 'c'
Even though the line is being declared in line 12.
struct customer c = empty_customer;
#include <stdio.h>
#include <stdlib.h>
#include "sales.h"
int main(void)
{
menu_update();
return 0;
}
void menu_update(void){
struct customer c = empty_customer;
float f = 0.00;
int i = 0;
while (1) {
cls();
print_Menu_Info();
switch(get_User_Input()){
case 1:
printf("Customer First Name: ");
scanf("%s",c.first_Name);
break;
case 2:
printf("Customer Last Name: ");
scanf("%s",c.last_Name);
break;
case 3:
printf("Customer Amount: ");
scanf("%f",&f);
c.amount = f;
break;
case 4:
printf("Customer Card Number: ");
scanf("%s",c.card_Number);
break;
case 5:
printf("Customer Card Expiration Month: ");
scanf("%i",&i);
c.card_Expiration_Month = i;
break;
case 6:
printf("Customer Card Expiration Year: ");
scanf("%i",&i);
c.card_Expiration_Year = i;
break;
case 7:
printf("Sales Person: ");
scanf("%s",c.sales_Person);
break;
case 8:
print_Customer_Table(&c);
break;
case 9:
c = empty_customer;
break;
case 10:
exit(0);
default:
printf("Incorrect action ID!\n");
break;
}
}
}
void cls(void){
printf("\033c");
}
int get_User_Input(){
int a = 0;
char line[32];
printf("Please input an action ID: ");
if (fgets(line, sizeof(line), stdin) != NULL) {
sscanf(line, "%i", &a);
cls();
}
return a;
}
void print_Customer_Table(struct customer *cp){
printf(
"1. Customer First Name: %s\n"
"2. Customer Last Name: %s\n"
"3. Customer Amount: $%.2f\n"
"4. Customer Card Number: %s\n"
"5. Customer Card Expiration Month: %i\n"
"6. Customer Card Expiration Year: %i\n"
"7. Sales Person: %s\n",
cp->first_Name,
cp->last_Name,
cp->amount,
cp->card_Number,
cp->card_Expiration_Month,
cp->card_Expiration_Year,
cp->sales_Person);
}
void print_Menu_Info(void){
print_Customer_Table(&c);
printf(
"If you would like to update a value type the value you would like to update(1-7)\n"
"If you would like to send this as a email type 8\n"
"If you would like to clear all customer data type 9\n"
"If you would like to exit this program type 10\n");
} and sales.h
#define NAME_LEN_MAX 128
#define CARD_NUM_LEN_MAX 16
struct customer {
char first_Name[NAME_LEN_MAX + 1];
char last_Name[NAME_LEN_MAX + 1];
float amount;
char card_Number[CARD_NUM_LEN_MAX + 1];
int card_Expiration_Month;
int card_Expiration_Year;
char sales_Person[NAME_LEN_MAX + 1];
};
const static struct customer empty_customer = {{""},{""},0.00,{""},0,0,{""}};
void print_Customer_Table(struct customer *cp);
void print_Menu_Info(void);
void cls(void);
int get_User_Input();
void menu_update(void);Videos
Hi CS50 gang!
I'm really enjoying the course, but my brain hurts already at week 2. I've noticed an error when compiling a programme in C when I try to declare a variable inside a FOR loop in C (example below).
I just move my variable declaration to above the FOR loop, but I have two questions about this:
-
Why does this happen? Is it because the loop tries to declare the variable each time it iterates?
-
Does it cause problems to declare the variable (int letter_count) above the loop, like I have done in the example below? Or is there a better convention for declaring variables?
int letter_count = 0;
//loop to break text string into chars
for (int i = 0, n = strlen(text); i < n; i++)
{
//check if char i is lower or upper and increment letter-count by 1 if true
if (isupper(text[i]) || islower(text[i]))
{
letter_count++;
}
printf("%c", text[i]);
}Thank you!
They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:
Missing header
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
To fix it, we must include the header:
#include <iostream>
int main() {
std::cout << "Hello world!" << std::endl;
return 0;
}
If you wrote the header and included it correctly, the header may contain the wrong include guard.
To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.
Misspelled variable
Another common source of beginner's error occur when you misspelled a variable:
int main() {
int aComplicatedName;
AComplicatedName = 1; /* mind the uppercase A */
return 0;
}
Incorrect scope
For example, this code would give an error, because you need to use std::string:
#include <string>
int main() {
std::string s1 = "Hello"; // Correct.
string s2 = "world"; // WRONG - would give error.
}
Use before declaration
void f() { g(); }
void g() { }
g has not been declared before its first use. To fix it, either move the definition of g before f:
void g() { }
void f() { g(); }
Or add a declaration of g before f:
void g(); // declaration
void f() { g(); }
void g() { } // definition
stdafx.h not on top (VS-specific)
This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:
#include <iostream>
#include "stdafx.h"
The #include <iostream> would be ignored. You need to move it below:
#include "stdafx.h"
#include <iostream>
Feel free to edit this answer.
Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.
The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.
If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.
Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.
int total = 0;
for (int i= 0; word[i] != '\0'; i++)
;
{
char c = word[i];
if (isalpha(c))
total += POINTS[get_index(c)];
}
return total;
}
int get_index(char c)
{
return tolower(c) - 'a';When I compile the code I get undeclared identifier 'i'. But when declare it I get declaration shadows a variable in the global scope [-Werror,-Wshadow]. Please help
In C, at least back in the old days, variable declarations have to come at the top of the block. C++ is different in that regard.
edit — apparently C99 is different from C90 in this respect (C99 being essentially the same as C++ on this issue).
Objects may only be declared at the top of a statement block in ISO C90. You can therefore do this:
#include<stdio.h>
int main()
{
int val1,val2;
printf("Enter the first value");
scanf("%d",val1);
scanf("%d",&val2);
// New statement block
{
int c;
c=val1 + val2;
printf(" the value is : %d", c);
}
return 0; // 0 means no error
}
Though it would perhaps be unusual to do so. Contrary to somewhat popular belief, the start of a function is not the only place you can declare an automatic variable. It is more common, rather than creating a dummy block, to use existing statement blocks introduced as part of an if or for construct for example.
It is useful to enclose case blocks in { ... }, even though not normally necessary, so that you can introduce temporary case specific variables:
switch( x )
{
case SOMETHING :
{
int case_local = 0 ;
}
break ;
...
}
As you are not using any braces for the inner for loop,so the value of i is not known to that printf statement.In conditional and looping statements,if there are no braces (or block created )for them,then they can only manipulate and have scope restricted only upto the statement just after there declaration.
for (int i = 1; i < m; i++)
new_u[i] = old_u[i] + d/(h*h)*(old_u[i-1] - 2*old_u[i] + old_u[i+1]); //i is known here
printf("%f", new_u[i]); //i is not available for this
use braces like this
for (int i = 1; i < m; i++)
{
new_u[i] = old_u[i] + d/(h*h)*(old_u[i-1] - 2*old_u[i] + old_u[i+1]);
printf("%f", new_u[i]);
}
The unknown identifier is i, not new_u; you need braces:
for (int i = 1; i < m; i++) {
new_u[i] = old_u[i] + d/(h*h)*(old_u[i-1]
- 2*old_u[i] + old_u[i+1]);
if (i>1) putchar(' ');
printf("%f", new_u[i]);
}
In your wrong version of the code i was visible only by the assignment to new_u[i] not by your printf
BTW, you probably want a space to be put between your numbers, like I did with putchar. Otherwise the output could be unreadable, e.g. 1.233.45 for two numbers 1.23 and 3.45
Since you have a for (int i=1; loop the scope of i is only the (test, increment and) body of your for; in your case that body was a single assignment.
The problem is the semicolon on line 138:
for (int column = 0; column < d - 1; column++);
In C, a semicolon is used to terminate a statement. The body of the for loop (the part in curly braces) is part of the loop statement. If you add a semicolon after the declaration, but before the curly braces, the compiler considers the loop statement to be terminated before the body even starts, producing the for loop has empty body warning.
Now, the loop variable column that you declared in line 138 is only in-scope during the body of the loop. Since you accidentally terminated the loop before giving it a body, column leaves scope immediately after the declaration on line 138. Since you haven't previously declared column in the scope outside of the loop, you get the use of undeclared identifier error.
If you delete the semicolon from the end of line 138 it should fix at least two of those errors; since row is also raising an error, it's likely you have the same problem in an earlier loop declaration. In general, you don't need to use semicolons after for loop declarations or body blocks in C. You only need them in the declaration itself and on the individual statements inside the body. Like this:
for (int i = 0; i < 5; i++)
{
printf('%i\n', i);
}
Notice there is no semicolon at the end of the loop declaration or after the curly brace that closes the loop body.
The above user has identified the syntax errors in your code and also explained the reason.Here is some more info on the error:
Most of the time use of an undeclared identifier error occurs when you try to use a variable without first declaring it.
To declare a variable one must write its type followed by its name (also known as identifier). Once you've declared it, only then you are allowed to use it in expressions or assignments.For example:
int num; //variable declaration
num=10;
The above code will not give an error but if you skip the declaration part,most probably the compiler will through a use of an undeclared identifier error.
Another way this error pops up is when you try to access a variable outside the block(a piece of code written between curly braces { } ) where it was originally defined.This is the case when you define a variable in a function body or a loop body and try to access it outside the loop or function.
Here, the compiler will throw an error because for it these variables cease to exist once the block has executed.Such variables are called local variables.The code outside their respective block is said to be outside of their scope. You can read more about this when you study C-Scope rules.Here's a useful link on Scope rules in C: http://www.geeksforgeeks.org/scope-rules-in-c/